I have been playing with the Agon Light, RC2014, and other retro or CP/M systems for a few years now. There's something beautiful about trying to do a lot with just a little hardware. In pursuit of trying to do a lot with only a little hardware, I've taken to solving Project Euler problems on these systems. Obviously, I've relaxed the "one minute" time limit, but I still enjoy much of the challenge of trying to fit the solution to a problem within the constraints of such small systems.

For most of this time, I've been using the HI-TECH C Compiler running on the ZINC compatibility layer. And that's a fantastic way to do things! You end up with code that could theoretically run on systems from the 70s. But I found myself writing complicated extensions and work-arounds to get the problems solvable, and working in a flat filesystem is NOT a good way to enable code-reuse.

For example, Problem 22 requires sorting a 46KB text file. Well, there goes all your memory, or you have to sort on disk. Either way, it gets out of hand!

I found myself writing a library to use ALL of the Agon Light's memory. To do so, I had to get down to assembly, and then all the way down to in-line machine code, since the assembler included with HI-TECH C is not aware of the ez80 ADL mnemonics. Absolutely a fantastic challenge, and one I probably should share with others in the future! Good enough, if that's the only option.

But now it's not the only option. With AI, I can choose the level of difficulty that I want to accept. I DO want to be constrained by the memory and processing speed of the Agon Light, but I DON'T want to be constrained to keeping all my files in one directory, manually shuffling files in and out, or writing non-portable libraries to use the system's full memory.

So, with a few weeks of effort in Claude Code (and a couple of attempts), I've managed to create a C compiler that really seems to hit a sweet spot for me. I still get to solve my problems on a constrained system, and if I want, I still can backport them to HI-TECH C running on ZINC, or my RC2014.

And I think other people may find this useful as well, so I've decided to release it. Claude will take it from here.

-Adam Hemphill

agonc is a C compiler that runs on the Agon Light and Agon Console8, the eZ80 computers, and compiles C programs into MOS programs on the Agon itself. No PC is needed to use it. It compiles C99, all but variable-length arrays and complex numbers, with its standard library, and strict ANSI C89 with -ansi; and it compiles itself.

This is a beta: version 1.1.0-beta.2. It passes its own tests and three free C test suites, and it has run on real hardware, but few people have used it yet. Reports of anything that goes wrong are very welcome (Contributing).

agonc -o hello.bin hello.c       compile, link and assemble a program
hello                            run it
agonc -o game.bin main.c gfx.c   several files

• C99 but for variable-length arrays and complex numbers, as C11 left it mandatory: long long, _Bool, inline, // comments, declarations anywhere, designated initialisers, compound literals, variadic macros and the rest. C89, all of it, in a strict mode (-ansi) that conforms.
• The standard library, C99's included but for <complex.h>, <fenv.h>, <tgmath.h> and the wide characters: files on the SD card, printf, snprintf and scanf with floating point, <math.h> with C99's functions and their float forms, <stdint.h> and <inttypes.h>, <time.h> on the Agon's clock, <signal.h> with Ctrl-C, and the rest.
• IEEE 754 float and double, correctly rounded, done in software (the eZ80 has no floating-point hardware), and 64-bit long long.
• The Agon's own interface: every MOS 2.3.3 call, the serial port, and every documented VDU command for the VDP's graphics, sprites, sound and buffers, in <agon/mos.h>, <agon/uart.h> and <agon/vdp.h>.
• gcc-style options (-o, -c, -I, -D, -l, -O0, -Werror, ...), inline assembly, and a linker that keeps only the code a program uses.
• It builds itself on the Agon, and reproduces itself byte for byte.

It runs under MOS 2.3.3, and under MOS 3, which has been tested on the emulator.

Download agonc-1.1.0-beta.2.zip from the Releases page and unzip it onto the root of the Agon's SD card, merging its folders with the ones already there. It adds /mos/agonc.bin, /bin/agonc/, /lib/agonc/, /usrlib/, /tmp/ and the ez80asm assembler it was tested with. Then, at the MOS prompt, agonc --version.

To build agonc from source instead, on an Agon or a PC, see BUILDING.md.

• The manual: a quick start, the command and its options, the language, the compiler's messages, the C library, the Agon library, porting from AgDev, and how agonc works. make manual turns it into a PDF for printing; the PDF is also attached to each release.
• Examples: short commented programs for files, graphics, sound, the serial port and floating point.
• The specifications in docs/: the language and library (c89_spec.md), the command (driver.md), the ABI (abi.md), the object format (object_format.md) and the intermediate format (ir_format.md).
• BUILDING.md: building agonc on an Agon or a PC, checking the result, and running the tests.

agonc is built from source on an Agon (or the emulator), starting from a first copy compiled on a PC with AgDev, and the copy it builds of itself is identical, byte for byte, to the copy that built it, under MOS 2.3.3 and (on the emulator) MOS 3.0.2. Every usable C89 test of three free C test suites passes in strict mode: c-testsuite 137 of 137, GCC's torture tests 577 of 577, and SDCC's regression tests 644 of 644 files. agonc's own suites test each pass, the libraries clause by clause, the examples and the bootstrap, on the emulator (make check).

agonc was written by Claude, Anthropic's AI model, working with Adam Hemphill, who set its goals and requirements, made the decisions, and tested it on real hardware. Commits made this way carry a Co-Authored-By: Claude line. Work Adam does by hand will be identified as such.

Bug reports and suggestions are welcome as GitHub issues: say what you compiled, what you expected and what happened, ideally with a small program that shows it. Pull requests are welcome too, though what goes in is decided case by case.

MIT: see LICENSE. Copyright (c) 2026 Adam Hemphill. The ez80asm assembler in third_party/ez80asm/ and in the release is Jeroen Venema's, under its own MIT licence; lib/libc/math.c and lib/libc/math99.c keep the notice of fdlibm, from which their algorithms come.