PSX Registers

Peach :

Yeah, s means saved, referring to their function in the ABI.
saved (or in more common naming, non-volatile) means that if a subroutine/function uses that register, it must restore it to its old value before exiting.

• On MIPS though, there's also the t registers (temporary, or volatile in more common naming)
  Subroutines can do whatever the fuck they want with them, compilers will assume volatile registers are thrashed by function calls* and not rely on their values not changing

*This is not true depending on compiler optimizations

Then you've got :

• k or kernel registers - Reserved for the kernel, please don't touch this unless you know what you're doing

• a or argument registers - Your parameters go in here on function calls. For example if you have a function with the signature myFunc (int a, int b), a will go in $a0 and b will go in $a1. In more modern architectures, the "argument registers" are usually a sub-section of the volatiles.

• $at: This is, as its name implies a temporary register for the assembler.
  There's some assembler macros like lw $dest, 32_BIT_ADDRESS.
  For example load32 $v0, 0x1F801814 to get GPUSTAT.
  This is not a valid instruction, it's a macro that your assembler might compile to ...

  lui $at, 0x1F80
  lw $v0, 0x1814($at)

  ...if it supports macros like this. It's a bullshit name and it's volatile.

• $sp: Stack pointer. MIPS doesn't have a traditional stack. It's stack is essentially having a conventional stack pointer register and using sp-relative loads/stores.

• $ra: The return address will be stored here when calling a function, so you can return with jr $ra

• $v0, $v1: First and second return values respectively

• $gp: A global pointer that points to a global memory area so it's easily accessible.

x86_64 example

For example on x86-64, the stack pointer (rsp) is considered saved on every ABI.
Errr rsp is a bad example, let's use rbp instead...
If I want to do :

my_asm_func:
  add rbp, 69
  ret

I'll fuck up any HLL code that jumps to this function, because I'm corrupting a register that the ABI says is saved/non-volatile.
On the other hand:

push rbp ; save rbp in stack
<do things with rbp here>
pop rbp ; restore rbp
ret

is perfectly valid. I am corrupting a non-volatile register but restoring its value before returning

Source

https://discord.com/channels/642647820683444236/663664210525290507/862413015764041788