fscsr

RISC-V fscsr Pseudo-Instruction Details

Assembler pseudo-instruction

fscsr expands to CSRRW to write fcsr. The two-operand form reads and returns the old value only when rd is not x0; the one-operand form uses x0.

What You Write

fscsr rd, rs fscsr rs

Expansion By Form

fscsr rd, rs: csrrw rd, fcsr, rs
fscsr rs: csrrw x0, fcsr, rs

What This Pseudo Instruction Is Saving You From Writing

fscsr rd, rs returns the old fcsr value when rd is not x0 and writes the new value. With rd=x0, including the one-operand form, it writes without reading fcsr.

Availability And Extension Conditions

Base Conditions

Applies where the described fcsr CSR is implemented.

How The Two Forms Differ

Form 1

fscsr rd, rs uses CSRRW to write fcsr. When rd is not x0, the old fcsr is read and copied to rd; with rd=x0, fcsr is not read.

Form 2

fscsr rs uses CSRRW with rd=x0 to write fcsr and does not read that CSR.

Assembly Forms And Real Instructions

This page records the official expansion only; the RISC-V Assembly Programmer's Manual lists these spellings as pseudoinstructions.

Official References And Reading Order

This page treats pseudo-instructions as assembler-level aliases or macros: first read what real instructions they expand to, then use the official ISA manual for the behavior of those real instructions. ABI, relocation, and linker-relaxation details follow the psABI document.

RISC-V Assembly Programmer's Manual v0.0.1, Chapter 30 Table 6

RISC-V Unprivileged ISA Manual 20260120, Sections 6.1 and 21.2

When To Think Of It First

Use the two-operand form with rd other than x0 to write fcsr and retain its old value.

Use the one-operand form to write fcsr.

Pitfalls / Common Confusions

The rd=x0 CSRRW form does not read the CSR or cause CSR-read side effects, but still writes it.

FAQ

Is fscsr a separate hardware opcode?

No. the RISC-V Assembly Programmer's Manual lists fscsr as an assembler pseudoinstruction; behavior comes from the listed CSR expansion.

What is the official fscsr expansion?

the RISC-V Assembly Programmer's Manual lists: csrrw rd, fcsr, rs; csrrw x0, fcsr, rs.

Does fscsr rd, rs read the old value when rd=x0?

No. With rd=x0, its CSRRW expansion does not read fcsr or cause CSR-read side effects, but still writes fcsr. It reads and returns the old value only when rd is not x0.