Learning PSP assembly

[LunaMoo]

For those you listed "v*" registers are simply returning function results(so if a syscall returns a value, it'll be there), "t*" registers are temporary(they get purged with syscall so if they store anything important it's to you to save it), general purpose registers.
Argument registers "a*" as their name suggests are used for function parameters, if a syscall does take any input, the data will be set or pointed to by those arguments. Most mips documents out there should list all register types and how they're used.

To understand syscalls you really need SDK, however obtaining proper, updated PSP SDK is impossible, there are some outdated sdk's on the net which people usually use when they code anything for psp and for most stuff it's good enough. Sources of jpcsp and ppsspp would be the most updated thing to check, unfortunately there are still some, maybe even quite many syscalls which noone really have a clue about, info we have might be either wrong or incomplete, might also differ betweed firmware versions. PPSSPP is also handling lots of functions too fast hence poorly coded games can suffer from timing related issues that might affect virtually anything from causing crashes to variable input timing.