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

Vol. 1 3-19

BASIC EXECUTION ENVIRONMENT

Each of the segment registers is associated with one of three types of storage: code,

data, or stack. For example, the CS register contains the segment selector for the

code segment, where the instructions being executed are stored. The processor

fetches instructions from the code segment, using a logical address that consists of

the segment selector in the CS register and the contents of the EIP register. The EIP

executed. The CS register cannot be loaded explicitly by an application program.

Instead, it is loaded implicitly by instructions or internal processor operations that

change program control (such as, procedure calls, interrupt handling, or task

switching).

The DS, ES, FS, and GS registers point to four data segments. The availability of

four data segments permits efficient and secure access to different types of data

structures. For example, four separate data segments might be created: one for the

data structures of the current module, another for the data exported from a higher-

level module, a third for a dynamically created data structure, and a fourth for data

shared with another program. To access additional data segments, the application

program must load segment selectors for these segments into the DS, ES, FS, and

GS registers, as needed.

The SS register contains the segment selector for the stack segment, where the

procedure stack is stored for the program, task, or handler currently being executed.

All stack operations use the SS register to find the stack segment. Unlike the CS

to set up multiple stacks and switch among them.

Figure 3-7. Use of Segment Registers in Segmented Memory Model

Segment Registers

Code

Segment

Data

Segment

Stack

Segment

Data

Segment

Data

Segment

Data

Segment

All segments

are mapped

to the same

linear-address

space