Yes that is really impressive.
Still it was done with 50% more memory, 1/3 of resolution and not implementing the whole game features.
Still it was done with 50% more memory, 1/3 of resolution and not implementing the whole game features.
ARM7TDMI takes 1-4 cycles to perform a simple 32bit x 32bit multiply, depending on the multiplier. I believe Cortex M33 takes just 1 cycle to do same. ARM7TDMI has no divide instruction and critically, no FPU that Quake requires.
GBA has only 32 kB of 0-wait state RAM (AKA internal working RAM). Versus 276 kB on the Arduino Nano.
GBA's 256 kB RAM block (external working RAM) has massive 6 cycle access time when loading a 32-bit value.
It's a true miracle someone managed to even get 1/3 of resolution on this weak hardware!
I guess FPU would not be even required with 120 pix horizontal resolution.
CM33 does in a single cycle even more: 2 16 bits multiplications, addition and accumulation, for instance.
Still it is the first time the "full" Quake was ported in less than 300 kB.
Quake performs one FPU divide per pixel for texture mapping perspective correction.
ARM7TDMI does not have any kind of divide, so perspective correction is tricky, even if it's just 120 px horizontally.
By the way, it's "d_scan.c" for anyone who's trying to web search for it.
Quake had to do this because it would have been too much especially for a low-end Pentium when it was released in 1996. Yes it is not even noticeable, especially at low res.
There are some user made maps however where this can be seen (e.g. i remember playing a map which was supposed to be inside a fantasy town and it used a bunch of wood-on-wall textures that made the distortion apparent).