2025/cable - Best imaginary emulator

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The International Obfuscated C Code Contest

2025/cable - Best imaginary emulator

Subleq computer

&larr; 2025/ayu<br>&uarr; 2025 &uarr;<br>2025/cesmoak &rarr;<br>C code<br>Makefile<br>Inventory<br>Original C

Author:

Name: Adrian Cable

Location: US - United States of America (United States)

Award: Best imaginary emulator

To build:

make all<br>To use:

./prog<br>Try:

./prog<br>Download a special vmlinux image and try it:

wget https://raw.githubusercontent.com/adriancable/eternal/main/ioccc/vmlinux.bootimage.xz<br>xz -d vmlinux.bootimage.xz<br>./prog<br>When the vmlinux.bootimage is running try:

export TZ=PST8PDT # or whatever is your timezome<br>~/fbclock -s -d

# ^C to exit

./pi

cd ~/doom<br>./doom<br>Judges’ remarks:

At 366 bytes, 2025/cable/prog.c also qualifies for the “Best Small<br>Program” award!

When looking at the entry, our thoughts were not “what does this program do?”, but “what good could this little piece of code do?”

As it turns out, quite a lot, including running some of the previous<br>IOCCC winners! Moreover, you can do a lot with just one instruction !

BTW : If you try running this entry under WSL, and it does not respond<br>to key presses, you may need to restart the WSL, or reboot your computer.

On purposes

Keep in mind that two of the purposes of this entry are to create a<br>sense of surprise and wonder: This wonderful winning entry is<br>NOT intended for production use. Adding bounds<br>checks to 2025/cable/prog.c would make<br>it larger and slower, and diminish both of these attributes.

See also the Eternal Software Initiative.

A fun challenge

Fun challenge 0

Use the toolchain to compile your own<br>software to run on this VM.

UPDATE : We are in the process of changing how we present fun challenges,<br>and present potential solutions to those challenges. Please stay tuned for an update.

Fun challenge 1

Produce variant of the vmlinux bootimage that contains a C compiler for this VM.

UPDATE : We are in the process of changing how we present fun challenges,<br>and present potential solutions to those challenges. Please stay tuned for an update.

Author’s remarks:

VM - eternal virtual machine

About

Virtual machines are notoriously large pieces of software (QEMU is around 2 million lines of code). Previous<br>IOCCC winners have pushed the boundaries on how small you can make a virtual machine: see for example<br>2024/macke (OpenRISC Linux, 4588 bytes). Or, there’s<br>2024/kurdyukov3 which runs DOOM and weighs in at an impressive<br>2495 bytes of C.

But with some creativity you can go much smaller, and thus, the author presents this year, for the delectation of<br>the judges, a virtual machine in 366 bytes of C, implementing:

32-bit CPU, 1.5GB RAM

800x512 resolution graphics card

real time clock, keyboard driver, pre-emptive multitasking, etc.

And yes, it runs DOOM (and Linux).

How to use

Only dependency is the SDL3 graphics library, to provide a portable way to write pixels to a screen. Install SDL3<br>with: apt install libsdl3-dev on Linux, brew install sdl3 on macOS.

Build:

make<br>Try (press q to quit):

./prog<br>Enjoy a classic game (player 1: a = up, z = down, player 2: k = up, m = down, press q to quit):

./prog<br>Then:

wget https://raw.githubusercontent.com/adriancable/eternal/main/ioccc/vmlinux.bootimage.xz<br>xz -d vmlinux.bootimage.xz<br>./prog<br>Once Linux is running, there’s lots to explore (just type exit when you’re done). You can try some of the<br>author’s favourite IOCCC winners from the past (have a look in ~/ioccc). Then try DOOM:

cd ~/doom<br>./doom<br>Or, enjoy a nice clock (remember to set your time zone):

export TZ=PST8PDT<br>~/fbclock -s -d<br>Or, try running a web server and browser:

ping 127.0.0.1<br>httpd -h /usr/share/www<br>htop<br>lynx http://127.0.0.1<br>Hint 1: build this entry with gcc rather than clang to get the best performance, at least on ARM.

Hint 2: the 8e5 number in the source controls the priority of CPU vs keyboard/display updates. Decrease it if<br>the keyboard/display feels laggy, increase it to improve CPU performance.

How it works

This VM implements an OISC - a One Instruction Set Computer. That instruction takes three signed 32-bit operands,<br>a, b and c, and runs a program from memory m[] as follows:

PC (program counter) starts at 0

Fetch the next instruction (32-bit signed operands a, b and c)

If the low bit on any operand is set, remove it, and replace that operand with m[operand] i.e., a dereference of that address

Set m[b] = m[b] - m[a]

If m[b] is 0 or negative, set the PC to c, otherwise increment PC by 3 words

Go to step 2

(This is a...