Editing risc-v/registers

{{risc-v title|Registers}}{{risc-v isa main}}
RISC-V defines a set of '''registers''' that are part of the core ISA.

==Overview ==
RISC-V base ISA consists of 32 [[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.

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.


:[[File:risc-v base integer regsiters.svg|500px]]

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

{| class="wikitable"
|-
! 5-bit Encoding (rx) !! 3-bit Compressed Encoding (rx') !! Register !! [[application binary interface|ABI]] Name !! Description !! Saved by Calle-
|-
| 0 || - || x0 || zero || [[zero register|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
|}