Segment Descriptor

  • Segment selectors point to segment descriptors
  • Segment Descriptor: An 8 byte field containing information describing a segment.
    • There are many types of segments the three most notable ones are code segment descriptors, data segment descriptors, task state segment descriptors (TSSD) and local state segment descriptors (LSSD)
      • Code Segment Descriptors: S=1 for a non system segment and refers to a code segment GDT or the LDT
      • Data Segment Descriptors: Refers to data segment.  S=1 GDT or the LDT
      • TSSD: Refers to the Task State Segment which is used to save contents of processor registers. Only in GDT. S=0. Type=11 (busy CPU/executing) or 9 (idle CPU)
      • LSSD: Refers to the segment containing a LDT. Only exists in the GDT.  Type=2, S=0

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