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PCMPESTRM

This instruction compares two operand strings (source and destination) based on the mode specified in the immediate operand. It performs a character-by-character comparison of the strings until a termination condition is met (such as finding a character not in a specified set or reaching the end of a string), returning the index of the first non-matching character and updating the destination register.

The table below covers what the source and destinations can be.

DO NOT support LOCK

This instruction is available only in 64-bit mode or compatibility mode. It requires the processor to support the SSE4.2 instruction set.

The immediate operand MUST be a valid mode identifier (0-15); using an unsupported immediate value will result in an invalid opcode exception. Since the instruction operates on 128-bit XMM registers, the input strings MUST be properly aligned to avoid performance penalties, although alignment is not strictly required for correctness. The instruction's behavior is heavily dependent on the ECX register, which MUST contain the maximum number of characters to be processed to prevent reading beyond the intended buffer boundaries. Failure to correctly set ECX may lead to reading memory beyond the allocated buffer, potentially causing a general protection fault.