EVEX prefix

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The EVEX prefix (Enhanced vector extension) and corresponding coding scheme is an extension to the 32-bit x86 (IA-32) and 64-bit x86-64 (Intel 64) instruction set architecture for Intel microprocessors.

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.

Features[edit]

EVEX coding can address 8 operand mask registers, 16 general-purpose registers and 32 vector registers in 64-bit mode (otherwise, 8 general-purpose and 8 vector), and can support up to 4 operands.

Like the VEX coding scheme, the EVEX prefix unifies existing opcode prefixes and escape codes, memory addressing and operand length modifiers of the x86 instruction set .

The following features are carried over from the VEX scheme:

  • Direct encoding of three SIMD registers (XMM, YMM, or ZMM) as source operands (MMX or x87 registers are not supported);
  • Compacted REX prefix for 64-bit mode;
  • Compacted SIMD prefix (66H, F2H, F3H), escape opcode (0FH) and two-byte escape (0F38H, 0F3AH);
  • Less strict memory alignment requirements for memory operand

EVEX also extends VEX with additional capabilities:

  • Extended SIMD register encoding: a total of 32 new 512-bit SIMD registers ZMM0-ZMM31 in 64-bit mode;
  • Operand mask encoding: 8 new 64-bit opmask registers k0-k7 for conditional execution and merging of destination operands;
  • Broadcasting from source to destination for instructions that take memory vector as a source operand: the second operand is broadcast before being used in the actual operation;
  • Direct embedded rounding control for instructions that operate on floating-point SIMD registers with rounding semantics;
  • Embedded exceptions control for floating-point instructions without rounding semantics;
  • Compressed displacement (DISP8*N), new memory addressing mode to improve encoding density of instruction byte stream; the scale factor N depends on vector length and broadcast mode.

For example, the EVEX encoding scheme allows conditional vector addition in the form of

VADDPS zmm1 {k1}{z}, zmm2, zmm3

where {k1} modifier next to the destination operand encodes the use of opmask register k1 for conditional processing and updates to destination, and {z} modifier (encoded by EVEX.z) provides the two types of masking (merging and zeroing), with merging as default when no modifier is attached.

Technical description[edit]

The EVEX coding scheme uses a code prefix consisting of 4 bytes; the first byte is always 62h and derives from an unused opcode of the 32-bit BOUND instruction, which is not supported in 64-bit mode. [1]

EVEX Prefix in the AVX-512 Instruction Format
# of bytes 4 1 1 1 4 / 1 1
[Prefixes] EVEX Opcode ModR/M [SIB] [Disp32] / [Disp8*N] [Immediate]

The ModR/M byte specifies the addressing of the source register with mod and r/m fields, which encode either 8 registers or 24 addressing modes, and the destination register is encoded with reg field. Base-plus-index and scale-plus-index addressing require the SIB byte, which encodes 2-bit scale factor as well as 3-bit index and 3-bit base registers. In certain SIB encodings, Disp32 contains displacements that need to be added to the base address.

The EVEX prefix retains some fields introduced in the VEX prefix:

  • Four bits R, X, B, and W from the REX prefix. 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. Comparing to the VEX prefix, RXB are provided in non-inversed forms.
  • Four bits named v, specifying a second non-destructive source register operand.
  • Bit L specifying 256-bit vector length.
  • Two bits named p to replace operand size prefixes and operand type prefixes (66, F2, F3).
  • Two of the m bits for replacing existing escape codes (0F, 0F 38 and 0F 3A;).
  • Bit V' is an additional source register index.

New functions of the existing fields:

  • Bit X now expands reg or r/m along with bit B when the SIB byte is not present.

There are several new bit fields:

  • Three bits named a, specifying the operand mask register (k0-k7) for vector instructions.
  • Bit z for specifying merging mode (merge or zero).
  • Bit b for source broadcast, rounding control (combined with L’L), or suppress exceptions
  • Bit L’ for specifying 512-bit vector length, or rounding control mode when combined with L

The encoding of the EVEX prefix is as follows:

7 6 5 4 3 2 1 0
Byte 0 (62h) 0 1 1 0 0 0 1 0
Byte 1 (P1) R X B R’ 0 0 m1 m0 P[7:0]
Byte 2 (P2) W v3 v2 v1 v0 1 p1 p0 P[15:8]
Byte 3 (P3) z L’ L b V’ a2 a1 a0 P[23:16]

The following table lists allowed register addressing combinations (bit 4 is always zero when encoding the 16 general purpose registers):

Register addressing in 64-bit mode using EVEX prefix
Addressing mode Bit 4 Bit 3 Bits [2:0] Register type Common usage
REG EVEX.R’ EVEX.R ModRM.reg General purpose, Vector Destination or Source
NDS/NDD EVEX.V’ EVEX.v3v2v1v0 GPR, Vector 2nd Source or Destination
RM EVEX.X EVEX.B ModRM.r/m GPR, Vector 1st Source or Destination
BASE 0 EVEX.B ModRM.r/m GPR Memory addressing
INDEX 0 EVEX.X SIB.index GPR Memory addressing
VIDX EVEX.V’ EVEX.X SIB.index Vector VSIB memory addressing
IS4 Imm8[3] Imm8[7:4] Vector 3rd Source

References[edit]