stem/CS/Processors.md

Execution Mode

Starts in Real Mode

Real Mode

• 16 bit
• 20 bit segmented memory
  • Max 1 MB memory
• Direct access to peripherals
• No memory protection or multitasking
• BIOS computers start in this mode

Protected Mode

• 16 bit & 32 bit
• Max 16 MB of physical memory
  • Max 1 GB virtual memory
• Privilege levels and protected memory

Long Mode

• 64 bit
• Largely an extension to 32 bit protected mode

System Management Mode

• 16 bit
• System-wide functions
  • Power management
  • System control hardware
  • Proprietary OEM code