0xff News - Future PSP releases; Time rewind in computer emulators

News via http://0xff.akop.org/2010/03/18/futu...ter-emulators/

ColEm will be the next emulator to be brought up to the latest version of the PSP library. It will also include the ‘time rewind’ feature found in several other emulators.

I have not completely given up on Castaway PSP (I’m still poking it from time to time), but I am rather running out of options and patience.

For those of you wondering if/when the ‘rewind’ feature will make it to other emulators, especially computer emulators such as fMSX and Fuse, the issue is as follows.


Consoles have a fixed memory size, which means that state for every game takes up the same amount of space, and generally speaking, this space is relatively small. Conversely, computer memory can vary (for example, on MSX anywhere from 64 kB to 4 MB).

‘Time rewind’ works by saving state once every few frames to memory. The feature usually dedicates 85% of the PSP’s remaining RAM for saving state, which generally adds up to about 12 MB or less on a “fat” PSP (PSP 1000). For console emulators this doesn’t tend to be a big deal, especially since save states are small, around 32 kB. While 12 MB/32 kB may seem like a lot of space, keep in mind that an emulator displays 60 frames per second, so if you only take a snapshot once every five frames, you will still exceed a megabyte after less than three seconds. The most common memory configuration for MSX is 128 kB, which is four times that – not including VRAM (which is another 64 kB), CPU registers, etc… Basically, on a computer emulator, this feature gives a much smaller bang for the proverbial buck.

The biggest issue, however, is not the size of the state footprint – it’s the fact that one has to be very careful about changes to the emulated machine that affect the size of the state’s footprint. Not adjusting the ‘time rewind’ engine appropriately will likely result in hard crashes.

All said, however, this task is far from impossible, and you can look forward to computer emulators getting this feature at some point, starting with Fuse (to appease my buddy ewgf, who originally suggested the idea).