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BASIC EXECUTION ENVIRONMENT

operand-size 66H prefix to toggle to a 16-bit operand size. However, setting REX.W takes precedence over the operand-size prefix (66H) when both are used.

In the case of SSE/SSE2/SSE3/SSSE3 SIMD instructions: the 66H, F2H, and F3H prefixes are mandatory for opcode extensions. In such a case, there is no interaction between a valid REX.W prefix and a 66H opcode extension prefix.

See Chapter 2, “Instruction Format,” in the Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 3A.

Table 3-4. Effective Operandand Address-Size Attributes in 64-Bit Mode

L Flag in Code Segment

 

 

 

 

 

 

 

 

Descriptor

1

1

1

1

1

1

1

1

 

 

 

 

 

 

 

 

 

REX.W Prefix

0

0

0

0

1

1

1

1

 

 

 

 

 

 

 

 

 

Operand-Size Prefix 66H

N

N

Y

Y

N

N

Y

Y

 

 

 

 

 

 

 

 

 

Address-Size Prefix 67H

N

Y

N

Y

N

Y

N

Y

 

 

 

 

 

 

 

 

 

Effective Operand Size

32

32

16

16

64

64

64

64

 

 

 

 

 

 

 

 

 

Effective Address Size

64

32

64

32

64

32

64

32

 

 

 

 

 

 

 

 

 

NOTES:

 

 

 

 

 

 

 

 

Y: Yes - this instruction prefix is present.

N: No - this instruction prefix is not present.

3.7OPERAND ADDRESSING

IA-32 machine-instructions act on zero or more operands. Some operands are specified explicitly and others are implicit. The data for a source operand can be located in:

the instruction itself (an immediate operand) a register

a memory location an I/O port

When an instruction returns data to a destination operand, it can be returned to:

a register

a memory location an I/O port

3.7.1Immediate Operands

Some instructions use data encoded in the instruction itself as a source operand. These operands are called immediate operands (or simply immediates). For

3-26 Vol. 1

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