Segment Selectors and Segmentation Registers

• Logical Address is made of two parts
  • Segment Identifier: 16 bit field called a segment selector points to a segment descriptor (covered next)
  • 
    • Index: The index in the GDT or LDT which this segment selector refers to
    • RPL: Represnts either 0 or 3 (kernel or user mode)
    • TI: Whether the table is from a local descriptor table (1) or global descriptor table (0) more on this later
  • Offset: 32 bit field determining offset from the base address of a segment

Segmentation Registers: Stores segment selectors (cs, ss, ds, es, fs, gs)

• cs: Code segment register, points to a segment which stores program instructions
  • cs makes use of the RPL field marking whether the code is from a kernel/user process
• ss: Stack segment register, points to a segment which stores a program’s stack
• ds: Data segment register, points to a segment which contains global/static data
• es, fs, gs: General purpose registers for arbitrary data segments