How does VWMULU.VX differ from VMUL or VMULH?
VWMULU.VX writes the full 2*SEW widening product. VMUL writes the low SEW bits, while VMULH-family instructions write the high SEW bits.
VWMULU.VX
RISC-V VWMULU.VX Instruction Details
Instruction ManualOPMVXVWMULU.VX produces full 2*SEW widening products for active elements and does not accumulate old vd.
Instruction Syntax
vwmulu.vx vd, vs2, rs1, vm
Operand Breakdown
vd is a wide destination with EEW=2*SEW and EMUL=2*LMUL<=8, aligned to EMUL; vs2 and the converted scalar supply SEW-bit multiplicands.
32-bit OP-V: opcode=1010111, funct6=111000, funct3=110 (OPMVX); vm bit25, vs2 bits24:20, rs1 bits19:15, vd bits11:7. Syntax source order: vs2, rs1. Optional v0.t means vm=0; omission means vm=1.
Multiply two unsigned sources and write the full 2*SEW product without accumulating old vd. First take low scalar bits for XLEN>SEW or sign-extend for XLEN<SEW; x0 supplies zero. Only mask-enabled vstart<=i<vl elements update; vstart>=vl (including vl=0) leaves vd unchanged.
A narrow source overlaps wide vd only with EMUL>=1 and equal highest registers, forcing mask/tail-agnostic behavior. Masked vd excludes v0, which cannot supply both mask and data. Prestart stays unchanged; vma/vta govern inactive/tail preservation or permitted all ones.
RV32/RV64 V 1.0; require enabled vector state, vill=0, supported SEW and 2*SEW. SEW=64 needs additional 128-bit integer elements. Success resets vstart=0 without saturation or vxsat change. The animation fixes LMUL=m1, VLEN=4096, vstart=0; stages are not processor cycles.
VVector OperationsInteger Arithmetic
Instruction Behavior
VWMULU.VX is a RISC-V V extension unsigned widening vector-scalar multiply instruction. Each active element interprets vs2 and x[rs1] after the official .vx scalar conversion as unsigned vs2[i] and unsigned x[rs1], computes the full 2*SEW product, and writes vd. With vm=0, only body elements whose v0.t bit is 1 execute the multiply; masked-off and tail elements follow the current vtype policies.
VWMULU.VX Decode And Execute Animation
Starts from OP-V encoding fields, then shows how VWMULU interprets SEW source elements as unsigned-by-unsigned and writes the full 2*SEW product to vd.
Instruction input
vwmulu.vx
Teaching context: V 1.0, enabled vector state and valid supported vtype; fix LMUL=m1, VLEN=4096, vstart=0. Require SEW and 2*SEW support; SEW=64 needs additional 128-bit integer elements. Enter nonnegative SEW bit patterns; interpret both sources as unsigned. .vx takes low bits when XLEN>SEW or sign-extends when XLEN<SEW; x0 supplies zero. Write the full product to 2*SEW vd without accumulating old vd, saturation or vxsat change. Destination EMUL=2 requires an even start; a narrow source may overlap only its highest register, forcing mask/tail-agnostic behavior. Equal sources require equal initial values; with v0.t, vd and data sources exclude v0. Only body is shown, without definite inactive/tail values. VL=0 updates nothing; success resets vstart=0. Stages are not processor cycles.
Execution Context
OP-V encodingvwmulu.vx
funct6
111000
vm
0
vs2
01000
rs1
01011
funct3
110
vd
00100
opcode
1010111
lane
0
1
2
3
4
5
6
7
v8
0xfffe
0x003a
0x004f
0xfffb
0x0079
0x008e
0xfff8
0x00b8
w*
w*
w*
w*
w*
w*
w*
w*
x11
0x0013
0x0013
0x0013
0x0013
0x0013
0x0013
0x0013
0x0013
v0.t
1
0
1
1
1
0
1
1
v4
...
-
...
...
...
-
...
...
Current step
Show OP-V 32-bit encoding fields
vwmulu.vx uses OP-V encoding. The animation places fixed fields, register fields, vm, and the rs1 field in one encoding strip.
This animation shows only ISA-visible relationships from the official V extension: OP-V field decode, active-element reads, 2*SEW widening integer unsigned widening multiplication, mask control, and vd writeback. It does not model pipelines, caches, or timing.
Quick Understanding & Search Notes
VWMULU.VX multiplies two SEW-width sources and writes the full 2*SEW product. It processes only active elements within the current vl, the second source comes from integer scalar register rs1 after the official .vx scalar conversion to SEW, and the signedness is unsigned vs2[i] and unsigned x[rs1].
VWMULU.VX uses OP-V multiply-class encoding; the .vx form is selected by funct3=110.
Each active element performs vd[i] = unsigned(vs2[i]) * unsigned(sew_scalar(x[rs1])), producing a 2*SEW-wide result written to widening destination vd. Here sew_scalar(x[rs1]) means the official .vx scalar rule: if XLEN>SEW, use the least-significant SEW bits of x[rs1]; if XLEN<SEW, sign-extend x[rs1] to SEW.
The destination register group EMUL is twice the source EMUL and must satisfy the V extension legality requirements for widening destinations.
Only body elements within the current vl are processed; vm=0 uses the v0.t mask, and masked-off plus tail elements follow the current vtype policies.
This instruction is not multiply-accumulate and does not read old vd; use the matching VWMACC* instruction when accumulation is required.
Requires enabled vector state, valid vtype and support for SEW and 2*SEW; widening SEW=64 needs additional 128-bit integer element support.
With vl=0, no destination element is updated. Destination EMUL=2*LMUL must be at most 8; a narrow source overlapping the wide destination must meet the highest-end overlap constraint, and legal different-EEW overlap forces mask/tail-agnostic behavior.
Vector Execution Context
vd is a wide destination with EEW=2*SEW and EMUL=2*LMUL<=8, aligned to EMUL; vs2 and the converted scalar supply SEW-bit multiplicands.
32-bit OP-V: opcode=1010111, funct6=111000, funct3=110 (OPMVX); vm bit25, vs2 bits24:20, rs1 bits19:15, vd bits11:7. Syntax source order: vs2, rs1. Optional v0.t means vm=0; omission means vm=1.
Multiply two unsigned sources and write the full 2*SEW product without accumulating old vd. First take low scalar bits for XLEN>SEW or sign-extend for XLEN<SEW; x0 supplies zero. Only mask-enabled vstart<=i<vl elements update; vstart>=vl (including vl=0) leaves vd unchanged.
A narrow source overlaps wide vd only with EMUL>=1 and equal highest registers, forcing mask/tail-agnostic behavior. Masked vd excludes v0, which cannot supply both mask and data. Prestart stays unchanged; vma/vta govern inactive/tail preservation or permitted all ones.
RV32/RV64 V 1.0; require enabled vector state, vill=0, supported SEW and 2*SEW. SEW=64 needs additional 128-bit integer elements. Success resets vstart=0 without saturation or vxsat change. The animation fixes LMUL=m1, VLEN=4096, vstart=0; stages are not processor cycles.
Common Usage Scenarios
Widening integer multiply
Assume enabled V/VS, valid supported e16,m1,tu,mu, supported 32-bit results, VLEN=4096, XLEN=64, vl=4, vstart=0; v12=[65535,2,32768,4], v0.mask[0..3]=1111. x5=3; vd[0]=65535×3=196605 → 0x0002fffd. Tails retain old values; success resets vstart=0.
V standard vector extension
Assume enabled V/VS, valid supported e16,m1,tu,mu, supported 32-bit results, VLEN=4096, XLEN=64, vl=4, vstart=0; v12=[65535,2,32768,4], v0.mask[0..3]=1111. x5=3; vd[0]=65535×3=196605 → 0x0002fffd. Tails retain old values; success resets vstart=0.
Pre-Use Checklist
Syntax Check
- vd is a wide destination with EEW=2*SEW and EMUL=2*LMUL<=8, aligned to EMUL; vs2 and the converted scalar supply SEW-bit multiplicands.
- 32-bit OP-V: opcode=1010111, funct6=111000, funct3=110 (OPMVX); vm bit25, vs2 bits24:20, rs1 bits19:15, vd bits11:7. Syntax source order: vs2, rs1. Optional v0.t means vm=0; omission means vm=1.
Semantic Check
- Multiply two unsigned sources and write the full 2*SEW product without accumulating old vd. First take low scalar bits for XLEN>SEW or sign-extend for XLEN<SEW; x0 supplies zero. Only mask-enabled vstart<=i<vl elements update; vstart>=vl (including vl=0) leaves vd unchanged.
- A narrow source overlaps wide vd only with EMUL>=1 and equal highest registers, forcing mask/tail-agnostic behavior. Masked vd excludes v0, which cannot supply both mask and data. Prestart stays unchanged; vma/vta govern inactive/tail preservation or permitted all ones.
- RV32/RV64 V 1.0; require enabled vector state, vill=0, supported SEW and 2*SEW. SEW=64 needs additional 128-bit integer elements. Success resets vstart=0 without saturation or vxsat change. The animation fixes LMUL=m1, VLEN=4096, vstart=0; stages are not processor cycles.
Pitfalls / Common Confusions
The result width is 2*SEW and destination EMUL is twice the source EMUL; this differs from single-width VMUL/VMULH.
VWMUL* writes only the product and does not read old vd as an accumulator; use VWMACC* for widening multiply-accumulate.
VWMULU.VX uses unsigned vs2[i] and unsigned x[rs1] interpretation; do not confuse it with the other VWMUL, VWMULU, and VWMULSU forms.
With vm=0, masked-off destination elements follow the current mask policy; do not assume fixed zeroing or preserving.
FAQ
Does VWMULU.VX read old vd for accumulation?
No. VWMULU.VX writes only the product; widening multiply-accumulate semantics belong to VWMACC* instructions.