RVOPcode
Home/Instructions/VNMSAC-VX
VNMSAC.VX

RISC-V VNMSAC.VX Instruction Details

Instruction ManualR-type

Vector integer negative multiply-accumulate using vd as the accumulator.

Instruction Syntax

vnmsac.vx vd, rs1, vs2, vm
Operand Breakdown
vd: destination vector register group.
vs2/vs1 or scalar source: selected by suffixes such as .vv, .vx, .vi, or .vf.
vm: when present, vm=0 uses v0 as the execution mask and vm=1 is unmasked.
VVector IntegerMultiply-Accumulate
Related Search Terms
VNMSAC.VX instructionvnmsac.vx syntaxvnmsac.vx multiply accumulateRVV multiply accumulatenegative multiply

Instruction Behavior

VNMSAC.VX computes vd[i] - (x[rs1] * vs2[i]) for active elements and writes the low SEW bits back to vd.

Quick Understanding & Search Notes

VNMSAC.VX is a destructive integer multiply-add class instruction; vd participates in the operation and receives the result.

The official operand order is vd, multiplier source, vs2, vm; do not swap vs1/rs1 with vs2.
The result is truncated to SEW width and does not raise integer overflow exceptions.
vm=0 uses v0 as the execution mask; vm=1 is unmasked.
RISC-V Unprivileged ISA Manual: V Standard Extension for Vector Operations

Vector Execution Context

When reading VNMSAC.VX, do not stop at the mnemonic. Official V-extension semantics also depend on the current vl, vtype, and mask state. .vx: one vector source and one integer scalar source participate.

Check vl first

The current vl determines the number of body elements. Typical code executes vsetvli, vsetivli, or vsetvl before this instruction.

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.

Official source: RISC-V V Standard Extension for Vector Operations

Common Usage Scenarios

Difference

e.g., sub t0, a0, a1 — compute the difference a0 - a1.

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

vnmsac subtracts the product from the old vd accumulator.
Old vd is the overwritten addend/accumulator; vnmsub instead overwrites old vd as a multiplicand.
The low SEW bits are written and no integer overflow exception is raised.

FAQ

Is vd only a write destination for VNMSAC.VX?

No. This class is destructive: the old vd value participates as an accumulator or multiplicand.