• Introduction
• Register Types
• UABI
• EABI

Introduction

Register Assignment for each ABI.

NOTE: Early Work in progress. Not all extension registers are included here yet.

The canonical ABI References are on the RISC-V GitHub:

• Standard ABI
  • Calling Convention
• Embedded EABI

Register Types

ABI Register Usage	Description	Saver	ABI Register Usage	Description	Saver
zero	Zero	immutable	ra	Return address	not_applicable
sp	Stack pointer	callee	gp	Global pointer	global
tp	Thread pointer	global	t0-t2, t3-t6	Temporary registers	none
s0-s1, s2-s11	Saved registers	callee	a0-a7	Argument/return value	caller

UABI

x0 zero	x5 t0	x10 a0	x18 s2	x28 t3
x1 ra	x6 t1	x11 a1	x19 s3	x29 t4
x2 sp	x7 t2	x12 a2	x20 s4	x30 t5
x3 gp	x8 s0	x13 a3	x21 s5	x31 t6
x4 tp	x9 s1	x14 a4	x22 s6
		x15 a5	x23 s7
		x16 a6	x24 s8
		x17 a7	x25 s9
			x26 s10
			x27 s11

EABI

x0 zero	x5 t0	x10 a0	x14 s2	x16 s5
x1 ra	x6 s3	x11 a1	x15 t1	x17 s6
x2 sp	x7 s4	x12 a2		x18 s7
x3 gp	x8 s0/fp	x13 a3		x19 s8
x4 tp	x9 s1			x20 s9
				x21 s10
				x22 s11
				x23 s12
				x24 s13
				x25 s14
				x26 s15
				x27 s16
				x28 s17
				x29 s18
				x30 s19
				x31 s20