Check vl first
The current vl determines the number of body elements. Typical code executes vsetvli, vsetivli, or vsetvl before this instruction.
VFNCVT.RTZ.XU.F.W
RISC-V VFNCVT.RTZ.XU.F.W Instruction Details
Instruction ManualR-typeNarrow 2*SEW FP source elements to SEW unsigned integers with fixed round-toward-zero.
Instruction Syntax
vfncvt.rtz.xu.f.w vd, vs2, vm
Operand Breakdown
vd: narrowing destination vector register group, with elements at the current SEW.
vs2: the only source vector register group, with source elements at 2*SEW; the assembly syntax has no vs1 source operand.
vs1[19:15]: the VFUNARY0 selector field in the encoding, used to distinguish the specific vfncvt.*.w conversion; it is not a source register.
vm: vm=0 uses v0.t as the execution mask and vm=1 is unmasked.
VVector Type ConvertFloating-Point
Instruction Behavior
VFNCVT.RTZ.XU.F.W is a fixed round-toward-zero RVV narrowing FP-to-unsigned-integer conversion. It reads 2*SEW FP source elements, fixes result rounding to round-toward-zero by instruction definition, and writes SEW unsigned-integer results; frm validity still follows the official vector FP rules.
VFNCVT.RTZ.XU.F.W Decode And Execute Animation
Decode the OP-V encoding and execute a VFUNARY0 narrowing conversion lane by lane: each active lane converts the 2*SEW FP value in vs2[i] to a unsigned integer.
Step 1 / 15
Read OP-V encoding fields
V-extension FP instructions use the OP-V major opcode, with funct6, source registers, vm, funct3, vd, and opcode fields.
Instruction input
vfncvt.rtz.xu.f.w
Execution context
rounding
RTZ
instruction-specified; frm validity still appliessource EEW
64
source element widthvta/vma
ta, ma
tail/inactive policyopcode
1010111
OP-V major opcodevs1
10110
VFUNARY0 conversion selectorEncoding fields
0x488b1257
31..26
25
24..20
19..15
14..12
11..7
6..0
010010
funct6
0
vm
01000
vs2
10110
vs1/sel
001
funct3
00100
vd
1010111
OP-V
Lane results
Long vectors scroll inside this module without page overflow.
i=0active
narrow_trunc_unsigned(0.25)
0
i=1active
narrow_trunc_unsigned(1.5)
1
i=2skip
v0.t=0, not executed
--
i=3active
narrow_trunc_unsigned(3.75)
3
i=4active
narrow_trunc_unsigned(7.125)
7
i=5active
narrow_trunc_unsigned(8.5)
8
i=6skip
v0.t=0, not executed
--
i=7active
narrow_trunc_unsigned(31.25)
31
source EEW=64 / binary64 -> destination width=32 demo values cover finite FP inputs whose rounded integer result fits this demo's integer range. The RTZ form's result rounding is fixed by the instruction to round-toward-zero; frm validity still follows the official vector FP rules. NaN, infinity, out-of-range cases, and fflags still follow the official RVV/scalar FP conversion rules; the 64-bit integer demo range avoids JavaScript unsafe-integer edges.
Quick Understanding & Search Notes
This is the fixed-RTZ form of VFNCVT.XU.F.W: same conversion direction, but rounding does not come from frm.
VFNCVT.RTZ.XU.F.W uses the RVV VFUNARY0 conversion encoding space: selector(vs1[19:15])=10110, funct6=010010, funct3=001 (OPFVV), and opcode=1010111 (OP-V). This vs1 field is an encoding selector, not a source register in the assembly syntax.
This family is narrowing: the vs2 source elements are 2*SEW wide and the vd destination elements are SEW wide; it is not single-width VFCVT or widening VFWCVT.
The narrowing source register group has twice the destination EMUL; by the official narrowing overlap rule, if the destination group overlaps the source group, the destination group's lowest-numbered register must equal the source group's lowest-numbered register.
Only active elements within vl execute; with vm=0, v0.t controls activity, and inactive/tail elements follow the current vma/vta policy.
Ordinary FP conversions use frm; rtz/rod forms have instruction-specified result rounding, while frm validity still follows the official vector FP rules; these conversions do not use fixed-point vxrm.
Vector Execution Context
When reading VFNCVT.RTZ.XU.F.W, do not stop at the mnemonic. Official V-extension semantics also depend on the current vl, vtype, and mask state. The suffix and operand form determine whether sources are vector, scalar, or immediate values.
Then check vtype
The current vtype supplies SEW, LMUL, tail policy, and mask policy; these affect element width, register-group size, and inactive/tail destination elements.
Then check vm/v0
For ordinary vector instructions with vm, vm=0 uses v0 as the execution mask and vm=1 is unmasked. A few forms such as VMERGE use v0 as data-selection input.
Common Usage Scenarios
Format compaction needing truncation semantics
Understand this scenario with real code like «vsetvli t0, a0, e32, m1, ta, ma vfncvt.rtz.xu.f.w v1, v2».
Wide FP to narrower unsigned integers
Understand this scenario with real code like «vsetvli t0, a0, e32, m1, ta, ma vfncvt.rtz.xu.f.w v1, v2».
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
The rtz suffix means result rounding is fixed by the instruction to round-toward-zero; frm validity still follows the official vector FP rules.
Negative, NaN, and out-of-range inputs still follow official FP conversion boundary rules, not an arbitrary saturation algorithm.
FAQ
How does VFNCVT differ from VFWCVT?
VFNCVT narrows from 2*SEW source elements to SEW destination elements; VFWCVT widens from SEW source elements to 2*SEW destination elements. The direction, register-group size, and allowed overlap rules differ.
Does VFNCVT use vxrm for rounding?
No. VFNCVT is a floating-point/integer conversion family: ordinary forms use floating-point frm, while rtz and rod are instruction-specified fixed rounding forms. vxrm is for fixed-point rounding instructions.