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armv6 Raspberry Pi Zero

The earliest target, and the only 32-bit one. It boots as far as userspace but does not reach a shell yet.

Building

nix build .#kernel-arm32-bcm2835          # also -debug and -dev
nix build .#kexts-arm32-bcm2835
nix build .#prelinked-arm32-bcm2835
nix build .#userland-arm32-bcm2835

There is no image target for this board in the flake yet. A custom disk image aimed at USB booting is being put together, and it is work in progress rather than something you can build today.

Why it is different

Everything else PureDarwin targets is 64-bit. ARMv6 needs a genuinely 32-bit XNU, which Apple last shipped a very long time ago, so this is a port rather than a configuration:

  • Memory barriers are CP15 operations, not the modern instructions.
  • No movw/movt, so address materialisation has to go through literal pools.
  • The kernel uses the older prelinked-kernel format rather than a fileset collection, built with prelink-builder.

State

The kernel boots, mounts root and gets into userspace: execution reaches dyld, which then complains about pthreads. That is a long way past where this target was when the port started, when the question was whether a 32-bit XNU would build at all and the error count was around 700.

Getting there meant going through pmap.c, which was the wall for a long time. The ARMv6 MMU model is different enough from the 64-bit one that the page table code needed real work rather than adaptation.

Kexts

Seven: corecrypto, pthread, ext4, IOStorageFamily, Ext4FileSystemDriver, PDBcm2835SD, PDArmPlatformExpert. corecrypto is not optional even here, because read_random() dereferences NULL without it.

Bootloader work uses lk-overlay, llvm-vc4-vce branch, which is still experimental and does not support this board properly. Build it from its own nix develop, then VC4_TOOLCHAIN=gcc ./build.sh <project>. Be aware that unreferenced modules link out silently, which makes a missing feature look like a broken one.