Does VMSBC.VXM write the add/subtract result?
No. It writes only carry or borrow mask bits; data results come from instructions such as VADC/VSBC.
VMSBC.VXM
RISC-V VMSBC.VXM Instruction Details
Instruction ManualR-typeCompute element-wise subtract borrow-out and write the Boolean result to mask destination vd.
Instruction Syntax
vmsbc.vxm vd, vs2, rs1, v0.t
Operand Breakdown
Destination rd: register receiving the operation result.
Source rs1: register holding the first operand.
Source rs2: register holding the second operand.
VVector IntegerCarry/Borrow
Instruction Behavior
VMSBC.VXM is a RISC-V V vector subtract borrow-out instruction. It does not write the integer sum or difference; it writes each element's borrow-out as a destination mask bit. The m-suffixed form uses v0.mask[i] as carry/borrow input. The destination is a mask register, not an ordinary SEW-width integer result.
Quick Understanding & Search Notes
VMSBC.VXM generates a borrow mask; in m forms, v0 is the carry/borrow input, not an ordinary execution mask.
Destination vd holds a one-bit mask result, one bit per element.
For m-suffixed forms, v0.mask[i] is an arithmetic input; the instruction is not using v0 to filter executed elements.
Pair with VADC/VSBC for multiword add/subtract.
Common Usage Scenarios
Multi-Precision Sub
Understand this scenario with real code like «vmsbc.vxm v1, v2, a1, v0.t».
BigInt
Understand this scenario with real code like «vmsbc.vxm v1, v2, a1, 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
Borrow=1 if pre-truncation diff negative
with borrow-in
The m-suffixed form operates on all body elements; v0 is carry/borrow input