EVEX prefix

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 (AMD64) instruction set architecture. EVEX is based on, but should not be confused with the MVEX prefix used by the Knights Corner processor.

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[]

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[]

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]

The ModR/M byte specifies one operand (always a register) with reg field, and the second operand is encoded with mod and r/m fields, specifying either a register or a location in memory. 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. Depending on the addressing mode, Disp8/Disp16/Disp32 field may follow with displacement that needs to be added to the address.

The EVEX prefix retains 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. Just like in VEX prefix, RXB are provided in inverted form.
• Four bits named v, specifying a second non-destructive source register operand. Just like in VEX prefix, vvvv is provided in inverted form.
• 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).

New functions of the existing fields:

• Bit X now expands r/m along with bit B when the SIB byte is not present, which allows 32 SIMD registers.

There are several new bit fields:

• Bit R’ expands reg. Like the R bit, R' is provided in inverted form.
• Bit V' expands vvvv. Like the vvvv bits, V' is provided in inverted form.
• 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:

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

A few VEX-encoded AVX blending instructions have 4 operands. To accommodate this, VEX has IS4 addressing mode, which encodes 4th operand (a vector register) in bits Imm8[7:4] of the immediate constant. Similar EVEX-encoded blend instructions have their 4th operand in a mask register. No EVEX-encoded instruction uses IS4 addressing mode encoding.

References[]