Protected Mode Setup: How To Run a Pentium

• Puts segmentation, paging, TLB h/w in a sane state, turn on segmentation, not paging - 17 instructions:

  • 1st instruction, executed in 16-bit mode: jump to BIOS startup ג€“ a standard part of x86 reset
  • 5 instructions: disable interrupts, clear TLB, set code and data segments to known values
  • 1 instruction: load a pointer to GDT (to manage addressing in segmented mode)
  • 4 instructions: turn on memory protection
  • 6 instructions: do remaining segment register setup for protected mode

• At this point we

  • are running in protected mode
  • can address 4GB of memory
  • are running a Pentium, not a 8086