3-9
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 register contains the linear address within the code segment
of the next instruction to be executed. The CS register cannot be loaded explicitly by an appli-
cation 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 a 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 register, the SS register can be loaded
explicitly, which permits application programs to set up multiple stacks and switch among them.
Refer to Section 3.3., “Memory Organization” for an overview of how the segment registers are
used in real-address mode.
Figure 3-6. Use of Segment Registers in Segmented Memory Model
Segment Registers
CS
DS
SS
ES
FS
GS
Code
Segment
Data
Segment
Stack
Segment
Data
Segment
Data
Segment
Data
Segment
All segments
are mapped
to the same
linear-address
space
