# CLMUL instruction set

Carry-less Multiplication (CLMUL) is an extension to the x86 instruction set used by microprocessors from Intel and AMD which was proposed by Intel in March 2008 and made available in the Intel Westmere processors announced in early 2010. Mathematically, the instruction implements multiplication of polynomials over the finite field GF(2) where the bitstring $a_{0}a_{1}\ldots a_{63}$ represents the polynomial $a_{0}+a_{1}X+a_{2}X^{2}+\cdots +a_{63}X^{63}$ . The CLMUL instruction also allows a more efficient implementation of the closely related multiplication of larger finite fields GF(2k) than the traditional instruction set.

One use of these instructions is to improve the speed of applications doing block cipher encryption in Galois/Counter Mode, which depends on finite field GF(2k) multiplication. Another application is the fast calculation of CRC values, including those used to implement the LZ77 sliding window DEFLATE algorithm in zlib and pngcrush.

## New instructions

The instruction computes the 128-bit carry-less product of two 64-bit values. The destination is a 128-bit XMM register. The source may be another XMM register or memory. An immediate operand specifies which halves of the 128-bit operands are multiplied. Mnemonics specifying specific values of the immediate operand are also defined:

Instruction Opcode Description
`PCLMULQDQ xmmreg,xmmrm,imm` `[rmi: 66 0f 3a 44 /r ib]` Perform a carry-less multiplication of two 64-bit polynomials over the finite field GF(2k).
`PCLMULLQLQDQ xmmreg,xmmrm` `[rm:  66 0f 3a 44 /r 00]` Multiply the low halves of the two registers.
`PCLMULHQLQDQ xmmreg,xmmrm` `[rm:  66 0f 3a 44 /r 01]` Multiply the high half of the destination register by the low half of the source register.
`PCLMULLQHQDQ xmmreg,xmmrm` `[rm:  66 0f 3a 44 /r 10]` Multiply the low half of the destination register by the high half of the source register.
`PCLMULHQHQDQ xmmreg,xmmrm` `[rm:  66 0f 3a 44 /r 11]` Multiply the high halves of the two registers.

## CPUs with CLMUL instruction set

The presence of the CLMUL instruction set can be checked by testing one of the CPU feature bits.