What does FMAX.Q do with one NaN operand?
Ordinary FMIN/FMAX returns the other non-NaN operand when exactly one operand is NaN; it returns canonical NaN when both operands are NaN.
FMAX.Q
RISC-V FMAX.Q Instruction Details
Instruction ManualR-typeFMAX.Q selects the numerically larger value from two quad-precision floating-point operands.
Instruction Syntax
fmax.q rd, rs1, rs2
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
FMAX.Q uses the OP-FP encoding with the Q format to select the numerically larger value from two quad-precision source operands. Ordinary FMIN/FMAX returns the non-NaN operand when exactly one input is NaN, returns canonical NaN when both inputs are NaN, and sets NV if either input is a signaling NaN. The official rule orders -0.0 below +0.0.
FMAX.Q Decode And Execute Animation
Shows the Q-extension quad-precision OP-FP flow: decode fields, read FP operands, perform FMAX semantics, then write rd.
rd
rs1
rs2
fmax.q
,
,
Execution Context
FLEN=128 / fmt=Q (11)
funct3: FMAX selects maximum
fflags: example fflags remains 0
31..27
26..25
24..20
19..15
14..12
11..7
6..0
00101
FMAX
11
fmt=Q
01100
rs2
01011
rs1
001
funct3=001
01010
rd
1010011
OP-FP
Execution Data Path
instruction
0x2EC59553
opcode
1010011 -> OP-FP
funct5/fmt
00101 + Q(11) -> FMAX.Q
funct3
001 -> FMAX selects maximum
rd/rs
fa0(f10) / fa1(f11) / fa2(f12)
read
-0.0, 0.0
FP op
FMAX(-0.0, 0.0) = 0.0
write
fa0(f10) = 0.0; example fflags remains 0
Current Step
Fetch: show the 32-bit OP-FP encoding
The word is split as an R-type OP-FP instruction; quad-precision OP-FP instructions use opcode 1010011.
encoding: 0x2EC59553
syntax : fmax.q fa0(f10), fa1(f11), fa2(f12)
result : fa0(f10) = 0.0; example fflags remains 0
The default example shows the official signed-zero ordering: -0.0 is less than +0.0. With NaNs, ordinary FMIN/FMAX returns the non-NaN operand for a single NaN and canonical NaN for two NaNs; signaling NaN sets NV.
Quad-precision Calculation
rs1
fa1(f11) = -0.0
rs2
fa2(f12) = 0.0
result
FMAX(-0.0, 0.0) = 0.0
The default example shows the official signed-zero ordering: -0.0 is less than +0.0. With NaNs, ordinary FMIN/FMAX returns the non-NaN operand for a single NaN and canonical NaN for two NaNs; signaling NaN sets NV.
This animation shows only Q-extension ISA-visible OP-FP encoding, example numeric results, and fflags relationship; it does not model FPU pipelines, latency, every IEEE 754 corner case, or microarchitecture.
Quick Understanding & Search Notes
FMAX.Q is not a compare that writes 0/1; it selects a quad-precision result in the floating-point register file. The key points are single-NaN returns non-NaN, two NaNs return canonical NaN, and -0.0 is ordered below +0.0.
The OP-FP fields use the Q format; funct5=00101, with funct3 distinguishing FMIN from FMAX.
Ordinary FMIN/FMAX NaN handling and the NV flag for signaling NaN come from the official F-extension semantics, applied to the H/Q format.
The instruction does not read rm and does not perform arithmetic that creates a newly rounded significand.
Common Usage Scenarios
Floating-Point Compare
Understand this scenario with real code like «fmax.q f0, f1, f2 # f0 = max(f1,f2)».
Numerical Computing
Understand this scenario with real code like «fmax.q f0, f1, f2 # f0 = max(f1,f2)».
Pre-Use Checklist
Syntax Check
- 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.
Semantic Check
- Confirm the floating-point format suffix (.H/.S/.D/.Q) matches the data width.
- Decide whether software must inspect fflags for NV, DZ, OF, UF, or NX.
Pitfalls / Common Confusions
Ordinary FMIN/FMAX returns the non-NaN operand when exactly one input is NaN; do not treat it as always propagating NaN.
A signaling NaN sets NV; quiet NaN and signaling NaN differ in exception-flag behavior.
-0.0 is ordered below +0.0 by the official rule, which affects the sign of FMIN/FMAX results.
FMIN/FMAX has no rm rounding field and is not rounded add/subtract/multiply/divide arithmetic.
FAQ
Does FMAX.Q use rm rounding?
No. FMIN/FMAX is a selection operation; its funct3 selects minimum or maximum semantics, not a rounding mode.