Can Claude write an Atari ST emulator in a morning?

At the weekend I wanted to find out whether Claude could write a full Atari ST emulator that runs in the browser. It took a morning.

You can try it at stuartervine.com/atari. It boots the open-source EmuTOS to the desktop. Drop in a floppy image and it runs. It has a 68000 CPU, the MFP, keyboard controller, sound chip and floppy controller, all driven by one master clock. I asked for cycle-exact timing from the start, because “just get it running” would have been a different and much easier project.

A few things stood out:

• TDD kept it honest. I asked for tests before code, and the emulator has thousands of them. That mattered most for the CPU, where a wrong flag in one instruction shows up as a crash three screens later.
• The hard bug wasn’t in the CPU. I wanted a keyboard joystick to move the eyeball in a STOS disk. Nothing worked until Claude found that STOS programs read the joystick from a fixed address in the old TOS 1.x memory map, which EmuTOS lays out differently. The fix is a compatibility mode that mirrors the joystick state to where those programs look. Claude found it by driving the emulator in Chrome until it understood what the program was reading.
• It’s not finished. Some of the cycle tables were typed from memory and I haven’t checked them against the manual. The DMA FIFO isn’t modelled, and I haven’t tried games that depend on raster timing. It runs the programs I threw at it, but “cycle-exact” is still a goal.

Even so, I’m still a bit amazed that a morning of nudging got me a working ST in a browser tab. WASD and F is joystick 1, if you want to play.