- 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.