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+# Cross building for ARM
+
+## Prepare Ubuntu RootFS
+
+Install required packages
+
+```
+sudo apt-get install qemu qemu-user-static binfmt-support debootstrap
+```
+
+Use `build_rootfs.sh` script to prepare Root File System. You should have `sudo`
+
+```
+sudo ./tools/cross/build_rootfs.sh arm
+```
+- supports `arm`(default) and `arm64` architecutre for now
+- supports `xenial`(default) and `trusty` release
+
+To see the options,
+```
+./tools/cross/build_rootfs.sh -h
+```
+
+RootFS will be prepared at `tools/cross/rootfs/arm` folder.
+
+## Prepare RootFS at alternative folder
+
+Use `ROOTFS_DIR` to a full path to prepare at alternative path.
+
+```
+ROOTFS_DIR=/home/user/rootfs/arm-xenial sudo ./tools/cross/build_rootfs.sh arm
+```
+
+## Using proxy
+
+If you need to use proxy server while building the rootfs, use `--setproxy` option.
+
+```
+# for example,
+sudo ./tools/cross/build_rootfs.sh arm --setproxy="1.2.3.4:8080"
+# or
+sudo ./tools/cross/build_rootfs.sh arm --setproxy="proxy.server.com:8888"
+```
+
+This will put `apt` proxy settings in `rootfs/etc/apt/apt.conf.d/90proxy` file
+for `http`, `https` and `ftp` protocol.
+
+## Install ARM Cross Toolchain
+
+We recommend you have g++ >= 6 installed on your system because NN generated tests require it.
+
+On Ubuntu 16.04 or older, follow the next steps:
+
+```
+cd ~/your/path
+wget https://releases.linaro.org/components/toolchain/binaries/7.2-2017.11/arm-linux-gnueabihf/gcc-linaro-7.2.1-2017.11-x86_64_arm-linux-gnueabihf.tar.xz
+tar xvf gcc-linaro-7.2.1-2017.11-x86_64_arm-linux-gnueabihf.tar.xz
+echo 'PATH=~/your/path/gcc-linaro-7.2.1-2017.11-x86_64_arm-linux-gnueabihf/bin:$PATH' >> ~/.bashrc
+```
+
+On Ubuntu 18.04 LTS, you can install using `apt-get`.
+Choose g++ version whatever you prefer: 6, 7 or 8.
+
+```
+sudo apt-get install g++-{6,7,8}-arm-linux-gnueabihf
+```
+
+Make sure you get `libstdc++.so` updated on your target with your new toolchain's corresponding one.
+
+For example, if you installed gcc-linaro-7.2.1-2017.11 above, do
+
+```
+wget https://releases.linaro.org/components/toolchain/binaries/7.2-2017.11/arm-linux-gnueabihf/runtime-gcc-linaro-7.2.1-2017.11-arm-linux-gnueabihf.tar.xz
+tar xvf runtime-gcc-linaro-7.2.1-2017.11-arm-linux-gnueabihf.tar.xz
+```
+
+Then, copy `libstdc++.so.6.0.24` into `/usr/lib/arm-linux-gnueabihf`, and update symbolic links on your device.
+
+## Build and install ARM Compute Library
+
+```
+TARGET_ARCH=armv7l make acl
+```
+Mostly you only need once of ACL build. This will build and install to `Product/(target_arch-os)/out/bin` folder.
+- this is required for ARM on Ubuntu
+
+## Build nnfw
+
+Give `TARGET_ARCH` variable to set the target architecture
+
+```
+CROSS_BUILD=1 TARGET_ARCH=armv7l make all install
+```
+- supports `armv7l` and `aarch64` for now
+
+If you used `ROOTFS_DIR` to prepare in alternative folder, you should also give this to makefile.
+
+```
+ROOTFS_DIR=ROOTFS_ARM=/path/to/your/rootfs/arm \
+CROSS_BUILD=1 TARGET_ARCH=armv7l make all install
+```
+
+## Run test
+
+```
+ ./tools/test_driver/test_driver.sh --artifactpath=.
+```