# null pointer

Ideas expressed here are nobody’s.

# Running Arch on the TI Nspire CX Calculator

This is a follow up to my previous post on how to run Debian on the Nspire, this time we will be going deeper into the matter and compiling the Linux Kernel ourselves, and doing such in a way that it’s compatible with Arch Linux ARM

## Hardware requirements

The requirements for this project are the same as for the last one, here’s what you’ll need:

• A Texas TI Nspire CX or CX CAS
• USB Hub
• Mini-B OTG USB Cable
• USB Drive

## Setting everything up

Since we will be building the Kernel ourselves this time we will need to do some special work on our setup, in particular to our build environment.

#### Cross Compiling

Firstly, we must get a cross-compiler working, in this case an arm-none-eabi type compiler, there’s a caveat though, in order for the Kernel to work with the Nspire we must build it on an outdated toolchain. There is a good tutorial over on Ndless SDK on how to do that, but we will be going over the steps here.

1. Clone the SDK
• git clone --recursive https://github.com/ndless-nspire/Ndless.git
2. Run the ndless-sdk/toolchain/build_toolchain.sh script.
• This will download the toolchain as well as build it for you. It will take a while.
3. The original guide recommends adding the binaries’ path to your PATH, but since I don’t think that amending your .bashrc for one-time projects like these is worth it, and having to manually edit your path can be boring, I have created this bash script that does that. Place it inside the Ndless folder, and then source it (make sure your current work path is the Ndless folder too, i.e. cd into it).
4. Your cross-compiler should be working now. Test it by running arm-none-eabi-gcc and see if you get any output.

If you managed to produce output in step 4. that means your Cross Compiler is working well. Remember to source that file whenever you want to use the binaries you compiled, or add them to your PATH on .bashrc if you think you might be using them often.

#### Getting the kernel

Every kernel compilation starts with a configuration file, this one is no different. Since setting the configuration so that it works on the Nspire is a hassle, I have set up a repository that will contain configurations for different versions of the Linux kernel. At the time of writing, solely 4.3.0 is there, but work on other versions is ongoing. Feel free to contact me if you’d like a config for a version of the kernel not yet supported. For the rest of this guide we will be using Kernel 4.3.0, although nothing should change for future versions.

1. Download the Linux kernel and untar it
2. Clone the configurations repository
• git clone https://github.com/bemeurer/nspire-kernel
3. Copy the configuration file to the kernel folder, name it .config
• cp nspire-kernel/4.3.0/config linux-4.3/.config

With this your kernel is ready for compilation, don’t worry, we will get to it in a moment.

## Compiling the kernel

Now that we have set up all the pieces that we will need for the compilation, we can start it. Make sure you have your .config file in the right place. Also confirm that you have the toolchain we compiled in your PATH.

1. ARCH=arm make -jX

• ARCH=arm specifies the architecture we’re building to
• -jX parallelizes compilation. Replace X with your thread count.
2. Check that you have the following files

• arch/arm/boot/zImage
• arch/arm/boot/dts/nspire-cx.dtb

The compilation step is now complete. We have built our kernel image (zImage) and out device tree file (nspire-cx.dtb) which are the files we needed.

## Preparing the system drive

In my last post we set op our rootfs using debian tools, this time it will be a bit different. Up until now this guide was distro-independent, apart from the fact that the config file is heavily influence by arch linux’s one, but we must shift this perspective now to focus on the subject of this post: Arch Linux. For the rest of this project we will be using ALARM, the Arch Linux port for the ARM architecture.

#### Getting the files

2. Make sure your USB drive uses an MBR partition table
3. Format the drive. Add some SWAP space, it will be useful for doing more memory-hungry operations.
• I’m using 512MiB Swap space on my drive, reason for which is the fact that my drive is small at 8GB.
4. Untar the file you downloaded into the drive
• sudo bsdtar xzf ArchLinuxARM-armv5-latest.tar.gz -C ~/mnt/ Replace ~/mnt with wherever you mounted your drive.

#### Installing modules

1. cd into the directory where we compiled linux
2. Make sure the arm-none-eabi toolchain is in your PATH
3. Install modules to the USB drive
• sudo ARCH=arm make modules_install INSTALL_MOD_PATH=~/mnt/

#### Changing root with QEMU

1. Make sure you have qemu-user-static and binfmt-support installed.
2. Copy QEMU binaries to the drive: sudo cp /usr/bin/qemu-arm-static ~/mnt/usr/bin
3. Register qemu-arm-static as an ARM interpreter in the kernel (must be root)
4. Change root into the new rootfs. If you are running arch and you have the arch-install-scripts package installed you can use arch-chroot
5. Synchronize package databases pacman -Syy
6. At this point you might want to install some recommended packages, such as sudo. If you have a USB WiFi Adapter connected to your hub you should also consider installing dialog and wpa_supplicant

#### Downgrading the kernel

Since we are using an outdated kernel version, 4.3.0, relative to the one on the ALARM image we extracted we must downgrade the kernel. Firstly you will need a kernel package. You can download the ones I compiled (linux-armv5-4.3.0-1-arm.pkg.tar.xz and linux-armv5-headers-4.3.0-1-arm.pkg.tar.xz) and install from the chroot with pacman -U [FILES], or compile it yourself. You can do that with either distcc (recommended) or with the following steps:

1. Clone the ALARM PKGBUILD repository
2. Roll it back to commit c82145d0d491c2e216ff49ec60d3e83c13e73230
• git reset --hard c82145d0d491c2e216ff49ec60d3e83c13e73230
3. Copy the core/linux-armv5 folder somewhere else
4. Replace the config file in the folder with our own
5. Get an MD5Sum of our config file
6. Replace the MD5Sum on the PKGBUILD file with out new one, it’s the last one in the list.
7. makepkg -cs --install

Whichever way you choose, you can check if it worked by:

• Running ls /lib/modules and seeing if you see the correct module folder there.
• Running pacman -Qs and checking if it lists the kernel as version 4.3.0-1

Lastly, add the packages linux-armv5 and linux-armv5-headers to your ignore list over on /etc/pacman.conf. To do so, uncomment the IgnorePkg line, and add them, separated by spaces, to it. This will guarantee that these changes won’t be later overwritten by some system upgrade.

### Setting up the calculator

To run the bootloader for the Linux kernel you will need to jailbreak your calculator. There are a lot of good online tutorials on how to do this, I, for one, recommend the guide from TIPlanet. With that out of the way we can get to setting up the files on the calculator’s flash memory.

3. cd into the folder containing Linux, where we performed the compilation steps.
4. Copy arch/arm/boot/zImage and arch/arm/boot/dts/nspire-cx.dtb somewhere
5. Concatenate the .tns extension to both of them

Since the TI Connectivity software does not work on Linux at all (not even under Wine), you will require either a Windows or MacOS machine to get files to and from your calculator, Virtual Machines work.

1. Edit the ndless.cfg.tns file and append ext.ll2 linuxloader2 to it.
• This associates our starting script to the bootloader program
2. Create a folder named linux
3. Place the files you downloaded, zImage.tns, and nspire-cx.dtb.tns into the linux folder

### Finishing up

Once the files have been placed on the calculator, the process is done. You may now start the OS by executing the bootloader. If you have any questions on how to do this you can reference the initial article. Below is a demo video of the calculator playing Doom on Linux.