AMD AMD-K6-2/400 User Guide - Page 81
Descriptors and Gates, application segment descriptor, system segment descriptor, gates
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23542A/0-September 2000 Preliminary Information AMD-K6™-2E+ Embedded Processor Data Sheet 31 12 11 10 9 8 7 6 5 4 3 2 1 0 Physical Page Base Address A P P UW V DA CW / / P L DT SR Symbol Description Bits AVL Available to Software 11-9 Reserved 8-7 D Dirty 6 A Accessed 5 PCD Page Cache Disable 4 PWT Page Writethrough 3 U/S User/Supervisor 2 W/R Write/Read 1 P Present (valid) 0 Figure 50. Page Table Entry (PTE) 3.5 Descriptors and Gates There are various types of structures and registers in the x86 architecture that define, protect, and isolate code segments, data segments, task state segments, and gates. These structures are called descriptors. s The application segment descriptor is used to point to either a data or code segment. Figure 51 on page 60 shows the application segment descriptor format. Table 9 on page 60 contains information describing the memory segment type to which the descriptor points. s The system segment descriptor is used to point to a task state segment, a call gate, or a local descriptor table. Figure 52 on page 61 shows the system segment descriptor format. Table 10 on page 61 contains information describing the type of segment or gate to which the descriptor points. s The AMD-K6-2E+ processor uses gates to transfer control between executable segments with different privilege levels. Figure 53 on page 62 shows the format of the gate descriptor types. Table 10 on page 61 contains information describing the type of segment or gate to which the descriptor points. Chapter 3 Software Environment 59