Register Guide
RISC-V fs4 / f20 Register: Floating saved register
f20/fs4 is a floating-point register used by F/D/Q/Zfh instructions for operands, results, or FP call arguments.
ABI name: fs4; physical register: f20.
Save rule: Callee.
Read it through its psABI role first, then inspect how each instruction reads or writes it.
Physical Name f20ABI Name fs4Save Rule Callee
Role
Floating saved register
Convention
Callee-saved
Remember This First
t* registers are caller-saved, so callees may overwrite them.
psABI Reference
ABI register
f20/fs4 is a floating-point register used by F/D/Q/Zfh instructions for operands, results, or FP call arguments.
Preserved across calls: Callee
RISC-V psABI integer register conventionWhen It Fits Best
- - Hold short-lived temporary computation results.
- - Use them for local computations that do not need to survive calls.
- - Use them as caller-managed scratch registers.
When Not To Use It This Way
- - Do not keep values in t* across a call unless you save them yourself first.
- - Do not expect callees to restore t* for you.
What Happens Around A Call
1
t* registers are caller-saved, so callees may overwrite them.
2
If the caller still needs a t* value after a call, it must save it in advance.
Protect Temporary Values Before A Call
Examples explain the rule, not a complete programaddi t0, a0, 1
# If t0 is needed after the call, save it to an ABI-valid location first
call foo
# restore t0 from that saved location after the call returns