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

3-30 Vol. 1

BASIC EXECUTION ENVIRONMENT

At the machine level, a segment override is specified with a segment-override prefix,

which is a byte placed at the beginning of an instruction. The following default

segment selections cannot be overridden:

• Instruction fetches must be made from the code segment.

• Destination strings in string instructions must be stored in the data segment

pointed to by the ES register.

• Push and pop operations must always reference the SS segment.

Some instructions require a segment selector to be specified explicitly. In these

cases, the 16-bit segment selector can be located in a memory location or in a 16-bit

located in register BX into segment register DS:

MOV DS, BX

Segment selectors can also be specified explicitly as part of a 48-bit far pointer in

memory. Here, the first doubleword in memory contains the offset and the next word

contains the segment selector.

3.7.4.1 Segmentation in 64-Bit Mode

In IA-32e mode, the effects of segmentation depend on whether the processor is

running in compatibility mode or 64-bit mode. In compatibility mode, segmentation

functions just as it does in legacy IA-32 mode, using the 16-bit or 32-bit protected

mode semantics described above.

In 64-bit mode, segmentation is generally (but not completely) disabled, creating a

flat 64-bit linear-address space. The processor treats the segment base of CS, DS,

ES, SS as zero, creating a linear address that is equal to the effective address. The

exceptions are the FS and GS segments, whose segment registers (which hold the

segment base) can be used as additional base registers in some linear address calcu-

lations.

3.7.5 Specifying an Offset

The offset part of a memory address can be specified directly as a static value (called

a displacement) or through an address computation made up of one or more of the

following components:

• Displacement — An 8-, 16-, or 32-bit value.

• Base — The value in a general-purpose register.

• Index — The value in a general-purpose register.

• Scale factor — A value of 2, 4, or 8 that is multiplied by the index value.

The offset which results from adding these components is called an effective

address. Each of these components can have either a positive or negative (2s

complement) value, with the exception of the scaling factor. Figure 3-11 shows all