Does clmulh 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.
CLMULH
RISC-V CLMULH Instruction Details
Instruction ManualR-typeRV32/RV64 Zbc/Zbkc carry-less multiply, returning high XLEN bits of the 2*XLEN-bit product.
Instruction Syntax
clmulh rd, rs1, rs2
RV32/RV64 Zbc or Zbkc 1.0; syntax clmulh rd, rs1, rs2, with no rs3 or immediate.
32-bit R-type: funct7=0000101, funct3=011, 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-1:XLEN. Sources use old XLEN-bit patterns; x0 reads zero and discards writes.
ZbcZbkcBit ManipulationCrypto
Related Search Terms
Instruction Behavior
clmulh performs GF(2) carry-less multiplication and returns the high XLEN bits.
CLMULH 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
011
11..7
rd
01010
6..0
opcode
0110011
Execution data path
funct7=0000101, funct3=011
x10 <- pending
Quick Understanding & Search Notes
clmulh performs GF(2) carry-less multiplication and returns the high XLEN bits.
RV32/RV64: Zbc/Zbkc 1.0; read two XLEN-bit integer-register patterns and combine partial products with XOR.
Select high XLEN bits 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/Zbkc, x11=0x80000000 and x12=2: full carry-less product=0x0000000100000000; clmulh writes x10=1.
Pre-Use Checklist
Syntax Check
- RV32/RV64 Zbc or Zbkc 1.0; syntax clmulh rd, rs1, rs2, with no rs3 or immediate.
- 32-bit R-type: funct7=0000101, funct3=011, 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-1:XLEN. Sources use old XLEN-bit patterns; x0 reads zero and discards writes.
Pitfalls / Common Confusions
MULH/MULHU select integer-product high bits; clmulh selects high bits of the carry-less product formed with XOR partial products.
FAQ
Does clmulh 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.