Does clmulr access memory?
No. It reads integer register operands and writes rd only.
CLMULR
RISC-V CLMULR Instruction Details
Instruction ManualR-typeCarry-less multiply (reversed, for CRC)
Instruction Syntax
clmulr 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.
ZbcBit ManipulationCrypto
Instruction Behavior
clmulr returns bits 2*XLEN-2:XLEN-1 of the 2*XLEN carry-less product and can accelerate CRC calculations.
CLMULR Decode & Execute Animation
This model shows ISA-visible GF(2) XOR partial products; it does not model integer carries, a pipeline, or a cryptographic protocol.
Execution context
32-bit encoding fields
31..25
funct7
0000101
24..20
rs2
01100
19..15
rs1
01011
14..12
funct3
010
11..7
rd
01010
6..0
opcode
0110011
Execution data path
funct7=0000101, funct3=010
x10 <- pending
Quick Understanding & Search Notes
clmulr returns bits 2*XLEN-2:XLEN-1 of the 2*XLEN carry-less product and can accelerate CRC calculations.
These instructions are part of a B-extension subset and operate on XLEN-wide integer register values.
clmulr is not ordinary integer multiplication with reversed operands; it is the Zbc-defined bit selection.
Common Usage Scenarios
Bit Manipulation
Understand this scenario with real code like «clmulr x10, x11, x12 ; x10 = bits[2XLEN-2:XLEN-1] of clmul product».
Pre-Use Checklist
Syntax Check
- Verify rd, rs1, rs2 (and rs3) are valid GPRs.
- Confirm funct3 and funct7 encoding is correct.
Semantic Check
- Check if the result affects subsequent branches or address calculations.
- Ensure the rd register is not overwritten by another instruction.
Pitfalls / Common Confusions
clmulr is not ordinary integer multiplication with reversed operands; it is the Zbc-defined bit selection.
FAQ
Does clmulr interpret signed numbers?
These bit operations work on bit patterns directly; aside from the specified word-width selection, arithmetic signed magnitude is not used.