Loading and streaming

Entering a level streams 15 to 25 MB out of CRASH.BD - the level itself plus the neighbouring areas the game keeps resident. Three separate things made that slow, and the mod addresses all three.

The loader

0x17DDC8   background loader, called once per frame
   │
   ├─ 0x2ABB48   flush queued commands to the IOP   (once per pass)
   │
   └─ 0x2AB150   chunk manager step, looped while it makes progress
                 G_ChunkLoadingManager_ = *(0x309AD8)
                 word 0: bits 0-11 "wanted", bits 12-23 "done"

The loader also has a frame budget, taken from the game clock at 0x192360.

Reads waited for the next frame

The chunk manager queued disc reads and the loader only pushed the queue to the IOP once per frame. Every read therefore cost at least a frame even when the drive was idle. A 14-instruction stub in the code cave now flushes the queue before each chunk-manager step instead, hooked at 0x17DE40. That was the single biggest win.

Two other ideas were measured and dropped: spinning the outer loop while reads are pending gained nothing, and a 512 KB read buffer gained about half a second (2 MB broke the boot - it is heap).

The data was on the slow part of the disc

See the archive page: CRASH.BD now lives at the outer edge.

Emulated drive speed

PCSX2's Fast CDVD halves emulated read times; the mod's game settings turn it on. It is a per-game setting, so it can be switched off in the game's properties.

Results

Measured with the rig, from its level warp until Crash can move, on a fresh boot (each figure includes about 1.5 s of the warp's own credits path):

Level	Retail	Modded ISO	Modded ISO + Fast CDVD
Earth hub (huba)	9.5 s	7.0 s	5.5 s
Classroom Chaos (crgpa08)	16.0 s	10.5 s	8.0 s

The first two changes are changes to the disc image, so they should help on real hardware too - though that has not been tested. What is left is mostly the drive: reads arrive 128 KB at a time, quantised to frames.

Frame timing

The 480p / 60 Hz patch switches the output to 60 Hz, but the game still set its internal frame rate from the PAL region flag - 50. That number is used for exactly one thing: how much of each frame the background loader may use. Believing a frame lasted 20 ms instead of 16.7 ms, the loader overran wherever it stayed busy, and roughly every fifth frame was shown twice.

One word at 0x17D6F0 sets it to 60. In the Earth hub, 57-61 of 300 frames were doubled before and 1 of 360 after.