CLMUL instruction set

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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[1] and made available in the Intel Westmere processors announced in early 2010. The purpose is to improve the speed of applications doing block cipher encryption in Galois/Counter Mode, which depends on finite field multiplication. Finite field (GF(2k)) multiplication can be implemented more efficiently[2] with the new CLMUL instructions than with the traditional instruction set.[3] Another application is the fast calculation of CRC values.[4]

New instructions[edit]

The instruction computes the 128-bit 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(2).
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[edit]

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

See also[edit]

References[edit]