![]() The VEX prefix provides a compact representation of the REX prefix, as well as various other prefixes, to expand the addressing mode, register enumeration and operand size and width: The REX prefix does not use half of its 8 bits. Bit-field W changes the operand size to 64 bits, R expands reg to 4 bits, B expands r/m (or opreg in the few opcodes that encode the register in the 3 lowest opcode bits, such as "POP reg"), and X and B expand index and base in the SIB byte. To use 64-bit addressing and additional registers present in the x86-64 architecture, the REX prefix has been introduced which provides additional space for encoding addressing modes. The base-plus-index and scale-plus-index forms of 32-bit addressing (encoded with r/m = 100 and mod ≠ 11) require another addressing byte, the SIB byte. r/m, bits - can specify a register as an operand, or combine with the mod field to encode an addressing mode.reg/opcode, bits - depending on primary opcode byte, specifies either a register or three more bits of opcode information.Also encodes opcode information for some instructions. mod, bits - combined with the r/m field, encodes either 8 registers or 24 addressing modes.In x86 architecture, instructions with a memory operand may use the ModR/M byte which specifies the addressing mode. The VEX coding scheme uses a code prefix consisting of two or three bytes, which may be added to existing or new instruction codes. ![]() Technical description Intel 64 instruction format using VEX prefix The remaining three m bits are reserved for future use, such as specifying vector lengths >256 bits, specifying different instruction lengths, or extending the opcode space however, as of 2013, Intel decided to introduce a new encoding scheme, the EVEX prefix, rather than expand the remaining m bits.Two of the m bits are used for replacing existing escape codes and for specifying the length of the instruction.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). It allows existing two-operand instructions to be modified into non-destructive three-operand forms where the destination register is different from both source registers.There is room for further extensions of the register size. It allows the size of SIMD vector registers to be extended from the 128- bit XMM registers to the 256-bit YMM registers.It allows instruction codes to have up to four operands (plus immediate), where the original scheme allows only two operands (plus immediate).The opcode map is extended to make space for future instructions. ![]() The VEX coding scheme allows the definition of new instructions and the extension or modification of previously existing instruction codes. The VEX prefix (from "vector extensions") and VEX coding scheme are an extension to the x86 and x86-64 instruction set architecture for microprocessors from Intel, AMD and others. Please consider expanding the lead to provide an accessible overview of all important aspects of the article. This article's lead section may be too short to adequately summarize the key points.
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