Thuc notes © 2021

Processor Modes

Part of series osdev |

Processor modes

  • Create an environment to manage, restrict and scope system memory that processors and tasks see and use.
  • Have three famous modes: real mode, protected mode, virtual real mode.

Processor real mode

  • Support unlimited direct software access to address memory, I/O address and peripheral hardware.
  • No support memory protection, multi-tasking or code privilege levels.
  • Limit 1 MB of RAM.
  • Addresses in real mode correspond to real locations in memory.
  • In real mode, a process can jump to other process memory to modify instruction and data.

Processor protected mode

  • Solve security issues of real mode. Processes can’t access other memory spaces.
  • Support virtual memory, paging and multi-tasking.
  • Program run in protected mode only see and use virtual memory. The virtual memory is assigned to programs and maps to real locations in memory.
  • Addresses in protected mode is virtual address locations, not real locations in memory.

Virtual real mode

  • Processor runs in the protected mode that’s emulated 16bit real mode machine.