AMD AMD-K6-2/500AFX Data Sheet - Page 44
Segment Registers, Segment Usage
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AMD-K6®-2 Processor Data Sheet Preliminary Information 21850J/0-February 2000 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 9 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 9. 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 46. Figure 10 on page 25 shows segment usage for Real mode and Protected mode memory models. 24 Software Environment Chapter 3