The Process Model

Figure 9: A UNIX Process Context

The operating system must know specific information about processes in order to manage and control them. Such information is usually grouped into two categories:

• process state information
  • CPU registers (general purpose and special purpose); used by the process will include:
    • memory access registers such as a stack pointer and a heap pointer, a stack frame pointer (points at a data block on the stack holding data exchanged between caller and callee functions),
    • a processor status register, possibly a register to hold return addresses,
  • program counter; this is a pointer to the program memory (text) location where the next instruction for this process resides
• process control information
  • scheduling priority, this describes the rules enforced when determining access to a processor by this process, and can include the identity of the ``process ready to run'' queue that this process is placed in when it is ready to take CPU time
  • resource use information, this information records the use of CPU time, elapsed time, process identity number, user or account identity number, etc.
  • I/O status information, this can include a list of I/O devices used by the process, a list of open files and any buffers associated with them
  • memory allocated, this can describe the region of memory in use (a base address and a size), the page tables (a description of which pieces of memory are ``mapped'' into the single region used by the process)
• This collection of process information is represented in the operating system by a data structure element called process control block (PCB) or a task control block. Consists of:
  • An executable program (code), which is usually referred to as the text section
  • Associated data needed by the program (global data, stack)
    • the global data variables and constants, which are usually referred to as the data section
    • the dynamic storage memory used to hold temporary variables and pass function call arguments and results, usually referred to as the stack
    • the dynamic storage memory used by C++ new/delete operators and C calls to malloc()/free(), usually referred to as the heap
  • Execution context (or state) of the program;
    • contents of data registers,
    • program counter,
    • stack pointer state (waiting on an event?),
    • memory allocation,
    • status of open files,

Figure 10: Process Control Block (PCB), Processes from main memory to registers