Testing

islet has over 600 tests. casectl has 313. cable-club has 19 accuracy tests that compare framebuffer output pixel-by-pixel against a reference emulator. These are different projects in different languages solving different problems, and they all test heavily. This is not an accident. The principle If you cannot verify a claim automatically, you do not actually know if it is true. You think it works. You believe it works. You tested it manually once and it seemed fine. None of that counts. What counts is a command you can run that tells you, definitively, whether the system does what it is supposed to do. ...

When I rewrote cable-club, the first thing I built was not the CPU or the renderer. It was the test harness. An emulator without accuracy tests is a program that produces plausible-looking output with no way to know if it is correct. I had already learned that lesson once. What accuracy testing means A GBA emulator can pass every standard CPU test suite — every ARM instruction, every Thumb instruction, every edge case in isolation — and still produce the wrong output when running a real game. The test suites verify that individual instructions behave correctly. They do not verify that the CPU, memory bus, DMA controller, interrupt system, and PPU all interact correctly over thousands of frames. ...