Does clmulr access memory?
It does not access data memory. It reads two integer registers and delivers the selected product bits to rd; rd=x0 discards the result.
CLMULR
RISC-V CLMULR Instruction Details
Instruction ManualR-typeRV32/RV64 Zbc carry-less multiply, returning bits 2*XLEN-2:XLEN-1 of the 2*XLEN-bit product.
Instruction Syntax
clmulr rd, rs1, rs2
RV32/RV64 Zbc 1.0; syntax clmulr rd, rs1, rs2, with no rs3 or immediate.
32-bit R-type: funct7=0000101, funct3=010, opcode=0110011; register fields select x0–x31.
Select shifted rs1 terms with set rs2 bits and combine with XOR; select full-product bits 2*XLEN-2:XLEN-1. Sources use old XLEN-bit patterns; x0 reads zero and discards writes.
ZbcBit Manipulation
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.
RV32/RV64: Zbc 1.0; read two XLEN-bit integer-register patterns and combine partial products with XOR.
Select bits 2*XLEN-2:XLEN-1 from the full 2*XLEN-bit carry-less product, without polynomial modular reduction.
x0 reads as zero and discards writes; overlapping source and destination registers use the pre-writeback sources.
Common Usage Scenarios
Bit Manipulation
RV32 Zbc, x11=0x80000000 and x12=1: full carry-less product=0x0000000080000000; selecting bits62:31, clmulr writes x10=1.
Pre-Use Checklist
Syntax Check
- RV32/RV64 Zbc 1.0; syntax clmulr rd, rs1, rs2, with no rs3 or immediate.
- 32-bit R-type: funct7=0000101, funct3=010, opcode=0110011; register fields select x0–x31.
Semantic Check
- Select shifted rs1 terms with set rs2 bits and combine with XOR; select full-product bits 2*XLEN-2:XLEN-1. Sources use old XLEN-bit patterns; x0 reads zero and discards writes.
Pitfalls / Common Confusions
Selects bits 2*XLEN-2:XLEN-1, one bit lower than the clmulh high-half slice; the selection rules are not interchangeable.
FAQ
Does clmulr interpret signed numbers?
It treats XLEN-bit patterns as polynomial operands for carry-less multiplication, without signed-integer multiplication semantics; partial products combine with XOR.