Vex 4 extension8/7/2023 ![]() ![]() The three X̅, B̅, and W bits, also similar to the corresponding bits in the REX prefix.The three-byte VEX prefix additionally contains: ![]() Two p bits that replace operand size prefixes and operand type prefixes (0圆6, 0xF2, 0xF3).An L bit, specifying 256-bit vector length.Four v̅ bits, specifying a second source register operand.The R̅ bit, similar to the REX.R prefix bit used in the x86-64 instruction set extension.The two-byte VEX prefix contains the following components: In 32-bit mode VEX encoded instructions can only access the first 8 YMM/XMM registers the encodings for the other registers would be interpreted as the legacy LDS and LES instructions that are not supported in 64-bit mode. In other cases, the VEX-encoded version is longer or shorter than the legacy code. ![]() In many cases, the number of prefix bytes and escape bytes that are replaced is the same as the number of bytes in the VEX prefix, so that the total length of the VEX-encoded instruction is the same as the length of the legacy instruction code. The VEX prefix replaces the most commonly used instruction prefix bytes and escape codes. For example, c ← a + b instead of a ← a + b (where register a is changed by the instruction).
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