1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
|
<html><head><title>NASM Manual</title></head>
<body><h1 align=center>The Netwide Assembler: NASM</h1>
<p align=center><a href="nasmdoc5.html">Next Chapter</a> |
<a href="nasmdoc3.html">Previous Chapter</a> |
<a href="nasmdoc0.html">Contents</a> |
<a href="nasmdoci.html">Index</a>
<h2><a name="chapter-4">Chapter 4: The NASM Preprocessor</a></h2>
<p>NASM contains a powerful macro processor, which supports conditional
assembly, multi-level file inclusion, two forms of macro (single-line and
multi-line), and a `context stack' mechanism for extra macro power.
Preprocessor directives all begin with a <code><nobr>%</nobr></code> sign.
<p>The preprocessor collapses all lines which end with a backslash (\)
character into a single line. Thus:
<p><pre>
%define THIS_VERY_LONG_MACRO_NAME_IS_DEFINED_TO \
THIS_VALUE
</pre>
<p>will work like a single-line macro without the backslash-newline
sequence.
<h3><a name="section-4.1">4.1 Single-Line Macros</a></h3>
<h4><a name="section-4.1.1">4.1.1 The Normal Way: <code><nobr>%define</nobr></code></a></h4>
<p>Single-line macros are defined using the
<code><nobr>%define</nobr></code> preprocessor directive. The definitions
work in a similar way to C; so you can do things like
<p><pre>
%define ctrl 0x1F &
%define param(a,b) ((a)+(a)*(b))
mov byte [param(2,ebx)], ctrl 'D'
</pre>
<p>which will expand to
<p><pre>
mov byte [(2)+(2)*(ebx)], 0x1F & 'D'
</pre>
<p>When the expansion of a single-line macro contains tokens which invoke
another macro, the expansion is performed at invocation time, not at
definition time. Thus the code
<p><pre>
%define a(x) 1+b(x)
%define b(x) 2*x
mov ax,a(8)
</pre>
<p>will evaluate in the expected way to
<code><nobr>mov ax,1+2*8</nobr></code>, even though the macro
<code><nobr>b</nobr></code> wasn't defined at the time of definition of
<code><nobr>a</nobr></code>.
<p>Macros defined with <code><nobr>%define</nobr></code> are case
sensitive: after <code><nobr>%define foo bar</nobr></code>, only
<code><nobr>foo</nobr></code> will expand to <code><nobr>bar</nobr></code>:
<code><nobr>Foo</nobr></code> or <code><nobr>FOO</nobr></code> will not. By
using <code><nobr>%idefine</nobr></code> instead of
<code><nobr>%define</nobr></code> (the `i' stands for `insensitive') you
can define all the case variants of a macro at once, so that
<code><nobr>%idefine foo bar</nobr></code> would cause
<code><nobr>foo</nobr></code>, <code><nobr>Foo</nobr></code>,
<code><nobr>FOO</nobr></code>, <code><nobr>fOO</nobr></code> and so on all
to expand to <code><nobr>bar</nobr></code>.
<p>There is a mechanism which detects when a macro call has occurred as a
result of a previous expansion of the same macro, to guard against circular
references and infinite loops. If this happens, the preprocessor will only
expand the first occurrence of the macro. Hence, if you code
<p><pre>
%define a(x) 1+a(x)
mov ax,a(3)
</pre>
<p>the macro <code><nobr>a(3)</nobr></code> will expand once, becoming
<code><nobr>1+a(3)</nobr></code>, and will then expand no further. This
behaviour can be useful: see <a href="nasmdoc9.html#section-9.1">section
9.1</a> for an example of its use.
<p>You can overload single-line macros: if you write
<p><pre>
%define foo(x) 1+x
%define foo(x,y) 1+x*y
</pre>
<p>the preprocessor will be able to handle both types of macro call, by
counting the parameters you pass; so <code><nobr>foo(3)</nobr></code> will
become <code><nobr>1+3</nobr></code> whereas
<code><nobr>foo(ebx,2)</nobr></code> will become
<code><nobr>1+ebx*2</nobr></code>. However, if you define
<p><pre>
%define foo bar
</pre>
<p>then no other definition of <code><nobr>foo</nobr></code> will be
accepted: a macro with no parameters prohibits the definition of the same
name as a macro <em>with</em> parameters, and vice versa.
<p>This doesn't prevent single-line macros being <em>redefined</em>: you
can perfectly well define a macro with
<p><pre>
%define foo bar
</pre>
<p>and then re-define it later in the same source file with
<p><pre>
%define foo baz
</pre>
<p>Then everywhere the macro <code><nobr>foo</nobr></code> is invoked, it
will be expanded according to the most recent definition. This is
particularly useful when defining single-line macros with
<code><nobr>%assign</nobr></code> (see <a href="#section-4.1.7">section
4.1.7</a>).
<p>You can pre-define single-line macros using the `-d' option on the NASM
command line: see <a href="nasmdoc2.html#section-2.1.18">section
2.1.18</a>.
<h4><a name="section-4.1.2">4.1.2 Resolving <code><nobr>%define</nobr></code>: <code><nobr>%xdefine</nobr></code></a></h4>
<p>To have a reference to an embedded single-line macro resolved at the
time that the embedding macro is <em>defined</em>, as opposed to when the
embedding macro is <em>expanded</em>, you need a different mechanism to the
one offered by <code><nobr>%define</nobr></code>. The solution is to use
<code><nobr>%xdefine</nobr></code>, or it's case-insensitive counterpart
<code><nobr>%ixdefine</nobr></code>.
<p>Suppose you have the following code:
<p><pre>
%define isTrue 1
%define isFalse isTrue
%define isTrue 0
val1: db isFalse
%define isTrue 1
val2: db isFalse
</pre>
<p>In this case, <code><nobr>val1</nobr></code> is equal to 0, and
<code><nobr>val2</nobr></code> is equal to 1. This is because, when a
single-line macro is defined using <code><nobr>%define</nobr></code>, it is
expanded only when it is called. As <code><nobr>isFalse</nobr></code>
expands to <code><nobr>isTrue</nobr></code>, the expansion will be the
current value of <code><nobr>isTrue</nobr></code>. The first time it is
called that is 0, and the second time it is 1.
<p>If you wanted <code><nobr>isFalse</nobr></code> to expand to the value
assigned to the embedded macro <code><nobr>isTrue</nobr></code> at the time
that <code><nobr>isFalse</nobr></code> was defined, you need to change the
above code to use <code><nobr>%xdefine</nobr></code>.
<p><pre>
%xdefine isTrue 1
%xdefine isFalse isTrue
%xdefine isTrue 0
val1: db isFalse
%xdefine isTrue 1
val2: db isFalse
</pre>
<p>Now, each time that <code><nobr>isFalse</nobr></code> is called, it
expands to 1, as that is what the embedded macro
<code><nobr>isTrue</nobr></code> expanded to at the time that
<code><nobr>isFalse</nobr></code> was defined.
<h4><a name="section-4.1.3">4.1.3 Macro Indirection: <code><nobr>%[...]</nobr></code></a></h4>
<p>The <code><nobr>%[...]</nobr></code> construct can be used to expand
macros in contexts where macro expansion would otherwise not occur,
including in the names other macros. For example, if you have a set of
macros named <code><nobr>Foo16</nobr></code>,
<code><nobr>Foo32</nobr></code> and <code><nobr>Foo64</nobr></code>, you
could write:
<p><pre>
mov ax,Foo%[__BITS__] ; The Foo value
</pre>
<p>to use the builtin macro <code><nobr>__BITS__</nobr></code> (see
<a href="#section-4.11.5">section 4.11.5</a>) to automatically select
between them. Similarly, the two statements:
<p><pre>
%xdefine Bar Quux ; Expands due to %xdefine
%define Bar %[Quux] ; Expands due to %[...]
</pre>
<p>have, in fact, exactly the same effect.
<p><code><nobr>%[...]</nobr></code> concatenates to adjacent tokens in the
same way that multi-line macro parameters do, see
<a href="#section-4.3.8">section 4.3.8</a> for details.
<h4><a name="section-4.1.4">4.1.4 Concatenating Single Line Macro Tokens: <code><nobr>%+</nobr></code></a></h4>
<p>Individual tokens in single line macros can be concatenated, to produce
longer tokens for later processing. This can be useful if there are several
similar macros that perform similar functions.
<p>Please note that a space is required after <code><nobr>%+</nobr></code>,
in order to disambiguate it from the syntax <code><nobr>%+1</nobr></code>
used in multiline macros.
<p>As an example, consider the following:
<p><pre>
%define BDASTART 400h ; Start of BIOS data area
</pre>
<p><pre>
struc tBIOSDA ; its structure
.COM1addr RESW 1
.COM2addr RESW 1
; ..and so on
endstruc
</pre>
<p>Now, if we need to access the elements of tBIOSDA in different places,
we can end up with:
<p><pre>
mov ax,BDASTART + tBIOSDA.COM1addr
mov bx,BDASTART + tBIOSDA.COM2addr
</pre>
<p>This will become pretty ugly (and tedious) if used in many places, and
can be reduced in size significantly by using the following macro:
<p><pre>
; Macro to access BIOS variables by their names (from tBDA):
</pre>
<p><pre>
%define BDA(x) BDASTART + tBIOSDA. %+ x
</pre>
<p>Now the above code can be written as:
<p><pre>
mov ax,BDA(COM1addr)
mov bx,BDA(COM2addr)
</pre>
<p>Using this feature, we can simplify references to a lot of macros (and,
in turn, reduce typing errors).
<h4><a name="section-4.1.5">4.1.5 The Macro Name Itself: <code><nobr>%?</nobr></code> and <code><nobr>%??</nobr></code></a></h4>
<p>The special symbols <code><nobr>%?</nobr></code> and
<code><nobr>%??</nobr></code> can be used to reference the macro name
itself inside a macro expansion, this is supported for both single-and
multi-line macros. <code><nobr>%?</nobr></code> refers to the macro name as
<em>invoked</em>, whereas <code><nobr>%??</nobr></code> refers to the macro
name as <em>declared</em>. The two are always the same for case-sensitive
macros, but for case-insensitive macros, they can differ.
<p>For example:
<p><pre>
%idefine Foo mov %?,%??
foo
FOO
</pre>
<p>will expand to:
<p><pre>
mov foo,Foo
mov FOO,Foo
</pre>
<p>The sequence:
<p><pre>
%idefine keyword $%?
</pre>
<p>can be used to make a keyword "disappear", for example in case a new
instruction has been used as a label in older code. For example:
<p><pre>
%idefine pause $%? ; Hide the PAUSE instruction
</pre>
<h4><a name="section-4.1.6">4.1.6 Undefining Single-Line Macros: <code><nobr>%undef</nobr></code></a></h4>
<p>Single-line macros can be removed with the
<code><nobr>%undef</nobr></code> directive. For example, the following
sequence:
<p><pre>
%define foo bar
%undef foo
mov eax, foo
</pre>
<p>will expand to the instruction <code><nobr>mov eax, foo</nobr></code>,
since after <code><nobr>%undef</nobr></code> the macro
<code><nobr>foo</nobr></code> is no longer defined.
<p>Macros that would otherwise be pre-defined can be undefined on the
command-line using the `-u' option on the NASM command line: see
<a href="nasmdoc2.html#section-2.1.19">section 2.1.19</a>.
<h4><a name="section-4.1.7">4.1.7 Preprocessor Variables: <code><nobr>%assign</nobr></code></a></h4>
<p>An alternative way to define single-line macros is by means of the
<code><nobr>%assign</nobr></code> command (and its case-insensitive
counterpart <code><nobr>%iassign</nobr></code>, which differs from
<code><nobr>%assign</nobr></code> in exactly the same way that
<code><nobr>%idefine</nobr></code> differs from
<code><nobr>%define</nobr></code>).
<p><code><nobr>%assign</nobr></code> is used to define single-line macros
which take no parameters and have a numeric value. This value can be
specified in the form of an expression, and it will be evaluated once, when
the <code><nobr>%assign</nobr></code> directive is processed.
<p>Like <code><nobr>%define</nobr></code>, macros defined using
<code><nobr>%assign</nobr></code> can be re-defined later, so you can do
things like
<p><pre>
%assign i i+1
</pre>
<p>to increment the numeric value of a macro.
<p><code><nobr>%assign</nobr></code> is useful for controlling the
termination of <code><nobr>%rep</nobr></code> preprocessor loops: see
<a href="#section-4.5">section 4.5</a> for an example of this. Another use
for <code><nobr>%assign</nobr></code> is given in
<a href="nasmdoc8.html#section-8.4">section 8.4</a> and
<a href="nasmdoc9.html#section-9.1">section 9.1</a>.
<p>The expression passed to <code><nobr>%assign</nobr></code> is a critical
expression (see <a href="nasmdoc3.html#section-3.8">section 3.8</a>), and
must also evaluate to a pure number (rather than a relocatable reference
such as a code or data address, or anything involving a register).
<h4><a name="section-4.1.8">4.1.8 Defining Strings: <code><nobr>%defstr</nobr></code></a></h4>
<p><code><nobr>%defstr</nobr></code>, and its case-insensitive counterpart
<code><nobr>%idefstr</nobr></code>, define or redefine a single-line macro
without parameters but converts the entire right-hand side, after macro
expansion, to a quoted string before definition.
<p>For example:
<p><pre>
%defstr test TEST
</pre>
<p>is equivalent to
<p><pre>
%define test 'TEST'
</pre>
<p>This can be used, for example, with the <code><nobr>%!</nobr></code>
construct (see <a href="#section-4.10.2">section 4.10.2</a>):
<p><pre>
%defstr PATH %!PATH ; The operating system PATH variable
</pre>
<h4><a name="section-4.1.9">4.1.9 Defining Tokens: <code><nobr>%deftok</nobr></code></a></h4>
<p><code><nobr>%deftok</nobr></code>, and its case-insensitive counterpart
<code><nobr>%ideftok</nobr></code>, define or redefine a single-line macro
without parameters but converts the second parameter, after string
conversion, to a sequence of tokens.
<p>For example:
<p><pre>
%deftok test 'TEST'
</pre>
<p>is equivalent to
<p><pre>
%define test TEST
</pre>
<h3><a name="section-4.2">4.2 String Manipulation in Macros</a></h3>
<p>It's often useful to be able to handle strings in macros. NASM supports
a few simple string handling macro operators from which more complex
operations can be constructed.
<p>All the string operators define or redefine a value (either a string or
a numeric value) to a single-line macro. When producing a string value, it
may change the style of quoting of the input string or strings, and
possibly use <code><nobr>\</nobr></code>-escapes inside
<code><nobr>`</nobr></code>-quoted strings.
<h4><a name="section-4.2.1">4.2.1 Concatenating Strings: <code><nobr>%strcat</nobr></code></a></h4>
<p>The <code><nobr>%strcat</nobr></code> operator concatenates quoted
strings and assign them to a single-line macro.
<p>For example:
<p><pre>
%strcat alpha "Alpha: ", '12" screen'
</pre>
<p>... would assign the value <code><nobr>'Alpha: 12" screen'</nobr></code>
to <code><nobr>alpha</nobr></code>. Similarly:
<p><pre>
%strcat beta '"foo"\', "'bar'"
</pre>
<p>... would assign the value <code><nobr>`"foo"\\'bar'`</nobr></code> to
<code><nobr>beta</nobr></code>.
<p>The use of commas to separate strings is permitted but optional.
<h4><a name="section-4.2.2">4.2.2 String Length: <code><nobr>%strlen</nobr></code></a></h4>
<p>The <code><nobr>%strlen</nobr></code> operator assigns the length of a
string to a macro. For example:
<p><pre>
%strlen charcnt 'my string'
</pre>
<p>In this example, <code><nobr>charcnt</nobr></code> would receive the
value 9, just as if an <code><nobr>%assign</nobr></code> had been used. In
this example, <code><nobr>'my string'</nobr></code> was a literal string
but it could also have been a single-line macro that expands to a string,
as in the following example:
<p><pre>
%define sometext 'my string'
%strlen charcnt sometext
</pre>
<p>As in the first case, this would result in
<code><nobr>charcnt</nobr></code> being assigned the value of 9.
<h4><a name="section-4.2.3">4.2.3 Extracting Substrings: <code><nobr>%substr</nobr></code></a></h4>
<p>Individual letters or substrings in strings can be extracted using the
<code><nobr>%substr</nobr></code> operator. An example of its use is
probably more useful than the description:
<p><pre>
%substr mychar 'xyzw' 1 ; equivalent to %define mychar 'x'
%substr mychar 'xyzw' 2 ; equivalent to %define mychar 'y'
%substr mychar 'xyzw' 3 ; equivalent to %define mychar 'z'
%substr mychar 'xyzw' 2,2 ; equivalent to %define mychar 'yz'
%substr mychar 'xyzw' 2,-1 ; equivalent to %define mychar 'yzw'
%substr mychar 'xyzw' 2,-2 ; equivalent to %define mychar 'yz'
</pre>
<p>As with <code><nobr>%strlen</nobr></code> (see
<a href="#section-4.2.2">section 4.2.2</a>), the first parameter is the
single-line macro to be created and the second is the string. The third
parameter specifies the first character to be selected, and the optional
fourth parameter preceeded by comma) is the length. Note that the first
index is 1, not 0 and the last index is equal to the value that
<code><nobr>%strlen</nobr></code> would assign given the same string. Index
values out of range result in an empty string. A negative length means
"until N-1 characters before the end of string", i.e.
<code><nobr>-1</nobr></code> means until end of string,
<code><nobr>-2</nobr></code> until one character before, etc.
<h3><a name="section-4.3">4.3 Multi-Line Macros: <code><nobr>%macro</nobr></code></a></h3>
<p>Multi-line macros are much more like the type of macro seen in MASM and
TASM: a multi-line macro definition in NASM looks something like this.
<p><pre>
%macro prologue 1
push ebp
mov ebp,esp
sub esp,%1
%endmacro
</pre>
<p>This defines a C-like function prologue as a macro: so you would invoke
the macro with a call such as
<p><pre>
myfunc: prologue 12
</pre>
<p>which would expand to the three lines of code
<p><pre>
myfunc: push ebp
mov ebp,esp
sub esp,12
</pre>
<p>The number <code><nobr>1</nobr></code> after the macro name in the
<code><nobr>%macro</nobr></code> line defines the number of parameters the
macro <code><nobr>prologue</nobr></code> expects to receive. The use of
<code><nobr>%1</nobr></code> inside the macro definition refers to the
first parameter to the macro call. With a macro taking more than one
parameter, subsequent parameters would be referred to as
<code><nobr>%2</nobr></code>, <code><nobr>%3</nobr></code> and so on.
<p>Multi-line macros, like single-line macros, are case-sensitive, unless
you define them using the alternative directive
<code><nobr>%imacro</nobr></code>.
<p>If you need to pass a comma as <em>part</em> of a parameter to a
multi-line macro, you can do that by enclosing the entire parameter in
braces. So you could code things like
<p><pre>
%macro silly 2
%2: db %1
%endmacro
silly 'a', letter_a ; letter_a: db 'a'
silly 'ab', string_ab ; string_ab: db 'ab'
silly {13,10}, crlf ; crlf: db 13,10
</pre>
<h4><a name="section-4.3.1">4.3.1 Recursive Multi-Line Macros: <code><nobr>%rmacro</nobr></code></a></h4>
<p>A multi-line macro cannot be referenced within itself, in order to
prevent accidental infinite recursion.
<p>Recursive multi-line macros allow for self-referencing, with the caveat
that the user is aware of the existence, use and purpose of recursive
multi-line macros. There is also a generous, but sane, upper limit to the
number of recursions, in order to prevent run-away memory consumption in
case of accidental infinite recursion.
<p>As with non-recursive multi-line macros, recursive multi-line macros are
case-sensitive, unless you define them using the alternative directive
<code><nobr>%irmacro</nobr></code>.
<h4><a name="section-4.3.2">4.3.2 Overloading Multi-Line Macros</a></h4>
<p>As with single-line macros, multi-line macros can be overloaded by
defining the same macro name several times with different numbers of
parameters. This time, no exception is made for macros with no parameters
at all. So you could define
<p><pre>
%macro prologue 0
push ebp
mov ebp,esp
%endmacro
</pre>
<p>to define an alternative form of the function prologue which allocates
no local stack space.
<p>Sometimes, however, you might want to `overload' a machine instruction;
for example, you might want to define
<p><pre>
%macro push 2
push %1
push %2
%endmacro
</pre>
<p>so that you could code
<p><pre>
push ebx ; this line is not a macro call
push eax,ecx ; but this one is
</pre>
<p>Ordinarily, NASM will give a warning for the first of the above two
lines, since <code><nobr>push</nobr></code> is now defined to be a macro,
and is being invoked with a number of parameters for which no definition
has been given. The correct code will still be generated, but the assembler
will give a warning. This warning can be disabled by the use of the
<code><nobr>-w-macro-params</nobr></code> command-line option (see
<a href="nasmdoc2.html#section-2.1.24">section 2.1.24</a>).
<h4><a name="section-4.3.3">4.3.3 Macro-Local Labels</a></h4>
<p>NASM allows you to define labels within a multi-line macro definition in
such a way as to make them local to the macro call: so calling the same
macro multiple times will use a different label each time. You do this by
prefixing <code><nobr>%%</nobr></code> to the label name. So you can invent
an instruction which executes a <code><nobr>RET</nobr></code> if the
<code><nobr>Z</nobr></code> flag is set by doing this:
<p><pre>
%macro retz 0
jnz %%skip
ret
%%skip:
%endmacro
</pre>
<p>You can call this macro as many times as you want, and every time you
call it NASM will make up a different `real' name to substitute for the
label <code><nobr>%%skip</nobr></code>. The names NASM invents are of the
form <code><nobr>..@2345.skip</nobr></code>, where the number 2345 changes
with every macro call. The <code><nobr>..@</nobr></code> prefix prevents
macro-local labels from interfering with the local label mechanism, as
described in <a href="nasmdoc3.html#section-3.9">section 3.9</a>. You
should avoid defining your own labels in this form (the
<code><nobr>..@</nobr></code> prefix, then a number, then another period)
in case they interfere with macro-local labels.
<h4><a name="section-4.3.4">4.3.4 Greedy Macro Parameters</a></h4>
<p>Occasionally it is useful to define a macro which lumps its entire
command line into one parameter definition, possibly after extracting one
or two smaller parameters from the front. An example might be a macro to
write a text string to a file in MS-DOS, where you might want to be able to
write
<p><pre>
writefile [filehandle],"hello, world",13,10
</pre>
<p>NASM allows you to define the last parameter of a macro to be
<em>greedy</em>, meaning that if you invoke the macro with more parameters
than it expects, all the spare parameters get lumped into the last defined
one along with the separating commas. So if you code:
<p><pre>
%macro writefile 2+
jmp %%endstr
%%str: db %2
%%endstr:
mov dx,%%str
mov cx,%%endstr-%%str
mov bx,%1
mov ah,0x40
int 0x21
%endmacro
</pre>
<p>then the example call to <code><nobr>writefile</nobr></code> above will
work as expected: the text before the first comma,
<code><nobr>[filehandle]</nobr></code>, is used as the first macro
parameter and expanded when <code><nobr>%1</nobr></code> is referred to,
and all the subsequent text is lumped into <code><nobr>%2</nobr></code> and
placed after the <code><nobr>db</nobr></code>.
<p>The greedy nature of the macro is indicated to NASM by the use of the
<code><nobr>+</nobr></code> sign after the parameter count on the
<code><nobr>%macro</nobr></code> line.
<p>If you define a greedy macro, you are effectively telling NASM how it
should expand the macro given <em>any</em> number of parameters from the
actual number specified up to infinity; in this case, for example, NASM now
knows what to do when it sees a call to <code><nobr>writefile</nobr></code>
with 2, 3, 4 or more parameters. NASM will take this into account when
overloading macros, and will not allow you to define another form of
<code><nobr>writefile</nobr></code> taking 4 parameters (for example).
<p>Of course, the above macro could have been implemented as a non-greedy
macro, in which case the call to it would have had to look like
<p><pre>
writefile [filehandle], {"hello, world",13,10}
</pre>
<p>NASM provides both mechanisms for putting commas in macro parameters,
and you choose which one you prefer for each macro definition.
<p>See <a href="nasmdoc6.html#section-6.3.1">section 6.3.1</a> for a better
way to write the above macro.
<h4><a name="section-4.3.5">4.3.5 Default Macro Parameters</a></h4>
<p>NASM also allows you to define a multi-line macro with a <em>range</em>
of allowable parameter counts. If you do this, you can specify defaults for
omitted parameters. So, for example:
<p><pre>
%macro die 0-1 "Painful program death has occurred."
writefile 2,%1
mov ax,0x4c01
int 0x21
%endmacro
</pre>
<p>This macro (which makes use of the <code><nobr>writefile</nobr></code>
macro defined in <a href="#section-4.3.4">section 4.3.4</a>) can be called
with an explicit error message, which it will display on the error output
stream before exiting, or it can be called with no parameters, in which
case it will use the default error message supplied in the macro
definition.
<p>In general, you supply a minimum and maximum number of parameters for a
macro of this type; the minimum number of parameters are then required in
the macro call, and then you provide defaults for the optional ones. So if
a macro definition began with the line
<p><pre>
%macro foobar 1-3 eax,[ebx+2]
</pre>
<p>then it could be called with between one and three parameters, and
<code><nobr>%1</nobr></code> would always be taken from the macro call.
<code><nobr>%2</nobr></code>, if not specified by the macro call, would
default to <code><nobr>eax</nobr></code>, and <code><nobr>%3</nobr></code>
if not specified would default to <code><nobr>[ebx+2]</nobr></code>.
<p>You can provide extra information to a macro by providing too many
default parameters:
<p><pre>
%macro quux 1 something
</pre>
<p>This will trigger a warning by default; see
<a href="nasmdoc2.html#section-2.1.24">section 2.1.24</a> for more
information. When <code><nobr>quux</nobr></code> is invoked, it receives
not one but two parameters. <code><nobr>something</nobr></code> can be
referred to as <code><nobr>%2</nobr></code>. The difference between passing
<code><nobr>something</nobr></code> this way and writing
<code><nobr>something</nobr></code> in the macro body is that with this way
<code><nobr>something</nobr></code> is evaluated when the macro is defined,
not when it is expanded.
<p>You may omit parameter defaults from the macro definition, in which case
the parameter default is taken to be blank. This can be useful for macros
which can take a variable number of parameters, since the
<code><nobr>%0</nobr></code> token (see <a href="#section-4.3.6">section
4.3.6</a>) allows you to determine how many parameters were really passed
to the macro call.
<p>This defaulting mechanism can be combined with the greedy-parameter
mechanism; so the <code><nobr>die</nobr></code> macro above could be made
more powerful, and more useful, by changing the first line of the
definition to
<p><pre>
%macro die 0-1+ "Painful program death has occurred.",13,10
</pre>
<p>The maximum parameter count can be infinite, denoted by
<code><nobr>*</nobr></code>. In this case, of course, it is impossible to
provide a <em>full</em> set of default parameters. Examples of this usage
are shown in <a href="#section-4.3.7">section 4.3.7</a>.
<h4><a name="section-4.3.6">4.3.6 <code><nobr>%0</nobr></code>: Macro Parameter Counter</a></h4>
<p>The parameter reference <code><nobr>%0</nobr></code> will return a
numeric constant giving the number of parameters received, that is, if
<code><nobr>%0</nobr></code> is n then <code><nobr>%</nobr></code>n is the
last parameter. <code><nobr>%0</nobr></code> is mostly useful for macros
that can take a variable number of parameters. It can be used as an
argument to <code><nobr>%rep</nobr></code> (see
<a href="#section-4.5">section 4.5</a>) in order to iterate through all the
parameters of a macro. Examples are given in
<a href="#section-4.3.7">section 4.3.7</a>.
<h4><a name="section-4.3.7">4.3.7 <code><nobr>%rotate</nobr></code>: Rotating Macro Parameters</a></h4>
<p>Unix shell programmers will be familiar with the
<code><nobr>shift</nobr></code> shell command, which allows the arguments
passed to a shell script (referenced as <code><nobr>$1</nobr></code>,
<code><nobr>$2</nobr></code> and so on) to be moved left by one place, so
that the argument previously referenced as <code><nobr>$2</nobr></code>
becomes available as <code><nobr>$1</nobr></code>, and the argument
previously referenced as <code><nobr>$1</nobr></code> is no longer
available at all.
<p>NASM provides a similar mechanism, in the form of
<code><nobr>%rotate</nobr></code>. As its name suggests, it differs from
the Unix <code><nobr>shift</nobr></code> in that no parameters are lost:
parameters rotated off the left end of the argument list reappear on the
right, and vice versa.
<p><code><nobr>%rotate</nobr></code> is invoked with a single numeric
argument (which may be an expression). The macro parameters are rotated to
the left by that many places. If the argument to
<code><nobr>%rotate</nobr></code> is negative, the macro parameters are
rotated to the right.
<p>So a pair of macros to save and restore a set of registers might work as
follows:
<p><pre>
%macro multipush 1-*
%rep %0
push %1
%rotate 1
%endrep
%endmacro
</pre>
<p>This macro invokes the <code><nobr>PUSH</nobr></code> instruction on
each of its arguments in turn, from left to right. It begins by pushing its
first argument, <code><nobr>%1</nobr></code>, then invokes
<code><nobr>%rotate</nobr></code> to move all the arguments one place to
the left, so that the original second argument is now available as
<code><nobr>%1</nobr></code>. Repeating this procedure as many times as
there were arguments (achieved by supplying <code><nobr>%0</nobr></code> as
the argument to <code><nobr>%rep</nobr></code>) causes each argument in
turn to be pushed.
<p>Note also the use of <code><nobr>*</nobr></code> as the maximum
parameter count, indicating that there is no upper limit on the number of
parameters you may supply to the <code><nobr>multipush</nobr></code> macro.
<p>It would be convenient, when using this macro, to have a
<code><nobr>POP</nobr></code> equivalent, which <em>didn't</em> require the
arguments to be given in reverse order. Ideally, you would write the
<code><nobr>multipush</nobr></code> macro call, then cut-and-paste the line
to where the pop needed to be done, and change the name of the called macro
to <code><nobr>multipop</nobr></code>, and the macro would take care of
popping the registers in the opposite order from the one in which they were
pushed.
<p>This can be done by the following definition:
<p><pre>
%macro multipop 1-*
%rep %0
%rotate -1
pop %1
%endrep
%endmacro
</pre>
<p>This macro begins by rotating its arguments one place to the
<em>right</em>, so that the original <em>last</em> argument appears as
<code><nobr>%1</nobr></code>. This is then popped, and the arguments are
rotated right again, so the second-to-last argument becomes
<code><nobr>%1</nobr></code>. Thus the arguments are iterated through in
reverse order.
<h4><a name="section-4.3.8">4.3.8 Concatenating Macro Parameters</a></h4>
<p>NASM can concatenate macro parameters and macro indirection constructs
on to other text surrounding them. This allows you to declare a family of
symbols, for example, in a macro definition. If, for example, you wanted to
generate a table of key codes along with offsets into the table, you could
code something like
<p><pre>
%macro keytab_entry 2
keypos%1 equ $-keytab
db %2
%endmacro
keytab:
keytab_entry F1,128+1
keytab_entry F2,128+2
keytab_entry Return,13
</pre>
<p>which would expand to
<p><pre>
keytab:
keyposF1 equ $-keytab
db 128+1
keyposF2 equ $-keytab
db 128+2
keyposReturn equ $-keytab
db 13
</pre>
<p>You can just as easily concatenate text on to the other end of a macro
parameter, by writing <code><nobr>%1foo</nobr></code>.
<p>If you need to append a <em>digit</em> to a macro parameter, for example
defining labels <code><nobr>foo1</nobr></code> and
<code><nobr>foo2</nobr></code> when passed the parameter
<code><nobr>foo</nobr></code>, you can't code <code><nobr>%11</nobr></code>
because that would be taken as the eleventh macro parameter. Instead, you
must code <code><nobr>%{1}1</nobr></code>, which will separate the first
<code><nobr>1</nobr></code> (giving the number of the macro parameter) from
the second (literal text to be concatenated to the parameter).
<p>This concatenation can also be applied to other preprocessor in-line
objects, such as macro-local labels (<a href="#section-4.3.3">section
4.3.3</a>) and context-local labels (<a href="#section-4.7.2">section
4.7.2</a>). In all cases, ambiguities in syntax can be resolved by
enclosing everything after the <code><nobr>%</nobr></code> sign and before
the literal text in braces: so <code><nobr>%{%foo}bar</nobr></code>
concatenates the text <code><nobr>bar</nobr></code> to the end of the real
name of the macro-local label <code><nobr>%%foo</nobr></code>. (This is
unnecessary, since the form NASM uses for the real names of macro-local
labels means that the two usages <code><nobr>%{%foo}bar</nobr></code> and
<code><nobr>%%foobar</nobr></code> would both expand to the same thing
anyway; nevertheless, the capability is there.)
<p>The single-line macro indirection construct,
<code><nobr>%[...]</nobr></code> (<a href="#section-4.1.3">section
4.1.3</a>), behaves the same way as macro parameters for the purpose of
concatenation.
<p>See also the <code><nobr>%+</nobr></code> operator,
<a href="#section-4.1.4">section 4.1.4</a>.
<h4><a name="section-4.3.9">4.3.9 Condition Codes as Macro Parameters</a></h4>
<p>NASM can give special treatment to a macro parameter which contains a
condition code. For a start, you can refer to the macro parameter
<code><nobr>%1</nobr></code> by means of the alternative syntax
<code><nobr>%+1</nobr></code>, which informs NASM that this macro parameter
is supposed to contain a condition code, and will cause the preprocessor to
report an error message if the macro is called with a parameter which is
<em>not</em> a valid condition code.
<p>Far more usefully, though, you can refer to the macro parameter by means
of <code><nobr>%-1</nobr></code>, which NASM will expand as the
<em>inverse</em> condition code. So the <code><nobr>retz</nobr></code>
macro defined in <a href="#section-4.3.3">section 4.3.3</a> can be replaced
by a general conditional-return macro like this:
<p><pre>
%macro retc 1
j%-1 %%skip
ret
%%skip:
%endmacro
</pre>
<p>This macro can now be invoked using calls like
<code><nobr>retc ne</nobr></code>, which will cause the conditional-jump
instruction in the macro expansion to come out as
<code><nobr>JE</nobr></code>, or <code><nobr>retc po</nobr></code> which
will make the jump a <code><nobr>JPE</nobr></code>.
<p>The <code><nobr>%+1</nobr></code> macro-parameter reference is quite
happy to interpret the arguments <code><nobr>CXZ</nobr></code> and
<code><nobr>ECXZ</nobr></code> as valid condition codes; however,
<code><nobr>%-1</nobr></code> will report an error if passed either of
these, because no inverse condition code exists.
<h4><a name="section-4.3.10">4.3.10 Disabling Listing Expansion</a></h4>
<p>When NASM is generating a listing file from your program, it will
generally expand multi-line macros by means of writing the macro call and
then listing each line of the expansion. This allows you to see which
instructions in the macro expansion are generating what code; however, for
some macros this clutters the listing up unnecessarily.
<p>NASM therefore provides the <code><nobr>.nolist</nobr></code> qualifier,
which you can include in a macro definition to inhibit the expansion of the
macro in the listing file. The <code><nobr>.nolist</nobr></code> qualifier
comes directly after the number of parameters, like this:
<p><pre>
%macro foo 1.nolist
</pre>
<p>Or like this:
<p><pre>
%macro bar 1-5+.nolist a,b,c,d,e,f,g,h
</pre>
<h4><a name="section-4.3.11">4.3.11 Undefining Multi-Line Macros: <code><nobr>%unmacro</nobr></code></a></h4>
<p>Multi-line macros can be removed with the
<code><nobr>%unmacro</nobr></code> directive. Unlike the
<code><nobr>%undef</nobr></code> directive, however,
<code><nobr>%unmacro</nobr></code> takes an argument specification, and
will only remove exact matches with that argument specification.
<p>For example:
<p><pre>
%macro foo 1-3
; Do something
%endmacro
%unmacro foo 1-3
</pre>
<p>removes the previously defined macro <code><nobr>foo</nobr></code>, but
<p><pre>
%macro bar 1-3
; Do something
%endmacro
%unmacro bar 1
</pre>
<p>does <em>not</em> remove the macro <code><nobr>bar</nobr></code>, since
the argument specification does not match exactly.
<h4><a name="section-4.3.12">4.3.12 Exiting Multi-Line Macros: <code><nobr>%exitmacro</nobr></code></a></h4>
<p>Multi-line macro expansions can be arbitrarily terminated with the
<code><nobr>%exitmacro</nobr></code> directive.
<p>For example:
<p><pre>
%macro foo 1-3
; Do something
%if<condition>
%exitmacro
%endif
; Do something
%endmacro
</pre>
<h3><a name="section-4.4">4.4 Conditional Assembly</a></h3>
<p>Similarly to the C preprocessor, NASM allows sections of a source file
to be assembled only if certain conditions are met. The general syntax of
this feature looks like this:
<p><pre>
%if<condition>
; some code which only appears if <condition> is met
%elif<condition2>
; only appears if <condition> is not met but <condition2> is
%else
; this appears if neither <condition> nor <condition2> was met
%endif
</pre>
<p>The inverse forms <code><nobr>%ifn</nobr></code> and
<code><nobr>%elifn</nobr></code> are also supported.
<p>The <code><nobr>%else</nobr></code> clause is optional, as is the
<code><nobr>%elif</nobr></code> clause. You can have more than one
<code><nobr>%elif</nobr></code> clause as well.
<p>There are a number of variants of the <code><nobr>%if</nobr></code>
directive. Each has its corresponding <code><nobr>%elif</nobr></code>,
<code><nobr>%ifn</nobr></code>, and <code><nobr>%elifn</nobr></code>
directives; for example, the equivalents to the
<code><nobr>%ifdef</nobr></code> directive are
<code><nobr>%elifdef</nobr></code>, <code><nobr>%ifndef</nobr></code>, and
<code><nobr>%elifndef</nobr></code>.
<h4><a name="section-4.4.1">4.4.1 <code><nobr>%ifdef</nobr></code>: Testing Single-Line Macro Existence</a></h4>
<p>Beginning a conditional-assembly block with the line
<code><nobr>%ifdef MACRO</nobr></code> will assemble the subsequent code
if, and only if, a single-line macro called <code><nobr>MACRO</nobr></code>
is defined. If not, then the <code><nobr>%elif</nobr></code> and
<code><nobr>%else</nobr></code> blocks (if any) will be processed instead.
<p>For example, when debugging a program, you might want to write code such
as
<p><pre>
; perform some function
%ifdef DEBUG
writefile 2,"Function performed successfully",13,10
%endif
; go and do something else
</pre>
<p>Then you could use the command-line option
<code><nobr>-dDEBUG</nobr></code> to create a version of the program which
produced debugging messages, and remove the option to generate the final
release version of the program.
<p>You can test for a macro <em>not</em> being defined by using
<code><nobr>%ifndef</nobr></code> instead of
<code><nobr>%ifdef</nobr></code>. You can also test for macro definitions
in <code><nobr>%elif</nobr></code> blocks by using
<code><nobr>%elifdef</nobr></code> and <code><nobr>%elifndef</nobr></code>.
<h4><a name="section-4.4.2">4.4.2 <code><nobr>%ifmacro</nobr></code>: Testing Multi-Line Macro Existence</a></h4>
<p>The <code><nobr>%ifmacro</nobr></code> directive operates in the same
way as the <code><nobr>%ifdef</nobr></code> directive, except that it
checks for the existence of a multi-line macro.
<p>For example, you may be working with a large project and not have
control over the macros in a library. You may want to create a macro with
one name if it doesn't already exist, and another name if one with that
name does exist.
<p>The <code><nobr>%ifmacro</nobr></code> is considered true if defining a
macro with the given name and number of arguments would cause a definitions
conflict. For example:
<p><pre>
%ifmacro MyMacro 1-3
%error "MyMacro 1-3" causes a conflict with an existing macro.
%else
%macro MyMacro 1-3
; insert code to define the macro
%endmacro
%endif
</pre>
<p>This will create the macro "MyMacro 1-3" if no macro already exists
which would conflict with it, and emits a warning if there would be a
definition conflict.
<p>You can test for the macro not existing by using the
<code><nobr>%ifnmacro</nobr></code> instead of
<code><nobr>%ifmacro</nobr></code>. Additional tests can be performed in
<code><nobr>%elif</nobr></code> blocks by using
<code><nobr>%elifmacro</nobr></code> and
<code><nobr>%elifnmacro</nobr></code>.
<h4><a name="section-4.4.3">4.4.3 <code><nobr>%ifctx</nobr></code>: Testing the Context Stack</a></h4>
<p>The conditional-assembly construct <code><nobr>%ifctx</nobr></code> will
cause the subsequent code to be assembled if and only if the top context on
the preprocessor's context stack has the same name as one of the arguments.
As with <code><nobr>%ifdef</nobr></code>, the inverse and
<code><nobr>%elif</nobr></code> forms <code><nobr>%ifnctx</nobr></code>,
<code><nobr>%elifctx</nobr></code> and <code><nobr>%elifnctx</nobr></code>
are also supported.
<p>For more details of the context stack, see
<a href="#section-4.7">section 4.7</a>. For a sample use of
<code><nobr>%ifctx</nobr></code>, see <a href="#section-4.7.5">section
4.7.5</a>.
<h4><a name="section-4.4.4">4.4.4 <code><nobr>%if</nobr></code>: Testing Arbitrary Numeric Expressions</a></h4>
<p>The conditional-assembly construct <code><nobr>%if expr</nobr></code>
will cause the subsequent code to be assembled if and only if the value of
the numeric expression <code><nobr>expr</nobr></code> is non-zero. An
example of the use of this feature is in deciding when to break out of a
<code><nobr>%rep</nobr></code> preprocessor loop: see
<a href="#section-4.5">section 4.5</a> for a detailed example.
<p>The expression given to <code><nobr>%if</nobr></code>, and its
counterpart <code><nobr>%elif</nobr></code>, is a critical expression (see
<a href="nasmdoc3.html#section-3.8">section 3.8</a>).
<p><code><nobr>%if</nobr></code> extends the normal NASM expression syntax,
by providing a set of relational operators which are not normally available
in expressions. The operators <code><nobr>=</nobr></code>,
<code><nobr><</nobr></code>, <code><nobr>></nobr></code>,
<code><nobr><=</nobr></code>, <code><nobr>>=</nobr></code> and
<code><nobr><></nobr></code> test equality, less-than, greater-than,
less-or-equal, greater-or-equal and not-equal respectively. The C-like
forms <code><nobr>==</nobr></code> and <code><nobr>!=</nobr></code> are
supported as alternative forms of <code><nobr>=</nobr></code> and
<code><nobr><></nobr></code>. In addition, low-priority logical
operators <code><nobr>&&</nobr></code>,
<code><nobr>^^</nobr></code> and <code><nobr>||</nobr></code> are provided,
supplying logical AND, logical XOR and logical OR. These work like the C
logical operators (although C has no logical XOR), in that they always
return either 0 or 1, and treat any non-zero input as 1 (so that
<code><nobr>^^</nobr></code>, for example, returns 1 if exactly one of its
inputs is zero, and 0 otherwise). The relational operators also return 1
for true and 0 for false.
<p>Like other <code><nobr>%if</nobr></code> constructs,
<code><nobr>%if</nobr></code> has a counterpart
<code><nobr>%elif</nobr></code>, and negative forms
<code><nobr>%ifn</nobr></code> and <code><nobr>%elifn</nobr></code>.
<h4><a name="section-4.4.5">4.4.5 <code><nobr>%ifidn</nobr></code> and <code><nobr>%ifidni</nobr></code>: Testing Exact Text Identity</a></h4>
<p>The construct <code><nobr>%ifidn text1,text2</nobr></code> will cause
the subsequent code to be assembled if and only if
<code><nobr>text1</nobr></code> and <code><nobr>text2</nobr></code>, after
expanding single-line macros, are identical pieces of text. Differences in
white space are not counted.
<p><code><nobr>%ifidni</nobr></code> is similar to
<code><nobr>%ifidn</nobr></code>, but is case-insensitive.
<p>For example, the following macro pushes a register or number on the
stack, and allows you to treat <code><nobr>IP</nobr></code> as a real
register:
<p><pre>
%macro pushparam 1
%ifidni %1,ip
call %%label
%%label:
%else
push %1
%endif
%endmacro
</pre>
<p>Like other <code><nobr>%if</nobr></code> constructs,
<code><nobr>%ifidn</nobr></code> has a counterpart
<code><nobr>%elifidn</nobr></code>, and negative forms
<code><nobr>%ifnidn</nobr></code> and <code><nobr>%elifnidn</nobr></code>.
Similarly, <code><nobr>%ifidni</nobr></code> has counterparts
<code><nobr>%elifidni</nobr></code>, <code><nobr>%ifnidni</nobr></code> and
<code><nobr>%elifnidni</nobr></code>.
<h4><a name="section-4.4.6">4.4.6 <code><nobr>%ifid</nobr></code>, <code><nobr>%ifnum</nobr></code>, <code><nobr>%ifstr</nobr></code>: Testing Token Types</a></h4>
<p>Some macros will want to perform different tasks depending on whether
they are passed a number, a string, or an identifier. For example, a string
output macro might want to be able to cope with being passed either a
string constant or a pointer to an existing string.
<p>The conditional assembly construct <code><nobr>%ifid</nobr></code>,
taking one parameter (which may be blank), assembles the subsequent code if
and only if the first token in the parameter exists and is an identifier.
<code><nobr>%ifnum</nobr></code> works similarly, but tests for the token
being a numeric constant; <code><nobr>%ifstr</nobr></code> tests for it
being a string.
<p>For example, the <code><nobr>writefile</nobr></code> macro defined in
<a href="#section-4.3.4">section 4.3.4</a> can be extended to take
advantage of <code><nobr>%ifstr</nobr></code> in the following fashion:
<p><pre>
%macro writefile 2-3+
%ifstr %2
jmp %%endstr
%if %0 = 3
%%str: db %2,%3
%else
%%str: db %2
%endif
%%endstr: mov dx,%%str
mov cx,%%endstr-%%str
%else
mov dx,%2
mov cx,%3
%endif
mov bx,%1
mov ah,0x40
int 0x21
%endmacro
</pre>
<p>Then the <code><nobr>writefile</nobr></code> macro can cope with being
called in either of the following two ways:
<p><pre>
writefile [file], strpointer, length
writefile [file], "hello", 13, 10
</pre>
<p>In the first, <code><nobr>strpointer</nobr></code> is used as the
address of an already-declared string, and <code><nobr>length</nobr></code>
is used as its length; in the second, a string is given to the macro, which
therefore declares it itself and works out the address and length for
itself.
<p>Note the use of <code><nobr>%if</nobr></code> inside the
<code><nobr>%ifstr</nobr></code>: this is to detect whether the macro was
passed two arguments (so the string would be a single string constant, and
<code><nobr>db %2</nobr></code> would be adequate) or more (in which case,
all but the first two would be lumped together into
<code><nobr>%3</nobr></code>, and <code><nobr>db %2,%3</nobr></code> would
be required).
<p>The usual <code><nobr>%elif</nobr></code>...,
<code><nobr>%ifn</nobr></code>..., and <code><nobr>%elifn</nobr></code>...
versions exist for each of <code><nobr>%ifid</nobr></code>,
<code><nobr>%ifnum</nobr></code> and <code><nobr>%ifstr</nobr></code>.
<h4><a name="section-4.4.7">4.4.7 <code><nobr>%iftoken</nobr></code>: Test for a Single Token</a></h4>
<p>Some macros will want to do different things depending on if it is
passed a single token (e.g. paste it to something else using
<code><nobr>%+</nobr></code>) versus a multi-token sequence.
<p>The conditional assembly construct <code><nobr>%iftoken</nobr></code>
assembles the subsequent code if and only if the expanded parameters
consist of exactly one token, possibly surrounded by whitespace.
<p>For example:
<p><pre>
%iftoken 1
</pre>
<p>will assemble the subsequent code, but
<p><pre>
%iftoken -1
</pre>
<p>will not, since <code><nobr>-1</nobr></code> contains two tokens: the
unary minus operator <code><nobr>-</nobr></code>, and the number
<code><nobr>1</nobr></code>.
<p>The usual <code><nobr>%eliftoken</nobr></code>,
<code><nobr>%ifntoken</nobr></code>, and
<code><nobr>%elifntoken</nobr></code> variants are also provided.
<h4><a name="section-4.4.8">4.4.8 <code><nobr>%ifempty</nobr></code>: Test for Empty Expansion</a></h4>
<p>The conditional assembly construct <code><nobr>%ifempty</nobr></code>
assembles the subsequent code if and only if the expanded parameters do not
contain any tokens at all, whitespace excepted.
<p>The usual <code><nobr>%elifempty</nobr></code>,
<code><nobr>%ifnempty</nobr></code>, and
<code><nobr>%elifnempty</nobr></code> variants are also provided.
<h3><a name="section-4.5">4.5 Preprocessor Loops: <code><nobr>%rep</nobr></code></a></h3>
<p>NASM's <code><nobr>TIMES</nobr></code> prefix, though useful, cannot be
used to invoke a multi-line macro multiple times, because it is processed
by NASM after macros have already been expanded. Therefore NASM provides
another form of loop, this time at the preprocessor level:
<code><nobr>%rep</nobr></code>.
<p>The directives <code><nobr>%rep</nobr></code> and
<code><nobr>%endrep</nobr></code> (<code><nobr>%rep</nobr></code> takes a
numeric argument, which can be an expression;
<code><nobr>%endrep</nobr></code> takes no arguments) can be used to
enclose a chunk of code, which is then replicated as many times as
specified by the preprocessor:
<p><pre>
%assign i 0
%rep 64
inc word [table+2*i]
%assign i i+1
%endrep
</pre>
<p>This will generate a sequence of 64 <code><nobr>INC</nobr></code>
instructions, incrementing every word of memory from
<code><nobr>[table]</nobr></code> to <code><nobr>[table+126]</nobr></code>.
<p>For more complex termination conditions, or to break out of a repeat
loop part way along, you can use the <code><nobr>%exitrep</nobr></code>
directive to terminate the loop, like this:
<p><pre>
fibonacci:
%assign i 0
%assign j 1
%rep 100
%if j > 65535
%exitrep
%endif
dw j
%assign k j+i
%assign i j
%assign j k
%endrep
fib_number equ ($-fibonacci)/2
</pre>
<p>This produces a list of all the Fibonacci numbers that will fit in 16
bits. Note that a maximum repeat count must still be given to
<code><nobr>%rep</nobr></code>. This is to prevent the possibility of NASM
getting into an infinite loop in the preprocessor, which (on multitasking
or multi-user systems) would typically cause all the system memory to be
gradually used up and other applications to start crashing.
<h3><a name="section-4.6">4.6 Source Files and Dependencies</a></h3>
<p>These commands allow you to split your sources into multiple files.
<h4><a name="section-4.6.1">4.6.1 <code><nobr>%include</nobr></code>: Including Other Files</a></h4>
<p>Using, once again, a very similar syntax to the C preprocessor, NASM's
preprocessor lets you include other source files into your code. This is
done by the use of the <code><nobr>%include</nobr></code> directive:
<p><pre>
%include "macros.mac"
</pre>
<p>will include the contents of the file
<code><nobr>macros.mac</nobr></code> into the source file containing the
<code><nobr>%include</nobr></code> directive.
<p>Include files are searched for in the current directory (the directory
you're in when you run NASM, as opposed to the location of the NASM
executable or the location of the source file), plus any directories
specified on the NASM command line using the <code><nobr>-i</nobr></code>
option.
<p>The standard C idiom for preventing a file being included more than once
is just as applicable in NASM: if the file
<code><nobr>macros.mac</nobr></code> has the form
<p><pre>
%ifndef MACROS_MAC
%define MACROS_MAC
; now define some macros
%endif
</pre>
<p>then including the file more than once will not cause errors, because
the second time the file is included nothing will happen because the macro
<code><nobr>MACROS_MAC</nobr></code> will already be defined.
<p>You can force a file to be included even if there is no
<code><nobr>%include</nobr></code> directive that explicitly includes it,
by using the <code><nobr>-p</nobr></code> option on the NASM command line
(see <a href="nasmdoc2.html#section-2.1.17">section 2.1.17</a>).
<h4><a name="section-4.6.2">4.6.2 <code><nobr>%pathsearch</nobr></code>: Search the Include Path</a></h4>
<p>The <code><nobr>%pathsearch</nobr></code> directive takes a single-line
macro name and a filename, and declare or redefines the specified
single-line macro to be the include-path-resolved version of the filename,
if the file exists (otherwise, it is passed unchanged.)
<p>For example,
<p><pre>
%pathsearch MyFoo "foo.bin"
</pre>
<p>... with <code><nobr>-Ibins/</nobr></code> in the include path may end
up defining the macro <code><nobr>MyFoo</nobr></code> to be
<code><nobr>"bins/foo.bin"</nobr></code>.
<h4><a name="section-4.6.3">4.6.3 <code><nobr>%depend</nobr></code>: Add Dependent Files</a></h4>
<p>The <code><nobr>%depend</nobr></code> directive takes a filename and
adds it to the list of files to be emitted as dependency generation when
the <code><nobr>-M</nobr></code> options and its relatives (see
<a href="nasmdoc2.html#section-2.1.4">section 2.1.4</a>) are used. It
produces no output.
<p>This is generally used in conjunction with
<code><nobr>%pathsearch</nobr></code>. For example, a simplified version of
the standard macro wrapper for the <code><nobr>INCBIN</nobr></code>
directive looks like:
<p><pre>
%imacro incbin 1-2+ 0
%pathsearch dep %1
%depend dep
incbin dep,%2
%endmacro
</pre>
<p>This first resolves the location of the file into the macro
<code><nobr>dep</nobr></code>, then adds it to the dependency lists, and
finally issues the assembler-level <code><nobr>INCBIN</nobr></code>
directive.
<h4><a name="section-4.6.4">4.6.4 <code><nobr>%use</nobr></code>: Include Standard Macro Package</a></h4>
<p>The <code><nobr>%use</nobr></code> directive is similar to
<code><nobr>%include</nobr></code>, but rather than including the contents
of a file, it includes a named standard macro package. The standard macro
packages are part of NASM, and are described in
<a href="nasmdoc5.html">chapter 5</a>.
<p>Unlike the <code><nobr>%include</nobr></code> directive, package names
for the <code><nobr>%use</nobr></code> directive do not require quotes, but
quotes are permitted. In NASM 2.04 and 2.05 the unquoted form would be
macro-expanded; this is no longer true. Thus, the following lines are
equivalent:
<p><pre>
%use altreg
%use 'altreg'
</pre>
<p>Standard macro packages are protected from multiple inclusion. When a
standard macro package is used, a testable single-line macro of the form
<code><nobr>__USE_</nobr></code><em>package</em><code><nobr>__</nobr></code>
is also defined, see <a href="#section-4.11.8">section 4.11.8</a>.
<h3><a name="section-4.7">4.7 The Context Stack</a></h3>
<p>Having labels that are local to a macro definition is sometimes not
quite powerful enough: sometimes you want to be able to share labels
between several macro calls. An example might be a
<code><nobr>REPEAT</nobr></code> ... <code><nobr>UNTIL</nobr></code> loop,
in which the expansion of the <code><nobr>REPEAT</nobr></code> macro would
need to be able to refer to a label which the
<code><nobr>UNTIL</nobr></code> macro had defined. However, for such a
macro you would also want to be able to nest these loops.
<p>NASM provides this level of power by means of a <em>context stack</em>.
The preprocessor maintains a stack of <em>contexts</em>, each of which is
characterized by a name. You add a new context to the stack using the
<code><nobr>%push</nobr></code> directive, and remove one using
<code><nobr>%pop</nobr></code>. You can define labels that are local to a
particular context on the stack.
<h4><a name="section-4.7.1">4.7.1 <code><nobr>%push</nobr></code> and <code><nobr>%pop</nobr></code>: Creating and Removing Contexts</a></h4>
<p>The <code><nobr>%push</nobr></code> directive is used to create a new
context and place it on the top of the context stack.
<code><nobr>%push</nobr></code> takes an optional argument, which is the
name of the context. For example:
<p><pre>
%push foobar
</pre>
<p>This pushes a new context called <code><nobr>foobar</nobr></code> on the
stack. You can have several contexts on the stack with the same name: they
can still be distinguished. If no name is given, the context is unnamed
(this is normally used when both the <code><nobr>%push</nobr></code> and
the <code><nobr>%pop</nobr></code> are inside a single macro definition.)
<p>The directive <code><nobr>%pop</nobr></code>, taking one optional
argument, removes the top context from the context stack and destroys it,
along with any labels associated with it. If an argument is given, it must
match the name of the current context, otherwise it will issue an error.
<h4><a name="section-4.7.2">4.7.2 Context-Local Labels</a></h4>
<p>Just as the usage <code><nobr>%%foo</nobr></code> defines a label which
is local to the particular macro call in which it is used, the usage
<code><nobr>%$foo</nobr></code> is used to define a label which is local to
the context on the top of the context stack. So the
<code><nobr>REPEAT</nobr></code> and <code><nobr>UNTIL</nobr></code>
example given above could be implemented by means of:
<p><pre>
%macro repeat 0
%push repeat
%$begin:
%endmacro
%macro until 1
j%-1 %$begin
%pop
%endmacro
</pre>
<p>and invoked by means of, for example,
<p><pre>
mov cx,string
repeat
add cx,3
scasb
until e
</pre>
<p>which would scan every fourth byte of a string in search of the byte in
<code><nobr>AL</nobr></code>.
<p>If you need to define, or access, labels local to the context
<em>below</em> the top one on the stack, you can use
<code><nobr>%$$foo</nobr></code>, or <code><nobr>%$$$foo</nobr></code> for
the context below that, and so on.
<h4><a name="section-4.7.3">4.7.3 Context-Local Single-Line Macros</a></h4>
<p>NASM also allows you to define single-line macros which are local to a
particular context, in just the same way:
<p><pre>
%define %$localmac 3
</pre>
<p>will define the single-line macro <code><nobr>%$localmac</nobr></code>
to be local to the top context on the stack. Of course, after a subsequent
<code><nobr>%push</nobr></code>, it can then still be accessed by the name
<code><nobr>%$$localmac</nobr></code>.
<h4><a name="section-4.7.4">4.7.4 <code><nobr>%repl</nobr></code>: Renaming a Context</a></h4>
<p>If you need to change the name of the top context on the stack (in
order, for example, to have it respond differently to
<code><nobr>%ifctx</nobr></code>), you can execute a
<code><nobr>%pop</nobr></code> followed by a
<code><nobr>%push</nobr></code>; but this will have the side effect of
destroying all context-local labels and macros associated with the context
that was just popped.
<p>NASM provides the directive <code><nobr>%repl</nobr></code>, which
<em>replaces</em> a context with a different name, without touching the
associated macros and labels. So you could replace the destructive code
<p><pre>
%pop
%push newname
</pre>
<p>with the non-destructive version
<code><nobr>%repl newname</nobr></code>.
<h4><a name="section-4.7.5">4.7.5 Example Use of the Context Stack: Block IFs</a></h4>
<p>This example makes use of almost all the context-stack features,
including the conditional-assembly construct
<code><nobr>%ifctx</nobr></code>, to implement a block IF statement as a
set of macros.
<p><pre>
%macro if 1
%push if
j%-1 %$ifnot
%endmacro
%macro else 0
%ifctx if
%repl else
jmp %$ifend
%$ifnot:
%else
%error "expected `if' before `else'"
%endif
%endmacro
%macro endif 0
%ifctx if
%$ifnot:
%pop
%elifctx else
%$ifend:
%pop
%else
%error "expected `if' or `else' before `endif'"
%endif
%endmacro
</pre>
<p>This code is more robust than the <code><nobr>REPEAT</nobr></code> and
<code><nobr>UNTIL</nobr></code> macros given in
<a href="#section-4.7.2">section 4.7.2</a>, because it uses conditional
assembly to check that the macros are issued in the right order (for
example, not calling <code><nobr>endif</nobr></code> before
<code><nobr>if</nobr></code>) and issues a <code><nobr>%error</nobr></code>
if they're not.
<p>In addition, the <code><nobr>endif</nobr></code> macro has to be able to
cope with the two distinct cases of either directly following an
<code><nobr>if</nobr></code>, or following an
<code><nobr>else</nobr></code>. It achieves this, again, by using
conditional assembly to do different things depending on whether the
context on top of the stack is <code><nobr>if</nobr></code> or
<code><nobr>else</nobr></code>.
<p>The <code><nobr>else</nobr></code> macro has to preserve the context on
the stack, in order to have the <code><nobr>%$ifnot</nobr></code> referred
to by the <code><nobr>if</nobr></code> macro be the same as the one defined
by the <code><nobr>endif</nobr></code> macro, but has to change the
context's name so that <code><nobr>endif</nobr></code> will know there was
an intervening <code><nobr>else</nobr></code>. It does this by the use of
<code><nobr>%repl</nobr></code>.
<p>A sample usage of these macros might look like:
<p><pre>
cmp ax,bx
if ae
cmp bx,cx
if ae
mov ax,cx
else
mov ax,bx
endif
else
cmp ax,cx
if ae
mov ax,cx
endif
endif
</pre>
<p>The block-<code><nobr>IF</nobr></code> macros handle nesting quite
happily, by means of pushing another context, describing the inner
<code><nobr>if</nobr></code>, on top of the one describing the outer
<code><nobr>if</nobr></code>; thus <code><nobr>else</nobr></code> and
<code><nobr>endif</nobr></code> always refer to the last unmatched
<code><nobr>if</nobr></code> or <code><nobr>else</nobr></code>.
<h3><a name="section-4.8">4.8 Stack Relative Preprocessor Directives</a></h3>
<p>The following preprocessor directives provide a way to use labels to
refer to local variables allocated on the stack.
<ul>
<li><code><nobr>%arg</nobr></code> (see <a href="#section-4.8.1">section
4.8.1</a>)
<li><code><nobr>%stacksize</nobr></code> (see
<a href="#section-4.8.2">section 4.8.2</a>)
<li><code><nobr>%local</nobr></code> (see <a href="#section-4.8.3">section
4.8.3</a>)
</ul>
<h4><a name="section-4.8.1">4.8.1 <code><nobr>%arg</nobr></code> Directive</a></h4>
<p>The <code><nobr>%arg</nobr></code> directive is used to simplify the
handling of parameters passed on the stack. Stack based parameter passing
is used by many high level languages, including C, C++ and Pascal.
<p>While NASM has macros which attempt to duplicate this functionality (see
<a href="nasmdoc8.html#section-8.4.5">section 8.4.5</a>), the syntax is not
particularly convenient to use. and is not TASM compatible. Here is an
example which shows the use of <code><nobr>%arg</nobr></code> without any
external macros:
<p><pre>
some_function:
%push mycontext ; save the current context
%stacksize large ; tell NASM to use bp
%arg i:word, j_ptr:word
mov ax,[i]
mov bx,[j_ptr]
add ax,[bx]
ret
%pop ; restore original context
</pre>
<p>This is similar to the procedure defined in
<a href="nasmdoc8.html#section-8.4.5">section 8.4.5</a> and adds the value
in i to the value pointed to by j_ptr and returns the sum in the ax
register. See <a href="#section-4.7.1">section 4.7.1</a> for an explanation
of <code><nobr>push</nobr></code> and <code><nobr>pop</nobr></code> and the
use of context stacks.
<h4><a name="section-4.8.2">4.8.2 <code><nobr>%stacksize</nobr></code> Directive</a></h4>
<p>The <code><nobr>%stacksize</nobr></code> directive is used in
conjunction with the <code><nobr>%arg</nobr></code> (see
<a href="#section-4.8.1">section 4.8.1</a>) and the
<code><nobr>%local</nobr></code> (see <a href="#section-4.8.3">section
4.8.3</a>) directives. It tells NASM the default size to use for subsequent
<code><nobr>%arg</nobr></code> and <code><nobr>%local</nobr></code>
directives. The <code><nobr>%stacksize</nobr></code> directive takes one
required argument which is one of <code><nobr>flat</nobr></code>,
<code><nobr>flat64</nobr></code>, <code><nobr>large</nobr></code> or
<code><nobr>small</nobr></code>.
<p><pre>
%stacksize flat
</pre>
<p>This form causes NASM to use stack-based parameter addressing relative
to <code><nobr>ebp</nobr></code> and it assumes that a near form of call
was used to get to this label (i.e. that <code><nobr>eip</nobr></code> is
on the stack).
<p><pre>
%stacksize flat64
</pre>
<p>This form causes NASM to use stack-based parameter addressing relative
to <code><nobr>rbp</nobr></code> and it assumes that a near form of call
was used to get to this label (i.e. that <code><nobr>rip</nobr></code> is
on the stack).
<p><pre>
%stacksize large
</pre>
<p>This form uses <code><nobr>bp</nobr></code> to do stack-based parameter
addressing and assumes that a far form of call was used to get to this
address (i.e. that <code><nobr>ip</nobr></code> and
<code><nobr>cs</nobr></code> are on the stack).
<p><pre>
%stacksize small
</pre>
<p>This form also uses <code><nobr>bp</nobr></code> to address stack
parameters, but it is different from <code><nobr>large</nobr></code>
because it also assumes that the old value of bp is pushed onto the stack
(i.e. it expects an <code><nobr>ENTER</nobr></code> instruction). In other
words, it expects that <code><nobr>bp</nobr></code>,
<code><nobr>ip</nobr></code> and <code><nobr>cs</nobr></code> are on the
top of the stack, underneath any local space which may have been allocated
by <code><nobr>ENTER</nobr></code>. This form is probably most useful when
used in combination with the <code><nobr>%local</nobr></code> directive
(see <a href="#section-4.8.3">section 4.8.3</a>).
<h4><a name="section-4.8.3">4.8.3 <code><nobr>%local</nobr></code> Directive</a></h4>
<p>The <code><nobr>%local</nobr></code> directive is used to simplify the
use of local temporary stack variables allocated in a stack frame.
Automatic local variables in C are an example of this kind of variable. The
<code><nobr>%local</nobr></code> directive is most useful when used with
the <code><nobr>%stacksize</nobr></code> (see
<a href="#section-4.8.2">section 4.8.2</a> and is also compatible with the
<code><nobr>%arg</nobr></code> directive (see
<a href="#section-4.8.1">section 4.8.1</a>). It allows simplified reference
to variables on the stack which have been allocated typically by using the
<code><nobr>ENTER</nobr></code> instruction. An example of its use is the
following:
<p><pre>
silly_swap:
%push mycontext ; save the current context
%stacksize small ; tell NASM to use bp
%assign %$localsize 0 ; see text for explanation
%local old_ax:word, old_dx:word
enter %$localsize,0 ; see text for explanation
mov [old_ax],ax ; swap ax & bx
mov [old_dx],dx ; and swap dx & cx
mov ax,bx
mov dx,cx
mov bx,[old_ax]
mov cx,[old_dx]
leave ; restore old bp
ret ;
%pop ; restore original context
</pre>
<p>The <code><nobr>%$localsize</nobr></code> variable is used internally by
the <code><nobr>%local</nobr></code> directive and <em>must</em> be defined
within the current context before the <code><nobr>%local</nobr></code>
directive may be used. Failure to do so will result in one expression
syntax error for each <code><nobr>%local</nobr></code> variable declared.
It then may be used in the construction of an appropriately sized ENTER
instruction as shown in the example.
<h3><a name="section-4.9">4.9 Reporting User-Defined Errors: <code><nobr>%error</nobr></code>, <code><nobr>%warning</nobr></code>, <code><nobr>%fatal</nobr></code></a></h3>
<p>The preprocessor directive <code><nobr>%error</nobr></code> will cause
NASM to report an error if it occurs in assembled code. So if other users
are going to try to assemble your source files, you can ensure that they
define the right macros by means of code like this:
<p><pre>
%ifdef F1
; do some setup
%elifdef F2
; do some different setup
%else
%error "Neither F1 nor F2 was defined."
%endif
</pre>
<p>Then any user who fails to understand the way your code is supposed to
be assembled will be quickly warned of their mistake, rather than having to
wait until the program crashes on being run and then not knowing what went
wrong.
<p>Similarly, <code><nobr>%warning</nobr></code> issues a warning, but
allows assembly to continue:
<p><pre>
%ifdef F1
; do some setup
%elifdef F2
; do some different setup
%else
%warning "Neither F1 nor F2 was defined, assuming F1."
%define F1
%endif
</pre>
<p><code><nobr>%error</nobr></code> and <code><nobr>%warning</nobr></code>
are issued only on the final assembly pass. This makes them safe to use in
conjunction with tests that depend on symbol values.
<p><code><nobr>%fatal</nobr></code> terminates assembly immediately,
regardless of pass. This is useful when there is no point in continuing the
assembly further, and doing so is likely just going to cause a spew of
confusing error messages.
<p>It is optional for the message string after
<code><nobr>%error</nobr></code>, <code><nobr>%warning</nobr></code> or
<code><nobr>%fatal</nobr></code> to be quoted. If it is <em>not</em>, then
single-line macros are expanded in it, which can be used to display more
information to the user. For example:
<p><pre>
%if foo > 64
%assign foo_over foo-64
%error foo is foo_over bytes too large
%endif
</pre>
<h3><a name="section-4.10">4.10 Other Preprocessor Directives</a></h3>
<p>NASM also has preprocessor directives which allow access to information
from external sources. Currently they include:
<ul>
<li><code><nobr>%line</nobr></code> enables NASM to correctly handle the
output of another preprocessor (see <a href="#section-4.10.1">section
4.10.1</a>).
<li><code><nobr>%!</nobr></code> enables NASM to read in the value of an
environment variable, which can then be used in your program (see
<a href="#section-4.10.2">section 4.10.2</a>).
</ul>
<h4><a name="section-4.10.1">4.10.1 <code><nobr>%line</nobr></code> Directive</a></h4>
<p>The <code><nobr>%line</nobr></code> directive is used to notify NASM
that the input line corresponds to a specific line number in another file.
Typically this other file would be an original source file, with the
current NASM input being the output of a pre-processor. The
<code><nobr>%line</nobr></code> directive allows NASM to output messages
which indicate the line number of the original source file, instead of the
file that is being read by NASM.
<p>This preprocessor directive is not generally of use to programmers, by
may be of interest to preprocessor authors. The usage of the
<code><nobr>%line</nobr></code> preprocessor directive is as follows:
<p><pre>
%line nnn[+mmm] [filename]
</pre>
<p>In this directive, <code><nobr>nnn</nobr></code> identifies the line of
the original source file which this line corresponds to.
<code><nobr>mmm</nobr></code> is an optional parameter which specifies a
line increment value; each line of the input file read in is considered to
correspond to <code><nobr>mmm</nobr></code> lines of the original source
file. Finally, <code><nobr>filename</nobr></code> is an optional parameter
which specifies the file name of the original source file.
<p>After reading a <code><nobr>%line</nobr></code> preprocessor directive,
NASM will report all file name and line numbers relative to the values
specified therein.
<h4><a name="section-4.10.2">4.10.2 <code><nobr>%!</nobr></code><code><nobr><env></nobr></code>: Read an environment variable.</a></h4>
<p>The <code><nobr>%!<env></nobr></code> directive makes it possible
to read the value of an environment variable at assembly time. This could,
for example, be used to store the contents of an environment variable into
a string, which could be used at some other point in your code.
<p>For example, suppose that you have an environment variable
<code><nobr>FOO</nobr></code>, and you want the contents of
<code><nobr>FOO</nobr></code> to be embedded in your program. You could do
that as follows:
<p><pre>
%defstr FOO %!FOO
</pre>
<p>See <a href="#section-4.1.8">section 4.1.8</a> for notes on the
<code><nobr>%defstr</nobr></code> directive.
<h3><a name="section-4.11">4.11 Standard Macros</a></h3>
<p>NASM defines a set of standard macros, which are already defined when it
starts to process any source file. If you really need a program to be
assembled with no pre-defined macros, you can use the
<code><nobr>%clear</nobr></code> directive to empty the preprocessor of
everything but context-local preprocessor variables and single-line macros.
<p>Most user-level assembler directives (see
<a href="nasmdoc6.html">chapter 6</a>) are implemented as macros which
invoke primitive directives; these are described in
<a href="nasmdoc6.html">chapter 6</a>. The rest of the standard macro set
is described here.
<h4><a name="section-4.11.1">4.11.1 NASM Version Macros</a></h4>
<p>The single-line macros <code><nobr>__NASM_MAJOR__</nobr></code>,
<code><nobr>__NASM_MINOR__</nobr></code>,
<code><nobr>__NASM_SUBMINOR__</nobr></code> and
<code><nobr>___NASM_PATCHLEVEL__</nobr></code> expand to the major, minor,
subminor and patch level parts of the version number of NASM being used.
So, under NASM 0.98.32p1 for example,
<code><nobr>__NASM_MAJOR__</nobr></code> would be defined to be 0,
<code><nobr>__NASM_MINOR__</nobr></code> would be defined as 98,
<code><nobr>__NASM_SUBMINOR__</nobr></code> would be defined to 32, and
<code><nobr>___NASM_PATCHLEVEL__</nobr></code> would be defined as 1.
<p>Additionally, the macro <code><nobr>__NASM_SNAPSHOT__</nobr></code> is
defined for automatically generated snapshot releases <em>only</em>.
<h4><a name="section-4.11.2">4.11.2 <code><nobr>__NASM_VERSION_ID__</nobr></code>: NASM Version ID</a></h4>
<p>The single-line macro <code><nobr>__NASM_VERSION_ID__</nobr></code>
expands to a dword integer representing the full version number of the
version of nasm being used. The value is the equivalent to
<code><nobr>__NASM_MAJOR__</nobr></code>,
<code><nobr>__NASM_MINOR__</nobr></code>,
<code><nobr>__NASM_SUBMINOR__</nobr></code> and
<code><nobr>___NASM_PATCHLEVEL__</nobr></code> concatenated to produce a
single doubleword. Hence, for 0.98.32p1, the returned number would be
equivalent to:
<p><pre>
dd 0x00622001
</pre>
<p>or
<p><pre>
db 1,32,98,0
</pre>
<p>Note that the above lines are generate exactly the same code, the second
line is used just to give an indication of the order that the separate
values will be present in memory.
<h4><a name="section-4.11.3">4.11.3 <code><nobr>__NASM_VER__</nobr></code>: NASM Version string</a></h4>
<p>The single-line macro <code><nobr>__NASM_VER__</nobr></code> expands to
a string which defines the version number of nasm being used. So, under
NASM 0.98.32 for example,
<p><pre>
db __NASM_VER__
</pre>
<p>would expand to
<p><pre>
db "0.98.32"
</pre>
<h4><a name="section-4.11.4">4.11.4 <code><nobr>__FILE__</nobr></code> and <code><nobr>__LINE__</nobr></code>: File Name and Line Number</a></h4>
<p>Like the C preprocessor, NASM allows the user to find out the file name
and line number containing the current instruction. The macro
<code><nobr>__FILE__</nobr></code> expands to a string constant giving the
name of the current input file (which may change through the course of
assembly if <code><nobr>%include</nobr></code> directives are used), and
<code><nobr>__LINE__</nobr></code> expands to a numeric constant giving the
current line number in the input file.
<p>These macros could be used, for example, to communicate debugging
information to a macro, since invoking <code><nobr>__LINE__</nobr></code>
inside a macro definition (either single-line or multi-line) will return
the line number of the macro <em>call</em>, rather than
<em>definition</em>. So to determine where in a piece of code a crash is
occurring, for example, one could write a routine
<code><nobr>stillhere</nobr></code>, which is passed a line number in
<code><nobr>EAX</nobr></code> and outputs something like `line 155: still
here'. You could then write a macro
<p><pre>
%macro notdeadyet 0
push eax
mov eax,__LINE__
call stillhere
pop eax
%endmacro
</pre>
<p>and then pepper your code with calls to
<code><nobr>notdeadyet</nobr></code> until you find the crash point.
<h4><a name="section-4.11.5">4.11.5 <code><nobr>__BITS__</nobr></code>: Current BITS Mode</a></h4>
<p>The <code><nobr>__BITS__</nobr></code> standard macro is updated every
time that the BITS mode is set using the <code><nobr>BITS XX</nobr></code>
or <code><nobr>[BITS XX]</nobr></code> directive, where XX is a valid mode
number of 16, 32 or 64. <code><nobr>__BITS__</nobr></code> receives the
specified mode number and makes it globally available. This can be very
useful for those who utilize mode-dependent macros.
<h4><a name="section-4.11.6">4.11.6 <code><nobr>__OUTPUT_FORMAT__</nobr></code>: Current Output Format</a></h4>
<p>The <code><nobr>__OUTPUT_FORMAT__</nobr></code> standard macro holds the
current Output Format, as given by the <code><nobr>-f</nobr></code> option
or NASM's default. Type <code><nobr>nasm -hf</nobr></code> for a list.
<p><pre>
%ifidn __OUTPUT_FORMAT__, win32
%define NEWLINE 13, 10
%elifidn __OUTPUT_FORMAT__, elf32
%define NEWLINE 10
%endif
</pre>
<h4><a name="section-4.11.7">4.11.7 Assembly Date and Time Macros</a></h4>
<p>NASM provides a variety of macros that represent the timestamp of the
assembly session.
<ul>
<li>The <code><nobr>__DATE__</nobr></code> and
<code><nobr>__TIME__</nobr></code> macros give the assembly date and time
as strings, in ISO 8601 format (<code><nobr>"YYYY-MM-DD"</nobr></code> and
<code><nobr>"HH:MM:SS"</nobr></code>, respectively.)
<li>The <code><nobr>__DATE_NUM__</nobr></code> and
<code><nobr>__TIME_NUM__</nobr></code> macros give the assembly date and
time in numeric form; in the format <code><nobr>YYYYMMDD</nobr></code> and
<code><nobr>HHMMSS</nobr></code> respectively.
<li>The <code><nobr>__UTC_DATE__</nobr></code> and
<code><nobr>__UTC_TIME__</nobr></code> macros give the assembly date and
time in universal time (UTC) as strings, in ISO 8601 format
(<code><nobr>"YYYY-MM-DD"</nobr></code> and
<code><nobr>"HH:MM:SS"</nobr></code>, respectively.) If the host platform
doesn't provide UTC time, these macros are undefined.
<li>The <code><nobr>__UTC_DATE_NUM__</nobr></code> and
<code><nobr>__UTC_TIME_NUM__</nobr></code> macros give the assembly date
and time universal time (UTC) in numeric form; in the format
<code><nobr>YYYYMMDD</nobr></code> and <code><nobr>HHMMSS</nobr></code>
respectively. If the host platform doesn't provide UTC time, these macros
are undefined.
<li>The <code><nobr>__POSIX_TIME__</nobr></code> macro is defined as a
number containing the number of seconds since the POSIX epoch, 1 January
1970 00:00:00 UTC; excluding any leap seconds. This is computed using UTC
time if available on the host platform, otherwise it is computed using the
local time as if it was UTC.
</ul>
<p>All instances of time and date macros in the same assembly session
produce consistent output. For example, in an assembly session started at
42 seconds after midnight on January 1, 2010 in Moscow (timezone UTC+3)
these macros would have the following values, assuming, of course, a
properly configured environment with a correct clock:
<p><pre>
__DATE__ "2010-01-01"
__TIME__ "00:00:42"
__DATE_NUM__ 20100101
__TIME_NUM__ 000042
__UTC_DATE__ "2009-12-31"
__UTC_TIME__ "21:00:42"
__UTC_DATE_NUM__ 20091231
__UTC_TIME_NUM__ 210042
__POSIX_TIME__ 1262293242
</pre>
<h4><a name="section-4.11.8">4.11.8 <code><nobr>__USE_</nobr></code><em>package</em><code><nobr>__</nobr></code>: Package Include Test</a></h4>
<p>When a standard macro package (see <a href="nasmdoc5.html">chapter
5</a>) is included with the <code><nobr>%use</nobr></code> directive (see
<a href="#section-4.6.4">section 4.6.4</a>), a single-line macro of the
form
<code><nobr>__USE_</nobr></code><em>package</em><code><nobr>__</nobr></code>
is automatically defined. This allows testing if a particular package is
invoked or not.
<p>For example, if the <code><nobr>altreg</nobr></code> package is included
(see <a href="nasmdoc5.html#section-5.1">section 5.1</a>), then the macro
<code><nobr>__USE_ALTREG__</nobr></code> is defined.
<h4><a name="section-4.11.9">4.11.9 <code><nobr>__PASS__</nobr></code>: Assembly Pass</a></h4>
<p>The macro <code><nobr>__PASS__</nobr></code> is defined to be
<code><nobr>1</nobr></code> on preparatory passes, and
<code><nobr>2</nobr></code> on the final pass. In preprocess-only mode, it
is set to <code><nobr>3</nobr></code>, and when running only to generate
dependencies (due to the <code><nobr>-M</nobr></code> or
<code><nobr>-MG</nobr></code> option, see
<a href="nasmdoc2.html#section-2.1.4">section 2.1.4</a>) it is set to
<code><nobr>0</nobr></code>.
<p><em>Avoid using this macro if at all possible. It is tremendously easy
to generate very strange errors by misusing it, and the semantics may
change in future versions of NASM.</em>
<h4><a name="section-4.11.10">4.11.10 <code><nobr>STRUC</nobr></code> and <code><nobr>ENDSTRUC</nobr></code>: Declaring Structure Data Types</a></h4>
<p>The core of NASM contains no intrinsic means of defining data
structures; instead, the preprocessor is sufficiently powerful that data
structures can be implemented as a set of macros. The macros
<code><nobr>STRUC</nobr></code> and <code><nobr>ENDSTRUC</nobr></code> are
used to define a structure data type.
<p><code><nobr>STRUC</nobr></code> takes one or two parameters. The first
parameter is the name of the data type. The second, optional parameter is
the base offset of the structure. The name of the data type is defined as a
symbol with the value of the base offset, and the name of the data type
with the suffix <code><nobr>_size</nobr></code> appended to it is defined
as an <code><nobr>EQU</nobr></code> giving the size of the structure. Once
<code><nobr>STRUC</nobr></code> has been issued, you are defining the
structure, and should define fields using the
<code><nobr>RESB</nobr></code> family of pseudo-instructions, and then
invoke <code><nobr>ENDSTRUC</nobr></code> to finish the definition.
<p>For example, to define a structure called
<code><nobr>mytype</nobr></code> containing a longword, a word, a byte and
a string of bytes, you might code
<p><pre>
struc mytype
mt_long: resd 1
mt_word: resw 1
mt_byte: resb 1
mt_str: resb 32
endstruc
</pre>
<p>The above code defines six symbols: <code><nobr>mt_long</nobr></code> as
0 (the offset from the beginning of a <code><nobr>mytype</nobr></code>
structure to the longword field), <code><nobr>mt_word</nobr></code> as 4,
<code><nobr>mt_byte</nobr></code> as 6, <code><nobr>mt_str</nobr></code> as
7, <code><nobr>mytype_size</nobr></code> as 39, and
<code><nobr>mytype</nobr></code> itself as zero.
<p>The reason why the structure type name is defined at zero by default is
a side effect of allowing structures to work with the local label
mechanism: if your structure members tend to have the same names in more
than one structure, you can define the above structure like this:
<p><pre>
struc mytype
.long: resd 1
.word: resw 1
.byte: resb 1
.str: resb 32
endstruc
</pre>
<p>This defines the offsets to the structure fields as
<code><nobr>mytype.long</nobr></code>,
<code><nobr>mytype.word</nobr></code>,
<code><nobr>mytype.byte</nobr></code> and
<code><nobr>mytype.str</nobr></code>.
<p>NASM, since it has no <em>intrinsic</em> structure support, does not
support any form of period notation to refer to the elements of a structure
once you have one (except the above local-label notation), so code such as
<code><nobr>mov ax,[mystruc.mt_word]</nobr></code> is not valid.
<code><nobr>mt_word</nobr></code> is a constant just like any other
constant, so the correct syntax is
<code><nobr>mov ax,[mystruc+mt_word]</nobr></code> or
<code><nobr>mov ax,[mystruc+mytype.word]</nobr></code>.
<p>Sometimes you only have the address of the structure displaced by an
offset. For example, consider this standard stack frame setup:
<p><pre>
push ebp
mov ebp, esp
sub esp, 40
</pre>
<p>In this case, you could access an element by subtracting the offset:
<p><pre>
mov [ebp - 40 + mytype.word], ax
</pre>
<p>However, if you do not want to repeat this offset, you can use -40 as a
base offset:
<p><pre>
struc mytype, -40
</pre>
<p>And access an element this way:
<p><pre>
mov [ebp + mytype.word], ax
</pre>
<h4><a name="section-4.11.11">4.11.11 <code><nobr>ISTRUC</nobr></code>, <code><nobr>AT</nobr></code> and <code><nobr>IEND</nobr></code>: Declaring Instances of Structures</a></h4>
<p>Having defined a structure type, the next thing you typically want to do
is to declare instances of that structure in your data segment. NASM
provides an easy way to do this in the <code><nobr>ISTRUC</nobr></code>
mechanism. To declare a structure of type <code><nobr>mytype</nobr></code>
in a program, you code something like this:
<p><pre>
mystruc:
istruc mytype
at mt_long, dd 123456
at mt_word, dw 1024
at mt_byte, db 'x'
at mt_str, db 'hello, world', 13, 10, 0
iend
</pre>
<p>The function of the <code><nobr>AT</nobr></code> macro is to make use of
the <code><nobr>TIMES</nobr></code> prefix to advance the assembly position
to the correct point for the specified structure field, and then to declare
the specified data. Therefore the structure fields must be declared in the
same order as they were specified in the structure definition.
<p>If the data to go in a structure field requires more than one source
line to specify, the remaining source lines can easily come after the
<code><nobr>AT</nobr></code> line. For example:
<p><pre>
at mt_str, db 123,134,145,156,167,178,189
db 190,100,0
</pre>
<p>Depending on personal taste, you can also omit the code part of the
<code><nobr>AT</nobr></code> line completely, and start the structure field
on the next line:
<p><pre>
at mt_str
db 'hello, world'
db 13,10,0
</pre>
<h4><a name="section-4.11.12">4.11.12 <code><nobr>ALIGN</nobr></code> and <code><nobr>ALIGNB</nobr></code>: Data Alignment</a></h4>
<p>The <code><nobr>ALIGN</nobr></code> and <code><nobr>ALIGNB</nobr></code>
macros provides a convenient way to align code or data on a word, longword,
paragraph or other boundary. (Some assemblers call this directive
<code><nobr>EVEN</nobr></code>.) The syntax of the
<code><nobr>ALIGN</nobr></code> and <code><nobr>ALIGNB</nobr></code> macros
is
<p><pre>
align 4 ; align on 4-byte boundary
align 16 ; align on 16-byte boundary
align 8,db 0 ; pad with 0s rather than NOPs
align 4,resb 1 ; align to 4 in the BSS
alignb 4 ; equivalent to previous line
</pre>
<p>Both macros require their first argument to be a power of two; they both
compute the number of additional bytes required to bring the length of the
current section up to a multiple of that power of two, and then apply the
<code><nobr>TIMES</nobr></code> prefix to their second argument to perform
the alignment.
<p>If the second argument is not specified, the default for
<code><nobr>ALIGN</nobr></code> is <code><nobr>NOP</nobr></code>, and the
default for <code><nobr>ALIGNB</nobr></code> is
<code><nobr>RESB 1</nobr></code>. So if the second argument is specified,
the two macros are equivalent. Normally, you can just use
<code><nobr>ALIGN</nobr></code> in code and data sections and
<code><nobr>ALIGNB</nobr></code> in BSS sections, and never need the second
argument except for special purposes.
<p><code><nobr>ALIGN</nobr></code> and <code><nobr>ALIGNB</nobr></code>,
being simple macros, perform no error checking: they cannot warn you if
their first argument fails to be a power of two, or if their second
argument generates more than one byte of code. In each of these cases they
will silently do the wrong thing.
<p><code><nobr>ALIGNB</nobr></code> (or <code><nobr>ALIGN</nobr></code>
with a second argument of <code><nobr>RESB 1</nobr></code>) can be used
within structure definitions:
<p><pre>
struc mytype2
mt_byte:
resb 1
alignb 2
mt_word:
resw 1
alignb 4
mt_long:
resd 1
mt_str:
resb 32
endstruc
</pre>
<p>This will ensure that the structure members are sensibly aligned
relative to the base of the structure.
<p>A final caveat: <code><nobr>ALIGN</nobr></code> and
<code><nobr>ALIGNB</nobr></code> work relative to the beginning of the
<em>section</em>, not the beginning of the address space in the final
executable. Aligning to a 16-byte boundary when the section you're in is
only guaranteed to be aligned to a 4-byte boundary, for example, is a waste
of effort. Again, NASM does not check that the section's alignment
characteristics are sensible for the use of <code><nobr>ALIGN</nobr></code>
or <code><nobr>ALIGNB</nobr></code>.
<p>See also the <code><nobr>smartalign</nobr></code> standard macro
package, <a href="nasmdoc5.html#section-5.2">section 5.2</a>.
<p align=center><a href="nasmdoc5.html">Next Chapter</a> |
<a href="nasmdoc3.html">Previous Chapter</a> |
<a href="nasmdoc0.html">Contents</a> |
<a href="nasmdoci.html">Index</a>
</body></html>
|
