Does fclass.q modify the floating-point value?
No. It reads the floating-point source and writes the class mask to an integer register.
FCLASS.Q
RISC-V FCLASS.Q Instruction Details
Instruction ManualR-typeExamine quad FP value; return 10-bit class mask. R-type, fmt=Q(11).
Instruction Syntax
fclass.q rd, rs1
Operand Breakdown
Destination rd: register receiving the operation result.
Source rs1: register holding the first operand.
Source rs2: register holding the second operand.
QFloating-Point Compare
Instruction Behavior
fclass.q classifies the quad-precision value in f[rs1] and writes a 10-bit class mask to integer register rd.
Quick Understanding & Search Notes
fclass.q classifies the quad-precision value in f[rs1] and writes a 10-bit class mask to integer register rd.
FCLASS does not round and does not compare numeric magnitude; it returns a class bit mask.
Classes include negative/positive infinity, normal, subnormal, zero, signaling NaN, and quiet NaN categories.
Common Usage Scenarios
Floating Point Basic
Understand this scenario with real code like «fclass.q x10, f0 # x10 = class of f0».
Numerical Computing
Understand this scenario with real code like «fclass.q x10, f0 # x10 = class of f0».
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
FCLASS does not round and does not compare numeric magnitude; it returns a class bit mask.
Classes include negative/positive infinity, normal, subnormal, zero, signaling NaN, and quiet NaN categories.
FAQ
Is fclass.q affected by rm?
No. FCLASS has no rounded result.