Does clmulh access memory?
No. It reads integer register operands and writes rd only.
CLMULH
RISC-V CLMULH Instruction Details
Instruction ManualR-typeCarry-less multiply (high-part)
Instruction Syntax
clmulh rd, rs1, rs2
Operand Breakdown
Destination rd: register receiving the operation result.
Source rs1: register holding the first operand.
Source rs2: register holding the second operand.
BZbcBit ManipulationCrypto
Related Search Terms
Instruction Encoding
31..25
funct7
24..20
rs2
19..15
rs1
14..12
funct3
11..7
rd
6..0
opcode
CLMULH uses opcode 0110011 (0x33), funct3 011, funct7 0000101. The rs1 and rs2 fields select the two source registers, and rd selects the destination register.
Format: R-type
opcode: 0110011 (0x33)
funct3: 011 (0x3)
funct7: 0000101 (0x05)
Instruction Behavior
clmulh performs GF(2) carry-less multiplication and returns the high XLEN bits.
Quick Understanding & Search Notes
clmulh performs GF(2) carry-less multiplication and returns the high XLEN bits.
These instructions are part of a B-extension subset and operate on XLEN-wide integer register values.
The high-half selection is distinct from signed or unsigned integer MULH semantics.
Common Usage Scenarios
Crypto & Security
Understand this scenario with real code like «clmulh x10, x11, x12 ; x10 = high_half(clmul(x11, x12))».
Multiplication & Division
Understand this scenario with real code like «clmulh x10, x11, x12 ; x10 = high_half(clmul(x11, x12))».
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 high-half selection is distinct from signed or unsigned integer MULH semantics.
FAQ
Does clmulh interpret signed numbers?
These bit operations work on bit patterns directly; aside from the specified word-width selection, arithmetic signed magnitude is not used.