Notes on Processes, Context, Context Switching

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					Notes on Processes, Context,
     Context Switching

 The following slides contain partially
   quoted statements from various
           Wikipedia pages.
                               fork()
• In computing, when a process forks, it creates a copy of itself, which
  is called a "child process." The original process is then called the
  "parent process". More generally, a fork in a multithreading
  environment means that a thread of execution is duplicated,
  creating a child thread from the parent thread.
• Under Unix and Unix-like operating systems, the parent and the
  child processes can tell each other apart by examining the return
  value of the fork() system call. In the child process, the return value
  of fork() is 0, whereas the return value in the parent process is the
  PID of the newly-created child process.
• The fork operation creates a separate address space for the child.
  The child process has an exact copy of all the memory segments of
  the parent process, though if copy-on-write semantics are
  implemented actual physical memory may not be assigned (i.e.,
  both processes may share the same physical memory segments for
  a while). Both the parent and child processes possess the same
  code segments, but execute independently of each other.
         CreateProcess Function

• Creates a new process and its primary thread.
  The new process runs in the security context
  of the calling process.
Syntax
BOOL WINAPI CreateProcess(
  __in_opt LPCTSTR lpApplicationName,
  __inout_opt LPTSTR lpCommandLine,
  __in_opt LPSECURITY_ATTRIBUTES
                   lpProcessAttributes,
  __in_opt LPSECURITY_ATTRIBUTES
           lpThreadAttributes,
  __in BOOL bInheritHandles,
  __in DWORD dwCreationFlags,
  __in_opt LPVOID lpEnvironment,
  __in_opt LPCTSTR lpCurrentDirectory,
  __in LPSTARTUPINFO lpStartupInfo,
  __out LPPROCESS_INFORMATION
           lpProcessInformation );
                             Context
• In computer science, a task context (process, thread ...) is the
  minimal set of data used by this task that must be saved to allow a
  task interruption at a given date, and a continuation of this task at
  the point it has been interrupted and at an arbitrary future date.
• The concept of context assumes significance in the case of
  interruptible tasks, wherein upon being interrupted the processor
  saves the context and proceeds to serve the Interrupt service
  routine. Thus the smaller the context the smaller is the latency.
• These data are located in:
    – Processor registers
    – Memory used by the task
    – On some Operating systems, control registers used by the system to
      manage the task
• The storage memory (files) is not concerned by the "task context" in
  the case of a context switch; even if this can be stored for some
  uses (Checkpointing).
                Context Switch
• A context switch is the computing process of storing
  and restoring the state (context) of a CPU such that
  multiple processes can share a single CPU resource.
• The context switch is an essential feature of a
  multitasking operating system.
• Context switches are usually computationally intensive
  and much of the design of operating systems is to
  optimize the use of context switches.
• A context switch can mean a register context switch, a
  task context switch, a thread context switch, or a
  process context switch. What constitutes the context is
  determined by the processor and the operating system.
                     State
• In computer science and automata theory, a
  state is a unique configuration of information
  in a program or machine.
• One of the key concepts in computer
  programming is the idea of state, essentially a
  snapshot of the measure of various conditions
  in the system.
                       Interrupts
• In computing, an interrupt is an asynchronous signal from
  hardware indicating the need for attention or a
  synchronous event in software indicating the need for a
  change in execution.
• A hardware interrupt causes the processor to save its state
  of execution via a context switch, and begin execution of an
  interrupt handler.
• Software interrupts are usually implemented as instructions
  in the instruction set, which cause a context switch to an
  interrupt handler similar to a hardware interrupt.
• Interrupts are a commonly used technique for computer
  multitasking, especially in real-time computing. Such a
  system is said to be interrupt-driven. [1]
• An act of interrupting is referred to as an interrupt request
  ("IRQ").

				
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posted:3/14/2012
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