doomacracy
doom on an fpga
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doomacracy linux
3 folders · 8 files
doomfetch
overlay
patches
build.sh
buildroot_config
busybox_config
fhs.nix
kernel_config
post-build.sh
README.md
uclibc_config
README.md

Linux on doomacracy #

A no-MMU Linux 6.8 (cnlohr's mini-rv32ima fork) with busybox in an initramfs, configured for the DE1's 8 MB of SDRAM.

Build (on a Linux machine) #

cd linux
nix-build fhs.nix -o br-fhs      # NixOS only: a normal /usr layout for buildroot
./br-fhs/bin/br-fhs -c ./build.sh

The first build makes a cross toolchain and takes about an hour on two cores. Copy work/buildroot/output/images/Image to build/linux-Image.

Run in the emulator #

make linux-emu            # 8 MB, the board's real size

Changes from cnlohr's configs #

  • CONFIG_RISCV_MISALIGNED=y: our CPU traps on misaligned loads and stores (cnlohr's emulator quietly handles them), so the kernel has to fix them up.
  • busybox: FEATURE_PREFER_APPLETS and FEATURE_SH_STANDALONE, so hush runs simple applets in-process; vi turned on; a few applets dropped (unzip, top, tree, lsof, checksums, xxd, and login/getty, which pulled in 83 KB of password code nothing here uses). No-MMU loads a private copy of busybox for every process, rounded up to a power of two, so its load size (text + data + bss + stack, from flthdr) has to stay under 512 KB; it is 430 KB.
  • patches/uclibc/0001-riscv32-no-64bit-syscall-arg-alignment.patch: uClibc padded 64-bit syscall arguments ARM-style on riscv32, so ftruncate received a length of 0 and vi emptied every file it saved.
  • overlay/etc/inittab: a respawning root shell on the serial console and one on the screen (tty1).
  • Framebuffer console: FB_SIMPLE, FRAMEBUFFER_CONSOLE, FONT_6x8.
  • Keyboard: SERIO_ALTERA_PS2, KEYBOARD_ATKBD, and the PLIC driver.
  • patches/linux/0001-altera_ps2-receive-only.patch: the board's PS/2 port only receives, so the driver registers no write function; atkbd then skips probing the keyboard and treats it as plain set 2. The patch directory is listed after cnlohr's own (board/qemu/riscv32-virt/nommu/ patches), which carries the rv32 no-MMU support; replacing it instead of adding to it builds a kernel whose wait4 returns ENOSYS.
  • The kernel's built-in device tree is not used; tools/gen_dts.py writes it.

doomfetch #

doomfetch/doomfetch.c is a small neofetch, compiled into the image by post-build.sh and run from /etc/profile when a shell starts. It is C on purpose: every process here loads its own copy of its program, so a 64 KB binary is far cheaper than a script that runs busybox (430 KB) a dozen times. For the same reason a pipe of two external busybox commands can run out of memory; prefer shell builtins where you can.

Programs of your own are built the same way: the buildroot cross gcc with -fPIC -Wl,-elf2flt="-r -s STACK", and delete the .gdb file elf2flt leaves next to the binary.