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Interrupts

An interrupt indicates that an asynchronous event has occurred. CPU interrupts are a form of hardware interrupt that cause the CPU to stop processing its current task, save the state, and begin processing a new request. When the new task is complete, the CPU will complete the prior task.

Processes and Threads

A process is an executable program and its associated data loaded and running in memory. A “heavy weight process” (HWP) is also called a task. A parent process may spawn additional child processes called threads. A thread is a lightweight process (LWP). Threads can share memory, resulting in lower overhead compared to heavy weight processes.

Processes may exist in multiple states:

  • New: a process being created
  • Ready: process waiting to be executed by the CPU
  • Running: process being executed by the CPU
  • Blocked: waiting for I/O
  • Terminate: a completed process

Another process type is “zombie,” a child process whose parent is terminated.

Multitasking and Multiprocessing

Applications run as processes in memory, comprised of executable code and data. Multitasking allows multiple tasks (heavy weight processes) to run simultaneously on one CPU. Older and simpler operating systems, such as MS-DOS, are non-multitasking: they run one process at a time. Most modern operating systems, such as Linux, Windows 10, and OS X support multitasking.

Note:
Some sources refer to other terms related to multitasking, including multiprogramming and multithreading. Multiprogramming is multiple programs running simultaneously on one CPU; multitasking is multiple tasks (processes) running simultaneously on one CPU, and multithreading is multiple threads (lightweight processes) running simultaneously on one CPU.

Multiprogramming is an older form of multitasking; many sources use the two terms synonymously. This book will use the term “multitasking” to refer to multiple simultaneous processes on one CPU.

Multiprocessing has a fundamental difference from multitasking: it runs multiple processes on multiple CPUs. Two types of multiprocessing are Symmetric Multiprocessing (SMP) and Asymmetric Multiprocessing (AMP) (some sources use ASMP). SMP systems have one operating system to manage all CPUs. AMP systems have one operating system image per CPU, essentially acting as independent systems.

Watchdog Timers

A watchdog timer is designed to recover a system by rebooting after critical processes hang or crash. The watchdog timer reboots the system when it reaches zero; critical operating system processes continually reset the timer, so it never reaches zero as long as they are running. If a critical process hangs or crashes, they no longer reset the watchdog timer, which reaches zero, and the system reboots.