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The VEX prefix's initial-byte values, 0xC4 and 0xC5, are the same as the opcodes of the LDS and LES instructions. Not supported in 64-bit mode, the ambiguity is resolved in 32-bit mode by exploiting the fact that a legal LDS or LES's ModR/M byte cannot specify a register source operand; i.e., be of the form 11xxxxxx .
The EVEX scheme is a 4-byte extension to the VEX scheme which supports the AVX-512 instruction set and allows addressing new 512-bit ZMM registers and new 64-bit operand mask registers. With Advanced Performance Extensions , the Extended EVEX prefix redefines the semantics of several payload bits.
FMA4 instructions are encoded with the VEX prefix, on the form VEX.66.0F3A xx /r ib (no EVEX encodings are defined). The opcode byte xx uses its bottom bit to select floating-point format (0=FP32, 1=FP64) and the remaining bits to select one of the 10 fused-multiply-add operations to perform. For FMA4, operand ordering is controlled by the VEX ...
Instruction prefix to indicate end of hardware lock elision, used with memory atomic/store instructions only (for other instructions, the F3 prefix may have other meanings). When used with such instructions during hardware lock elision, will end the associated transaction instead of performing the store/atomic.
The TBM instructions are all encoded using the XOP prefix. They are all available in 32-bit and 64-bit forms, selected with the XOP.W bit (0=32bit, 1=64bit). (XOP.W is ignored outside 64-bit mode.) Like all instructions encoded with VEX/XOP prefixes, they are unavailable in Real Mode and Virtual-8086 mode.
The VEX prefix used by AVX and AVX2, while flexible, did not leave enough room for the features Intel wanted to add to AVX-512. This has led them to define a new prefix called EVEX. Compared to VEX, EVEX adds the following benefits: [7] Expanded register encoding allowing 32 512-bit registers.
The XOP instructions have an opcode byte 8F (hexadecimal), but otherwise almost identical coding scheme as AVX with the 3-byte VEX prefix. Commentators [4] have seen this as evidence that Intel has not allowed AMD to use any part of the large VEX coding space. AMD has been forced to use different codes in order to avoid using any code ...
The new VEX coding scheme introduces a new set of code prefixes that extends the opcode space, allows instructions to have more than two operands, and allows SIMD vector registers to be longer than 128 bits.