What determines the element count for vwmaccus.vx?
The current vl and vtype determine it, with vstart, LMUL, SEW, mask state, and tail policy also affecting execution.
VWMACCUS.VX
RISC-V VWMACCUS.VX Instruction Details
Instruction ManualR-typeVWMACCUS.VX widening unsigned-scalar by signed-vector multiply-accumulate into a 2*SEW-wide vd accumulator.
Instruction Syntax
vwmaccus.vx vd, rs1, vs2, vm
Operand Breakdown
Destination rd: register receiving the operation result.
Source rs1: register holding the first operand.
Source rs2: register holding the second operand.
VVector Operations
Instruction Behavior
vwmaccus.vx is a RISC-V V extension widening unsigned-scalar by signed-vector multiply-accumulate instruction. for each active element, unsigned x[rs1] times signed vs2[i] forms a 2*SEW product added to wide vd[i]. It updates active elements only; inactive elements follow the current policy.
Quick Understanding & Search Notes
vwmaccus.vx is a V extension vector instruction. Active elements are controlled by vl, vtype, vstart, and the optional v0.t mask; for each active element, unsigned x[rs1] times signed vs2[i] forms a 2*SEW product added to wide vd[i].
Official syntax is `vwmaccus.vx vd, rs1, vs2, vm`; without a mask operand it is unmasked, while `, v0.t` updates only selected active elements.
Tail elements and masked-off elements follow the current vtype tail/mask policy; the mnemonic alone does not imply zeroing.
This .vx form reverses the signedness direction of vwmaccsu.vx: rs1 is unsigned and vs2 is signed.
Common Usage Scenarios
Bit Operations & Masks
Understand this scenario with real code like «vwmaccus.vx v8, x5, v12, v0.t».
Data Storing
Understand this scenario with real code like «vwmaccus.vx v8, x5, v12, v0.t».
Vector Operations
Understand this scenario with real code like «vwmaccus.vx v8, x5, v12, v0.t».
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
vd is a wide accumulator and is read-modify-written; source/destination register groups must satisfy V extension overlap and EMUL constraints.
This .vx form reverses the signedness direction of vwmaccsu.vx: rs1 is unsigned and vs2 is signed.
Masking, vl, vtype, vstart, and tail policy affect which elements are updated.
FAQ
Does vwmaccus.vx always process the whole vector register?
No. V instructions operate on active elements; register grouping and inactive-element behavior are controlled by vtype and policy bits.