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INSTRUCTION FORMAT

Table 4-5. VEX.pp interpretation

pp

Implies this prefix after other

prefixes but before VEX

 

 

 

00B

None

 

 

01B

66

 

 

10B

F3

 

 

11B

F2

 

 

4.1.6The Opcode Byte

One (and only one) opcode byte follows the 2 or 3 byte VEX. Legal opcodes are specified in Appendix B, in color. Any instruction that uses illegal opcode will #UD.

4.1.7The MODRM, SIB, and Displacement Bytes

The encodings are unchanged but the interpretation of reg_field or rm_field differs (see above).

4.1.8The Third Source Operand (Immediate Byte)

VEX-encoded instructions can support instruction with a four operand syntax. VBLENDVPD, VBLENDVPS, and PBLENDVB use imm8[7:4] to encode one of the source registers.

4.1.9AVX Instructions and the Upper 128-bits of YMM registers

If an instruction with a destination XMM register is encoded with a VEX prefix, the processor zeroes the upper 128 bits of the equivalent YMM register. Legacy SSE instructions without VEX preserve the upper 128-bits.

4.1.9.1Vector Length Transition and Programming Considerations

An instruction encoded with a VEX.128 prefix that loads a YMM register operand operates as follows:

Data is loaded into bits 127:0 of the register Bits above bit 127 in the register are cleared.

Thus, such an instruction clears bits 255:128 of a destination YMM register on processors with a maximum vector-register width of 256 bits. In the event that future processors extend the vector registers to greater widths, an instruction encoded with a VEX.128 or VEX.256 prefix will also clear any bits beyond bit 255.

4-10

Ref. # 319433-011

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