AMD-K6 Datasheet, PDF (42/346 Pages) Advanced Micro Devices – AMD-K6 Processor

AMD-K6^® Processor Data Sheet
Preliminary Information
20695H/0—March 1998
Segment Registers
The six 16-bit segment registers are used as pointers to areas
(segments) of memory. Table 4 lists the segment registers and
their functions. Figure 8 shows the format for all six segment
registers.
Table 4. Segment Registers
Segment
Register
Segment Register Function
CS Code segment, where instructions are located
DS Data segment, where data is located
ES Data segment, where data is located
FS Data segment, where data is located
GS Data segment, where data is located
SS Stack segment
15
0
Segment Usage
Figure 8. Segment Register
The operating system determines the type of memory model
that is implemented. The segment register usage is determined
by the operating system’s memory model. In a Real mode
memory model the segment register points to the base address
in memory. In a Protected mode memory model the segment
register is called a selector and it selects a segment descriptor
in a descriptor table. This descriptor contains a pointer to the
base of the segment, the limit of the segment, and various
protection attributes. For more information on descriptor
formats, see “Descriptors and Gates” on page 45. Figure 9 on
page 25 shows segment usage for Real mode and Protected
mode memory models.
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Software Environment
Chapter 3