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Processes

Processes

A running program

  • Address space: the memory that the program can address. Stores instructions and data.
  • Registers: many programs explicitly read and update registers.
    • Program counter (PC): which instruction to execute next?
    • Stack pointer
    • Frame pointer
  • I/O information
    • A list of files that the program currently has open

Mechanism vs. Policy

Mechanisms are low-level methods or protocols that implement a needed piece of functionality.

  • Time-sharing mechanism: a CPU is used by one process for a given amount of time, then to another process, and so forth.
  • Space-sharing mechanism: a resource (e.g. disk space) is divided and shared by multiple programs.

Policies are algorithms for making decisions in the OS.

  • Scheduling policy: a CPU decides which program to run among a list of programs based on some sort of intelligence.x

Process Creation

  1. Create entry for process list
  2. Load code and static data into memory from disk or flash-based SSD
  3. Allocate memory for stack
  4. Allocate memory for heap
  5. Initialize file I/O
  6. Start the process by running main()

Process States

  • Running
  • Ready to run
  • Blocked

Different actions could transition from one state to another. E.g. blocked --> file I/O --> Ready.

Data Structures

  • Process list: a data structure that stores information about all processes in the OS
  • Process control block (PCB): a data structure that stores info about individual processes

The Process API in UNIX

fork()

  • Create a copy of the exact process
  • Everything is the same except for the PID returned by the call: parent get the PID of the child, child get 0
  • Indeterministic: parent and child run simultaneously on the CPU, so we don't know which will be executed first. This is determined by the CPU scheduler.

wait()

Wait for the child process to run and exit before continuing the parent process.

exec()

  • Overwrites the current code segment with the code and static data from the new program
  • Re-initialize stack and heap memory
  • Never returns if successful

Process Control in UNIX

Users/superusers can control processes by sending signals to cause jobs to stop, continue, or terminate. Each user can only control its own recourses, while a superuser (root) can control all processes.

Limited Direct Execution (LDE) Protocol

User mode vs. Kernel mode

  • The OS runs in the kernel mode, which can execute all privileged operations and restricted instructions.
  • Code that runs in user mode is restricted in what operations it can execute. To perform restricted instructions, it performs a system call.

System Call

A program calls a special trap instruction to perform a system call.

  1. CPU: save registers and PCs to kernel stack (a stack to store program state so that later we could pick up where we've left)
  2. CPU: move to kernel mode
  3. CPU: jump to trap handler (a place, whose address is known by CPU but not the user, that tells the OS which code to run. Initialized during booting)
  4. OS: handle trap, run code
  5. OS: execute return-from-trap
  6. CPU: restore registers and PCs from kernel stack
  7. CPU: move to user mode
  8. CPU: jump to PC after trap
  9. Program: resume execution