
Chapter 4: 128-Bit Media and Scientific Programming
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24593—Rev. 3.09—September 2003
AMD 64-Bit Technology
Software can interpret the data types in ways other than those
shown in Figure 4-14—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 85 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.
Register Operands.
Most 128-bit media instructions can access
source and destination operands in XMM registers. A few of
these instructions access the MMX registers, GPR registers,
rFLAGS register, or MXCSR register. The type of register
addressed is specified in the instruction syntax. 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 208.
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 16,
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