Check vl first
The current vl determines the number of body elements. Typical code executes vsetvli, vsetivli, or vsetvl before this instruction.
VMSBC.VV
RISC-V VMSBC.VV Instruction Details
Instruction ManualR-typeCompute element-wise subtract borrow-out and write the Boolean result to mask destination vd.
Instruction Syntax
vmsbc.vv vd, vs2, vs1
Operand Breakdown
vd: mask destination register, with one Boolean result bit per element.
vs2/vs1 or scalar source: compared at the current SEW.
vm: vm=0 uses v0 to restrict participating elements, and vm=1 is unmasked.
VVector IntegerCarry/Borrow
Instruction Behavior
VMSBC.VV 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 form without the m suffix has no borrow input and no ordinary execution-mask operand. The destination is a mask register, not an ordinary SEW-width integer result.
Quick Understanding & Search Notes
VMSBC.VV 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.
Forms without the m suffix do not read v0 as carry/borrow input.
Pair with VADC/VSBC for multiword add/subtract.
Vector Execution Context
When reading VMSBC.VV, do not stop at the mnemonic. Official V-extension semantics also depend on the current vl, vtype, and mask state. .vv: two vector sources participate element by element.
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
Multi-Precision Sub
Understand this scenario with real code like «vmsbc.vv v1, v2, v3».
BigInt
Understand this scenario with real code like «vmsbc.vv v1, v2, v3».
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
No borrow-in in the non-m form
FAQ
Does VMSBC.VV write the add/subtract result?
No. It writes only carry or borrow mask bits; data results come from instructions such as VADC/VSBC.