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Register Guide

RISC-V a5 / x15 Register: Argument

One of the a0-a7 integer argument registers in the standard psABI. It is not preserved across calls.

ABI name: a5; physical register: x15.
Save rule: Not preserved.
Read it through its psABI role first, then inspect how each instruction reads or writes it.
Related Search Terms
a5 registerx15 registerRISC-V a5a5 argument registera5 psABIa5 not preserveda5 save across calls
Physical Name x15ABI Name a5Save Rule Not preserved
Role
Argument
Convention
Standard psABI: not preserved across calls
Remember This First
The psABI marks a0-a7 as not preserved across calls.
psABI Reference

Argument register

The standard psABI uses a0-a7 as argument registers, with only a0-a1 also used for return values. They are not preserved across calls.

Preserved across calls: No
RISC-V psABI integer register convention
Quick Understanding & Search Notes

a5 / x15 belongs to the a0-a7 argument-register group. The psABI uses a0-a7 for arguments and a0-a1 for return values; these registers are not preserved across calls.

a0-a7 are argument registers, and a0-a1 are also used for return values.
After a call returns, do not assume the old a5 argument value still exists.
RISC-V psABI v1.0: Integer register convention

When It Fits Best

  • - a5 is one of the psABI a0-a7 integer argument registers.
  • - a5 is not listed as a return-value register in the psABI table; look first at a0/a1 for returns.
  • - Read a0-a7 first as function argument registers under the psABI.

When Not To Use It This Way

  • - Do not assume a* registers keep their old values after a call.
  • - If the original a* value is needed after a call, do not leave it in a* across that call.

What Happens Around A Call

1

The psABI marks a0-a7 as not preserved across calls.

2

After a call returns, do not assume the old a5 argument value still exists.

3

For a scalar of at most XLEN bits, the psABI passes it by value on the stack when no argument register is available.

Arguments And Return Values

Examples explain the rule, not a complete program
addi a5, zero, 3  # write a5
# a5 is an argument register, outside the a0-a1 return-value registers

FAQ

Does a5 / x15 keep its old argument value after a call?

Do not assume so. The standard psABI marks a0-a7 as not preserved across calls.

Which a registers carry return values?

The psABI uses a0-a1 for return values as well as arguments.