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{{risc-v title|Standard Extensions}}{{risc-v isa main}}
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{{risc-v title|Registers}}{{risc-v isa main}}
 
RISC-V defines a set of '''registers''' that are part of the core ISA.
 
RISC-V defines a set of '''registers''' that are part of the core ISA.
  
 
==Overview ==
 
==Overview ==
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RISC-V base ISA consists of 32 general-purpose registers <code>x1-x31</code> which hold integer values.
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RISC-V base ISA consists of 31 [[general-purpose registers]] <code>x1-x31</code> which hold integer values. The register <code>x0</code> is hardwired to the [[zero register|constant <code>0</code>]]. There is an additional user-visible [[program counter]] <code>pc</code> register which holds the address of the current instruction. RISC-V does not define a specific subroutine return address link register, but it does suggest that the standard software calling convention should use register <code>x1</code> to store the return address on a call.
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The width of those registers are defined by the RISC-V base variant used. That is, for RV32, the registers are 32 [[bits]] wide, for RV64, they are 64 bits, and for RV128, those registers are 128 bit wide.
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Note that RISC-V defines a special ISA ''E'' for resource-constrained embedded applications which only defines 16 registers.
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 +
 
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:[[File:risc-v base integer regsiters.svg|500px]]
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== Calling convention ==
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In conventional RISC-V software, the stack grows downward with the stack pointer always being 16-byte aligned.
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{| class="wikitable"
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|-
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! 5-bit Encoding (rx) !! 3-bit Compressed Encoding (rx') !! Register !! [[application binary interface|ABI]] Name !! Description !! Saved by Calle-
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|-
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| 0 || - || x0 || zero || [[zero register|hardwired zero]] || -
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|-
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| 1 || - || x1 || ra || return address || -R
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|-
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| 2 || - || x2 || sp || stack pointer || -E
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|-
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| 3 || - || x3 || gp || global pointer || -
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|-
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| 4 || - || x4 || tp || thread pointer || -
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|-
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| 5 || - || x5 || t0 || temporary register 0 || -R
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|-
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| 6 || - || x6 || t1 || temporary register 1 || -R
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|-
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| 7 || - || x7 || t2 || temporary register 2 || -R
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|-
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| 8 || 0 || x8 || s0 / fp || saved register 0 / frame pointer || -E
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|-
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| 9 || 1 || x9 || s1 || saved register 1 || -E
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|-
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| 10 || 2 || x10 || a0 || function argument 0 / return value 0 || -R
 +
|-
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| 11 || 3 || x11 || a1 || function argument 1 / return value 1 || -R
 +
|-
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| 12 || 4 || x12 || a2 || function argument 2 || -R
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|-
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| 13 || 5 || x13 || a3 || function argument 3 || -R
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|-
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| 14 || 6 || x14 || a4 || function argument 4 || -R
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|-
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| 15 || 7 || x15 || a5 || function argument 5 || -R
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|-
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| 16 || - || x16 || a6 || function argument 6 || -R
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|-
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| 17 || - || x17 || a7 || function argument 7 || -R
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|-
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| 18 || - || x18 || s2 || saved register 2 || -E
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|-
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| 19 || - || x19 || s3 || saved register 3 || -E
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|-
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| 20 || - || x20 || s4 || saved register 4 || -E
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|-
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| 21 || - || x21 || s5 || saved register 5 || -E
 +
|-
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| 22 || - || x22 || s6 || saved register 6 || -E
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|-
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| 23 || - || x23 || s7 || saved register 7 || -E
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|-
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| 24 || - || x24 || s8 || saved register 8 || -E
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|-
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| 25 || - || x25 || s9 || saved register 9 || -E
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|-
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| 26 || - || x26 || s10 || saved register 10 || -E
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|-
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| 27 || - || x27 || s11 || saved register 11 || -E
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|-
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| 28 || - || x28 || t3 || temporary register 3 || -R
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|-
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| 29 || - || x29 || t4 || temporary register 4 || -R
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|-
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| 30 || - || x30 || t5 || temporary register 5 || -R
 +
|-
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| 31 || - || x31 || t6 || temporary register 6 || -R
 +
|}

Latest revision as of 08:03, 1 March 2022

RISC-V
Instruction Set Architecture
General
Base Variants(base)
Standard Extensions(all)
Topics

v · d · e

RISC-V defines a set of registers that are part of the core ISA.

Overview[edit]

RISC-V base ISA consists of 31 general-purpose registers x1-x31 which hold integer values. The register x0 is hardwired to the constant 0. There is an additional user-visible program counter pc register which holds the address of the current instruction. RISC-V does not define a specific subroutine return address link register, but it does suggest that the standard software calling convention should use register x1 to store the return address on a call.

The width of those registers are defined by the RISC-V base variant used. That is, for RV32, the registers are 32 bits wide, for RV64, they are 64 bits, and for RV128, those registers are 128 bit wide.

Note that RISC-V defines a special ISA E for resource-constrained embedded applications which only defines 16 registers.


risc-v base integer regsiters.svg

Calling convention[edit]

In conventional RISC-V software, the stack grows downward with the stack pointer always being 16-byte aligned.

5-bit Encoding (rx) 3-bit Compressed Encoding (rx') Register ABI Name Description Saved by Calle-
0 - x0 zero hardwired zero -
1 - x1 ra return address -R
2 - x2 sp stack pointer -E
3 - x3 gp global pointer -
4 - x4 tp thread pointer -
5 - x5 t0 temporary register 0 -R
6 - x6 t1 temporary register 1 -R
7 - x7 t2 temporary register 2 -R
8 0 x8 s0 / fp saved register 0 / frame pointer -E
9 1 x9 s1 saved register 1 -E
10 2 x10 a0 function argument 0 / return value 0 -R
11 3 x11 a1 function argument 1 / return value 1 -R
12 4 x12 a2 function argument 2 -R
13 5 x13 a3 function argument 3 -R
14 6 x14 a4 function argument 4 -R
15 7 x15 a5 function argument 5 -R
16 - x16 a6 function argument 6 -R
17 - x17 a7 function argument 7 -R
18 - x18 s2 saved register 2 -E
19 - x19 s3 saved register 3 -E
20 - x20 s4 saved register 4 -E
21 - x21 s5 saved register 5 -E
22 - x22 s6 saved register 6 -E
23 - x23 s7 saved register 7 -E
24 - x24 s8 saved register 8 -E
25 - x25 s9 saved register 9 -E
26 - x26 s10 saved register 10 -E
27 - x27 s11 saved register 11 -E
28 - x28 t3 temporary register 3 -R
29 - x29 t4 temporary register 4 -R
30 - x30 t5 temporary register 5 -R
31 - x31 t6 temporary register 6 -R
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