Learning PSP assembly

[Yugisokubodai]

Thank you very much.
I have one question: in Mips emulator susch as Spim or Mars, syscall is executed with the corresponding value in $v0.
For example, to print text string to console, $v0 must be 4.
I wonder which register it is for the psp syscall?
In psx, it's $t1.