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The VEX prefix replaces the most commonly used instruction prefix bytes and escape bytes. 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.
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 ...
Write 8 bytes to shadow stack WRUSSD m32,r32: 66 0F 38 F5 /r: Write 4 bytes to user shadow stack 0 WRUSSQ m64,r64: 66 REX.W 0F 38 F5 /r: Write 8 bytes to user shadow stack SETSSBSY: F3 0F 01 E8: Mark shadow stack busy CLRSSBSY m64: F3 0F AE /6: Clear shadow stack busy flag CET_IBT Indirect Branch Tracking.
The EVEX prefix retains fields introduced in the VEX prefix: Four bits R̅, X̅, B̅ and W from the VEX prefix, stored in inverted form. W expands the operand size to 64 bits or serves as an additional opcode, R expands reg, B expands r/m or reg, and X and B expand index and base in the SIB byte. Four bits named v̅, stored in inverted form.
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.
Store random bytes to ES:[rDI], and increment ES:rDI accordingly. XSTORE will store currently-available bytes, which may be from 0 to 8 bytes. REP XSTORE and REP XRNG2 will write the number of random bytes specified by rCX, waiting for the random number generator when needed. [b] EDX specifies a "quality factor". [c] Nehemiah (stepping 3) REP ...
VEX coding is also used for instructions operating on the k0-k7 mask registers that were introduced with AVX-512. The alignment requirement of SIMD memory operands is relaxed. [5] Unlike their non-VEX coded counterparts, most VEX coded vector instructions no longer require their memory operands to be aligned to the vector size.
1000 8: YB: yottabyte 1000 9: RB: ronnabyte 1000 10: QB: ... Bit and byte prefixes; ... Manual of Style/Dates and numbers § Quantities of bytes and bits