Does VMADC.VXM write the add/subtract result?
No. It writes only carry or borrow mask bits; data results come from instructions such as VADC/VSBC.
VMADC.VXM
RISC-V VMADC.VXM Instruction Details
Instruction ManualR-typeCompute element-wise add carry-out and write the Boolean result to mask destination vd.
Instruction Syntax
vmadc.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
VMADC.VXM is a RISC-V V vector add carry-out instruction. It does not write the integer sum or difference; it writes each element's carry-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
VMADC.VXM generates a carry 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 Add
Understand this scenario with real code like «vmadc.vxm v1, v2, a1, v0.t».
BigInt
Understand this scenario with real code like «vmadc.vxm v1, v2, a1, v0.t».
Carry Propagation
Understand this scenario with real code like «vmadc.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
The m-suffixed form operates on all body elements; v0 is carry/borrow input
Carry-in always from v0