# GF2P8MULB — Galois Field Multiply Bytes

Opcode/Instruction | Op/En | 64/32 bit Mode Support | CPUID Feature Flag | Description
--- | --- | --- | --- | ---
66 0F38 CF /r GF2P8MULB xmm1, xmm2/m128 | A | V/V | GFNI | Multiplies elements in the finite field GF(2^8).
VEX.128.66.0F38.W0 CF /r VGF2P8MULB xmm1, xmm2, xmm3/m128 | B | V/V | AVX GFNI | Multiplies elements in the finite field GF(2^8).
VEX.256.66.0F38.W0 CF /r VGF2P8MULB ymm1, ymm2, ymm3/m256 | B | V/V | AVX GFNI | Multiplies elements in the finite field GF(2^8).
EVEX.128.66.0F38.W0 CF /r VGF2P8MULB xmm1{k1}{z}, xmm2, xmm3/m128 | C | V/V | AVX512VL GFNI | Multiplies elements in the finite field GF(2^8).
EVEX.256.66.0F38.W0 CF /r VGF2P8MULB ymm1{k1}{z}, ymm2, ymm3/m256 | C | V/V | AVX512VL GFNI | Multiplies elements in the finite field GF(2^8).
EVEX.512.66.0F38.W0 CF /r VGF2P8MULB zmm1{k1}{z}, zmm2, zmm3/m512 | C | V/V | AVX512F GFNI | Multiplies elements in the finite field GF(2^8).

## Instruction Operand Encoding ¶

Op/En | Tuple | Operand 1 | Operand 2 | Operand 3 | Operand 4
--- | --- | --- | --- | --- | ---
A | N/A | ModRM:reg (r, w) | ModRM:r/m (r) | N/A | N/A
B | N/A | ModRM:reg (w) | VEX.vvvv (r) | ModRM:r/m (r) | N/A
C | Full Mem | ModRM:reg (w) | EVEX.vvvv (r) | ModRM:r/m (r) | N/A

### Description ¶

The instruction multiplies elements in the finite field GF(28), operating on a byte (field element) in the first source operand and the corresponding byte in a second source operand. The field GF(28) is represented in polynomial representation with the reduction polynomial x8 + x4 + x3 + x + 1.

This instruction does not support broadcasting.

The EVEX encoded form of this instruction supports memory fault suppression. The SSE encoded forms of the instruction require16B alignment on their memory operations.

### Operation ¶

    define gf2p8mul_byte(src1byte, src2byte):
        tword := 0
        FOR i := 0 to 7:
            IF src2byte.bit[i]:
                tword := tword XOR (src1byte<< i)
            * carry out polynomial reduction by the characteristic polynomial p*
        FOR i := 14 downto 8:
            p := 0x11B << (i-8)
                    *0x11B = 0000_0001_0001_1011 in binary*
            IF tword.bit[i]:
                tword := tword XOR p
    return tword.byte[0]

#### VGF2P8MULB dest, src1, src2 (EVEX Encoded Version) ¶

    (KL, VL) = (16, 128), (32, 256), (64, 512)
    FOR j := 0 TO KL-1:
        IF k1[j] OR *no writemask*:
            DEST.byte[j] := gf2p8mul_byte(SRC1.byte[j], SRC2.byte[j])
        ELSE iF *zeroing*:
            DEST.byte[j] := 0
        * ELSE DEST.byte[j] remains unchanged*
    DEST[MAX_VL-1:VL] := 0

#### VGF2P8MULB dest, src1, src2 (128b and 256b VEX Encoded Versions) ¶

    (KL, VL) = (16, 128), (32, 256)
    FOR j := 0 TO KL-1:
        DEST.byte[j] := gf2p8mul_byte(SRC1.byte[j], SRC2.byte[j])
    DEST[MAX_VL-1:VL] := 0

#### GF2P8MULB srcdest, src1 (128b SSE Encoded Version) ¶

    FOR j := 0 TO 15:
        SRCDEST.byte[j] :=gf2p8mul_byte(SRCDEST.byte[j], SRC1.byte[j])

### Intel C/C++ Compiler Intrinsic Equivalent ¶

    (V)GF2P8MULB __m128i _mm_gf2p8mul_epi8(__m128i, __m128i);

    (V)GF2P8MULB __m128i _mm_mask_gf2p8mul_epi8(__m128i, __mmask16, __m128i, __m128i);

    (V)GF2P8MULB __m128i _mm_maskz_gf2p8mul_epi8(__mmask16, __m128i, __m128i);

    VGF2P8MULB __m256i _mm256_gf2p8mul_epi8(__m256i, __m256i);

    VGF2P8MULB __m256i _mm256_mask_gf2p8mul_epi8(__m256i, __mmask32, __m256i, __m256i);

    VGF2P8MULB __m256i _mm256_maskz_gf2p8mul_epi8(__mmask32, __m256i, __m256i);

    VGF2P8MULB __m512i _mm512_gf2p8mul_epi8(__m512i, __m512i);

    VGF2P8MULB __m512i _mm512_mask_gf2p8mul_epi8(__m512i, __mmask64, __m512i, __m512i);

    VGF2P8MULB __m512i _mm512_maskz_gf2p8mul_epi8(__mmask64, __m512i, __m512i);

### SIMD Floating-Point Exceptions ¶

None.

### Other Exceptions ¶

Legacy-encoded and VEX-encoded: See Table 2-21, “Type 4 Class Exception Conditions.”

EVEX-encoded: See Table 2-49, “Type E4 Class Exception Conditions.”