Modes of Operation

• Earlier we discussed a system call.
• System calls work because a CPU must have at least two modes of operation
  • A kernel mode
    • Also called supervisor, system or privileged mode
    • Only mode where execution of privileged instruction is permitted.
    • Examples:
      • Halt
      • Changing interrupt status
      • Interacting with peripheral devices. (read and write)
    • Some memory management instructions
  • A user mode
    • Most other instructions.
  • When the system boots, it is in kernel mode.
  • It switches to user mode when it starts processes.
  • When a system call occurs
    • The jump to the interrupt handler switches to kernel mode
    • The return from the interrupt handler switches back to user mode.
    • (from Silberschatz)
  • The intel CPU has four "rings", but articles I have looked at suggest that only the inner and outer are usually used.
• The Linux kernel operates with a timer.
  • This sends an interrupt, which generates a system call, which invokes the kernel at regular intervals.
  • This is what makes multitasking possible.
• 1.5 discusses Topics we have touched on already
  • This is a:
    1. tell 'em what you are going to tell 'em
    2. Tell 'em
    3. Tell em what you told 'em
    situation.
  • We are in the first stage.
  • Future chapters will address each of these.
  • Read it.
  • Process management
    • a program is an entry on a hard drive.
    • a process is a running program
    • a thread is a lightweight process
    • The kernel will support process:
      • Created/destroyed (including data structures and table entries)
      • Suspended and resumed
      • Communicate with each other.
      • Synchronize with each other.
  • Memory Management
    • A program thinks it owns all of memory.
    • And that it alone changes memory
    • The OS supports this idea.
    • But the OS
      • Givith and taketh away memory
      • Writes it to disk/brings it back into memory as needed.
      • Protects and keeps processes in their own memory.
      • But might let memory be shared when needed.
  • File Management
    • The OS provides the entire idea of files/directories and other "file type" things.
    • It Implements any protection/security involved with these.
    • It manages the devices on which these are stored.
    • It provides an abstraction for file manipulation
      • Creation and deletion
      • Organization
      • Alteration
  • Device management
  • I/O system management.