Does FROUNDNX.S write an integer register?
No. The result is written to floating-point register f[rd]; the floating-point value is integral.
FROUNDNX.S
RISC-V FROUNDNX.S Instruction Details
Instruction ManualR-typeRound a single-precision floating-point value to an integral value in the same FP format; set NX only when the input is not NaN and differs from the rounded result.
Instruction Syntax
froundnx.s rd, rs1, rm
Operand Breakdown
rd, rs1: FP registers f0–f31 or ABI names; f0 is writable and rd may overlap rs1.
rs1 is the only source; rs2=00101 is a fixed selector. Optional rm defaults to dyn; 111 uses frm. Static 101/110 and dynamic frm=101–111 have reserved behavior.
ZfaFFP Precision
Instruction Behavior
FROUNDNX.S rounds the single-precision floating-point value in f[rs1] to an integral value according to rm, but writes the result back to f[rd] in the same single-precision floating-point format. It sets NX only when the input is not NaN and differs from the rounded result. A signaling NaN sets NV; a quiet NaN produces the canonical NaN under the general FP NaN rule.
FROUNDNX.S Decode And Execute Animation
Shows the Zfa + F-extension single-precision OP-FP flow: decode fields, read FP operands, perform FROUNDNX semantics, then write rd.
rd
rs1
rm
froundnx.s
,
,
Leave rm empty to use the assembly default dyn encoding and fcsr.frm below.
Execution Context
FLEN>=32 / fmt=S (00)
rounding: RNE nearest-even
fflags: initial 0
31..27
26..25
24..20
19..15
14..12
11..7
6..0
01000
FROUNDNX
00
fmt=S
00101
rs2=5 (roundToIntegralExact)
01011
rs1
000
rm
01010
rd
1010011
OP-FP
Execution Data Path
instruction
0x40558553
opcode
1010011 -> OP-FP
funct5/fmt
01000 + S(00) -> FROUNDNX.S
rm
000 -> RNE nearest-even
rd/rs
fa0(f10) / fa1(f11) / rs2=00101 (fixed)
read
Source registers not read yet
FP op
Awaiting Execute
write
Not written yet; initial fflags=0
Current Step
Fetch: show the 32-bit OP-FP encoding
The word is split as an R-type OP-FP instruction; single-precision OP-FP instructions use opcode 1010011.
encoding: 0x40558553
syntax : froundnx.s fa0(f10), fa1(f11), rne
result : Not written yet; initial fflags=0
This bounded teaching model computes whitelisted finite examples plus zero, infinity, and NaN; FROUNDNX.S writes the integral value in S format. Zero and infinity are copied. NX is set when the input differs from the rounded result and is not NaN.
Single-precision Calculation
rs1
fa1(f11) = 1.5
result
Awaiting Execute
This bounded teaching model computes whitelisted finite examples plus zero, infinity, and NaN; FROUNDNX.S writes the integral value in S format. Zero and infinity are copied. NX is set when the input differs from the rounded result and is not NaN.
Assumes Zfa + F, FS enabled, correctly NaN-boxed inputs, and initial fflags=0. Accepts the listed exact finite whitelist, signed zeros, infinity, qNaN, and sNaN. Rounding uses rm/frm; zero and infinity are copied, and sNaN sets NV. NX is set when a non-NaN input differs from the rounded result. rd may overlap rs1, and f0 accepts writeback. Steps are teaching views.
This animation shows only Zfa + F-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
Unlike FP-to-integer FCVT.W.S, FROUNDNX.S writes an integral value in the same FP format to an FP register. FCVT.W.S writes an integer register. FROUNDNX sets NX when a non-NaN input changes.
RV32/RV64 requires Zfa+F and enabled FP state. 32-bit OP-FP: funct5=01000, fmt=00, rs2=00101, opcode=1010011; rs2 is a fixed selector.
Optional rm defaults to dyn; rm=111 selects frm. Static rm=101/110 and dynamic frm=101–111 have reserved behavior.
The result stays in the same FP format; signed zeros and infinities are copied, NaNs produce canonical NaN, and sNaN sets NV. Changed non-NaN values set NX, with no DZ, OF, or UF added; existing fflags are preserved.
For a source narrower than FLEN, check NaN-boxing: upper bits not all ones make the input a canonical NaN; narrower results get upper ones.
Common Usage Scenarios
Floating-Point Rounding
Assume required extensions and FP state enabled, f11=1.5 in this format (validly NaN-boxed when narrower than FLEN), and initial fflags=0: the rne example writes f10=2.0 in an FP register; the changed value sets NX=1.
Numeric Format Handling
Assume required extensions and FP state enabled, f11=1.5 in this format (validly NaN-boxed when narrower than FLEN), and initial fflags=0: the rne example writes f10=2.0 in an FP register; the changed value sets NX=1.
Pre-Use Checklist
Syntax Check
- rd, rs1: FP registers f0–f31 or ABI names; f0 is writable and rd may overlap rs1.
- rs1 is the only source; rs2=00101 is a fixed selector. Optional rm defaults to dyn; 111 uses frm. Static 101/110 and dynamic frm=101–111 have reserved behavior.
Semantic Check
- Confirm Zfa, the matching precision extension, enabled FP state, and valid NaN-boxing for narrower sources.
- The result stays in the same FP format. Zero and infinity are copied; NaN results are canonical, and sNaN adds NV. NX is added only when a non-NaN input differs from the rounded result. No DZ, OF or UF is added; existing fflags are preserved.
Pitfalls / Common Confusions
FROUNDNX sets NX only when the input is not NaN and differs from the rounded result; do not confuse it with FROUND, which does not set NX.
The result is still floating-point, not an integer-register result; use a floating-point-to-integer FCVT form for an integer-register result.
rm controls the direction for rounding to an integral value; static rm=101/110 and reserved frm values under DYN have reserved behavior.
FAQ
How does FROUNDNX.S differ from the other FROUND form?
FROUNDNX sets NX only when the input is not NaN and differs from the rounded result; FROUND does not set NX.