FSUB.D
RISC-V FSUB.D Instruction Details
Instruction ManualR-typeDouble-precision floating-point subtraction: rd = rs1-rs2.
Instruction Syntax
fsub.d rd, rs1, rs2, rm
Operand Breakdown
rd: destination floating-point register receiving the formatted result.
rs1/rs2: source floating-point registers; arithmetic/rounding forms with rm use rm or dynamic frm for rounding mode.
DFloating-Point Arithmetic
Instruction Behavior
FSUB.D performs double-precision floating-point subtraction: rd = rs1 − rs2. R-type, fmt=D(01). Note: rs2 is the subtrahend, not the minuend. Inputs must be properly NaN-boxed. Rounding mode by rm. Catastrophic cancellation may occur when operands are close.
Common Usage Scenarios
Basic Arithmetic
Understand this scenario with real code like «fsub.d f0, f1, f2 # f0 = f1-f2 (RNE)».
Pre-Use Checklist
Syntax Check
- Confirm the current instruction format is R-type.
- Confirm the operand order matches the example.
Semantic Check
- Ensure the destination register usage is compatible with the calling convention.
- Confirm this is not the lower-level form of a pseudo-instruction expansion.
Pitfalls / Common Confusions
catastrophic cancellation when operands close