User Guide

128-Bit Media and Scientific Programming 123

24592—Rev. 3.15—November 2009 AMD64 Technology

Software can interpret the data types in ways other than those shown in Figure 4-14 on page 122—

such as bit fields or fractional numbers—but the 128-bit media instructions do not directly support

such interpretations and software must handle them entirely on its own.

4.4.2 Operand Sizes and Overrides

Operand sizes for 128-bit media instructions are determined by instruction opcodes. Some of these

opcodes include an operand-size override prefix, but this prefix acts in a special way to modify the

opcode and is considered an integral part of the opcode. The general use of the 66h operand-size

override prefix described in “Instruction Prefixes” on page 71 does not apply to 128-bit media

instructions.

For details on the use of operand-size override prefixes in 128-bit media instructions, see the opcodes

in “128-Bit Media Instruction Reference” in Volume 4.

4.4.3 Operand Addressing

Depending on the 128-bit media instruction, referenced operands may be in registers or memory.

XMM registers. A few of these instructions access the MMX registers, GPR registers, rFLAGS

When addressing GPR or XMM registers, the REX instruction prefix can be used to access the

extended GPR or XMM registers, as described in “Instruction Prefixes” on page 175.

Memory Operands. Most 128-bit media instructions can read memory for source operands, and

some of the instructions can write results to memory. “Memory Addressing” on page 14, describes the

general methods for addressing memory operands.

Immediate Operands. Immediate operands are used in certain data-conversion, vector-shift, and

vector-compare instructions. Such instructions take 8-bit immediates, which provide control for the

operation.

I/O Ports. I/O ports in the I/O address space cannot be directly addressed by 128-bit media

instructions, and although memory-mapped I/O ports can be addressed by such instructions, doing so

may produce unpredictable results, depending on the hardware implementation of the architecture.

4.4.4 Data Alignment

Generally, 128-bit media instructions that access a 128-bit operand in memory trigger a general-

protection exception (#GP) if the operand is not aligned to a 16-byte boundary. For instance, 128-bit

media instructions that manipulate scalar operands never trigger a #GP, nor do the following

instructions:

• MASKMOVDQU—Masked Move Double Quadword Unaligned.

• MOVDQU—Move Unaligned Double Quadword.

• MOVUPD—Move Unaligned Packed Double-Precision Floating-Point.