RISC-V Registers

By Yoo Chung.

Published on 2023-03-10.

Updated on 2023-03-17.

Integer registers

General purpose

Register	ABI name	Convention	Saver
`x0`	`zero`	Hard-wired zero
`x1`	`ra`	Return address	caller
`x2`	`sp`	Stack pointer	caller
`x3`	`gp`	Global pointer
`x4`	`tp`	Thread pointer
`x5`	`t0`	Temporary or alternate link register	caller
`x6`	`t1`	Temporary	caller
`x7`	`t2`	Temporary	caller
`x8`	`s0`/`fp`	Saved register or frame pointer	callee
`x9`	`s1`	Saved register	callee
`x10`	`a0`	Function argument or return value	caller
`x11`	`a1`	Function argument or return value	caller
`x12`	`a2`	Function argument	caller
`x13`	`a3`	Function argument	caller
`x14`	`a4`	Function argument	caller
`x15`	`a5`	Function argument	caller
`x16`	`a6`	Function argument	caller
`x17`	`a7`	Function argument	caller
`x18`	`s2`	Saved register	callee
`x19`	`s3`	Saved register	callee
`x20`	`s4`	Saved register	callee
`x21`	`s5`	Saved register	callee
`x22`	`s6`	Saved register	callee
`x23`	`s7`	Saved register	callee
`x24`	`s8`	Saved register	callee
`x25`	`s9`	Saved register	callee
`x26`	`s10`	Saved register	callee
`x27`	`s11`	Saved register	callee
`x28`	`t3`	Temporary	caller
`x29`	`t4`	Temporary	caller
`x30`	`t5`	Temporary	caller
`x31`	`t6`	Temporary	caller

Standard calling convention

Hardware may treat these specially for better performance.

x1: Holds return address for the call
x2: Stack pointer
x5: Alternate link register

The optional compressed instruction format depends on these conventions for x1 and x2.

Notes

Writes to x0 are always discarded, and reads from x0 always returns 0.

Special purpose

pc: Program counter

References

[1]

Andrew Waterman and Krste Asanović (Eds.). 2019. The RISC-V instruction set manual, volume I: User-level ISA, document version 20191213. RISC-V Foundation. Document Version 20191213