Register | ABI Name | Description | Saver
----- | ----- | ----- | -----
x0 | zero | hardwired zero | -
x1 | ra | return address | Caller
x2 | sp | stack pointer | Callee
x3 | gp | global pointer | -
x4 | tp | thread pointer | -
x5-7 | t0-2 | temporary registers | Caller
x8 | s0 / fp | saved register / frame pointer | Callee
x9 | s1 | saved register | Callee
x10-11 | a0-1 | function arguments / return values | Caller
x12-17 | a2-7 | function arguments | Caller
x18-27 | s2-11 | saved registers | Callee
x28-31 | t3-6 | temporary registers | Caller