Does a0 / x10 keep its old argument value after a call?
Do not assume so. a0-a7 are not preserved across calls, and the callee may overwrite them.
Register Guide
RISC-V a0 / x10 Register: First Argument, Return Value, and Calling Convention
a0 is one of the RISC-V psABI argument registers and is also commonly used for return values. It is not preserved across calls, so after a call returns you should read a0 as a return value or as a register the callee was allowed to overwrite.
psABI: a0-a7 are argument registers, with a0-a1 also used for return values.
psABI: a0 is not preserved across calls.
Beginner rule: save the old a0 before a call if you still need it afterward.
Physical Name x10ABI Name a0Save Rule Caller
Role
Argument / Return value
Convention
Caller-saved
Remember This First
a* registers are caller-saved, so callees may overwrite them.
psABI Reference
Argument/return-value register
The psABI uses a0-a7 as argument registers, with a0-a1 also carrying return values. They are not callee-saved and may be overwritten by calls.
Preserved across calls: No
RISC-V psABI integer register conventionQuick Understanding & Search Notes
a0 / x10 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.
a0 is not preserved across calls, so a callee may overwrite it.
When It Fits Best
- - Carry function arguments before the call; a0 is the first argument register.
- - a0 is also part of the a0-a1 return-value register pair.
- - 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 an a* value must survive a call, save it into s* or the stack first.
What Happens Around A Call
1
a* registers are caller-saved, so callees may overwrite them.
2
After a call returns, a0 / a1 should usually be interpreted as return values, not old arguments.
3
When arguments exceed a0-a7, the extra ones go on the stack.
Arguments And Return Values
Examples explain the rule, not a complete programaddi a0, zero, 3 # first argument
addi a1, zero, 4 # second argument
call add2
# after return, a0 holds the return valueFAQ
Which a registers carry return values?
The psABI uses a0-a1 for return values as well as arguments.