Intel 64 and IA-32 Architectures Software Developers Manual Volume 1, Basic Architecture

Vol. 1 3-25
BASIC EXECUTION ENVIRONMENT
The operand-size attribute selects the size of operands. When the 16-bit operand-
size attribute is in force, operands can generally be either 8 bits or 16 bits, and when
the 32-bit operand-size attribute is in force, operands can generally be 8 bits or 32
bits.
The address-size attribute selects the sizes of addresses used to address memory:
16 bits or 32 bits. When the 16-bit address-size attribute is in force, segment offsets
and displacements are 16 bits. This restriction limits the size of a segment to 64
KBytes. When the 32-bit address-size attribute is in force, segment offsets and
displacements are 32 bits, allowing up to 4 GBytes to be addressed.
The default operand-size attribute and/or address-size attribute can be overridden
for a particular instruction by adding an operand-size and/or address-size prefix to an
instruction. See Chapter 2, “Instruction Format,” in the Intel® 64 and IA-32 Archi-
tectures Software Developer’s Manual, Volume 3A. The effect of this prefix applies
only to the targeted instruction.
Table 3-4 shows effective operand size and address size (when executing in
protected mode or compatibility mode) depending on the settings of the D flag and
the operand-size and address-size prefixes.
3.6.1 Operand Size and Address Size in 64-Bit Mode
In 64-bit mode, the default address size is 64 bits and the default operand size is 32
bits. Defaults can be overridden using prefixes. Address-size and operand-size
prefixes allow mixing of 32/64-bit data and 32/64-bit addresses on an instruction-
by-instruction basis. Table 3-4 shows valid combinations of the 66H instruction prefix
and the REX.W prefix that may be used to specify operand-size overrides in 64-bit
mode. Note that 16-bit addresses are not supported in 64-bit mode.
REX prefixes consist of 4-bit fields that form 16 different values. The W-bit field in the
REX prefixes is referred to as REX.W. If the REX.W field is properly set, the prefix
specifies an operand size override to 64 bits. Note that software can still use the
Table 3-3. Effective Operand- and Address-Size Attributes
D Flag in Code Segment
Descriptor
00001111
Operand-Size Prefix 66H N N Y Y N N Y Y
Address-Size Prefix 67H NYNYNYNY
Effective Operand Size 16 16 32 32 32 32 16 16
Effective Address Size 16 32 16 32 32 16 32 16
NOTES:
Y: Yes - this instruction prefix is present.
N: No - this instruction prefix is not present.