Processes

Memory Allocation: Running programs need memory for code, global variables (.bss), the stack, and the heap.
Virtual address: Address used directly by software
Processses are unprivileged

Privilege

Supervisor mode: Access to privileged instructions
User mode: No privileged instructions
- Attempting to execute a privileged instruction fromm user mode results in a trap (type of interupt)
Mode indicated by bits in status register
Mode changes on interrupt handling, system calls, or other traps

A mechanism represented by layers used to enforce different levels of access and permissions for software components running on a system.
Represented in numerical values. The lower the number, the higher the privilege.
Number of rings is dependent on architecture.
Most common implementation has four rings:
- Ring 0: Kernel Mode
  - Highest privilege level.
  - Unrestricted access to system resources.
  - Can execute privileged instructions.
  - Critical system functions such as memory management, process scheduling, device I/O handling.
- Rings 1 & 2: Reserved
  - Intermediate privilege levels.
  - Could be unused or reserved for future expansion
  - Most commonly used for device drivers, not so common in modern systems.
- Ring 3: User Mode
  - Lowest privilege level.
  - User-space applications run in this ring
  - Process’ have limited access to system resources and cannot directly execute privileged instructions.
  - Process’ here rely on system calls to request services from kernel. Kernel performs operations on their behalf.

System calls:

How unprivileged processes perform privileged operations
Privileged system (kernel) code called by unprivileged (user) process
Note called like ordinary functions:
1. Set arguments (including syscall #) in registers or memory
2. Invoke interrupt or syscall instruction