Privilege Level

Abstract

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• Determines what Instruction may be executed by CPU

3 Modes

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Machine Mode

• Free to do anything on the computer

Kernel Mode

• CPU is allowed to execute any supported Instruction and access any Main Memory
• Kernel & Device Driver run in this 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

Example

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x86-64

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

RISCV

• 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)