Does FSUB.Q raise integer exceptions?
No. RISC-V floating-point exceptions are accrued in fflags; software reads floating-point status state to observe them.
FSUB.Q
RISC-V FSUB.Q Instruction Details
Instruction ManualR-typeQuad-precision floating-point subtraction: rd = rs1 - rs2.
Instruction Syntax
fsub.q 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.
QFloating-Point Arithmetic
Instruction Behavior
FSUB.Q performs quad-precision floating-point subtraction for fsub.q, rounds the result according to rm or the dynamic rounding mode, and writes rd. Special values, NaNs, infinities, divide-by-zero cases, and exception flags follow the IEEE 754 rules adopted by the RISC-V floating-point extensions.
Quick Understanding & Search Notes
FSUB.Q is a Q-related quad-precision floating-point subtraction instruction. It produces a floating-register result, uses rm when rounding is required, and records floating-point exception flags such as NV, DZ, OF, UF, and NX in fflags.
The .Q suffix selects the operand and result floating-point format; narrower formats in wider FLEN registers follow NaN-boxing rules.
rm is a machine-instruction field; when assembly omits it, the assembler usually selects dynamic rounding or its default spelling.
Common Usage Scenarios
Floating Point Basic
Understand this scenario with real code like «fsub.q f0, f1, f2, rne # f0 = f1-f2».
Numerical Computing
Understand this scenario with real code like «fsub.q f0, f1, f2, rne # f0 = f1-f2».
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
Rounding comes from the rm field; rm=111 uses the dynamic frm rounding mode.
Floating-point exceptions are recorded in fflags, not as integer branch conditions or integer traps.
NaNs, infinities, overflow, underflow, and inexact results are reported through floating-point exception flags.
FAQ
What does rm mean for FSUB.Q?
rm selects the rounding mode; the dynamic rounding mode comes from frm. For exact results it does not change the value, but the field is still part of the instruction encoding.