3 Modes

Machine Mode

  • Free to do anything on the computer

Kernel Mode

User Mode

  • Only a subset of Instruction is allowed, I/O and Main Memory access are limited, and many CPU settings are locked
  • Applications run in this mode
  • If application trying to access resources they don’t have access to like Main Memory they don’t have access to, Segmentation Fault will occur



  • The privilege level can be read by CPU from a Registers called cs


  • The privilege level of the current execution mode is stored in the mstatus, a Control & Status Registers (CSRs)
  • Specifically, the mstatus.MPP (Mode Privilege Priority) field holds the current privilege level
  • The mstatus.MPP field is a 2-bit field that can have the following values:
    • 00: User mode (U-mode)
    • 01: Supervisor mode (S-mode)
    • 10: Machine mode (M-mode)