Architecture of the Playstation

Architecture of the Playstation
copetti.org

One of the hacks we created was a limited pre-emptive multi-tasking system on the original Playstation to run our physics engine at a constant frequency. We'd asked Sony US who escalated to Sony Japan and told us it couldn't be done and we should write our code in a cooperative multi-tasking style.

We eventually figured out how to use the vertical blank interrupt to save all the registers, modify the return address to return from the interrupt to our physics code, then our physics code would run (not in the interrupt context), then restore the registers and longjmp back to the main game code which had been originally interrupted by the vertical blank interrupt.

This wasn't general-purpose pre-emption and had only fixed divisors of the screen refresh rate available, but was enough to get our physics engine to run close to isochronously.

Side note 1: our game (NASCAR Racing) was comparatively terrible and I remember being in awe of what the Crash Bandicoot, Ridge Racer, and other early PSX teams accomplished in gameplay and graphics.

Side note 2: as unconventional as the PSX was, it was positively mainstream compared to its contemporary Sega Saturn.

The TMPR5900 (the MIPS chip developed by Toshiba to power the PS2) was sufficiently complex to develop that we developed a cycle-accurate simulator so Sony (and a few game developers) could start development before hardware was available.

However emulation isn't always the best approach: the PS2 (at least the first edition) implemented back compatibility by simply including a PS1 on board. That's a kind of Moore's law in that the cost of doing so had dropped so much since the original PS introduction that it was worth it.

To us the elephant in the room limitation of PS1 is not mentioned here as far as I could tell from a quick read-through: it had no Z-buffer. This meant that you either had to do heroic pre-computation (sort polygons ahead of time) or you ended up with lots of flickering polygons (bluedino's "jiggling and sparkling" comment below seems apt).

The other thing to note is that to make a truly high-performance game for PS1 you had to completely ditch Sony's sanctioned C APIs -- something they said would cause your game to get banned, but which we did anyway. Sony's libraries spent more time copying registers to and from the stack than they did doing the actual work, so they were a big lose.

Finally, that 2KB scratchpad was immensely powerful if you used it well. It's the only reason the 3D collision detection on Crash was fast enough; it took me 9 months to get that performant enough to ship. The CPU and RAM in those machines were just incredibly slow by today's standards.