1.4 Process  (Page 4/9)

Included information

Implementations differ, but in general a PCB will include, directly or indirectly:

• The identifier of the process (a process identifier, or PID)
• Register values for the process including, notably,

the Program Counter value for the process

• The address space for the process
• Priority
• A list of open files&sockets
• Process accounting information, such as when the process was last run, how much CPU time it has accumulated, etc.
• Pointer to the next PCB i.e. pointer to the PCB of the next process to run

During a context switch, the running process is stopped and another process is given a chance to run. The kernel must stop the execution of the running process, copy out the values in hardware registers to its PCB, and update the hardware registers with the values from the PCB of the new process.

Location of the pcb

Since the PCB contains the critical information for the process, it must be kept in an area of memory protected from normal user access. In some operating systems the PCB is placed in the beginning of the kernel stack of the process since that is a convenient protected location.

Process states

Processes go through various process states which determine how the process is handled by the operating system kernel. The specific implementations of these states vary in different operating systems, and the names of these states are not standard, but the general high-level functionality is the same.

When a process is created, it needs to wait for the process scheduler to set its status to "waiting" and load it into main memory from secondary storage device (such as a hard disk or a CD-ROM). Once the process has been assigned to a processor by a short-term scheduler, a context switch is performed (loading the process into the processor) and the process state is set to "running" - where the processor executes its instructions. If a process needs to wait for a resource (such as waiting for user input, or waiting for a file to become available), it is moved into the "blocked" state until it no longer needs to wait - then it is moved back into the "waiting" state. Once the process finishes execution, or is terminated by the operating system, it is moved to the "terminated" state where it waits to be removed from main memory

Process states

Context switch

A context switch is the computing process of storing and restoring the state 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.

How is switching code invoked?

Preemptive: user thread executing ® clock interrupt ® PC modified by hardware to “vector” to interrupt handler ® user thread state is saved for restart ® clock interrupt handler is invoked ® disable interrupt checking ® check whether current thread has run “long enough” ® if yes, post asynchronous software trap (AST) ® enable interrupt checking ® exit interrupt handler ® enter “return-to-user” code ® check whether AST was posted ® if not, restore user thread state and return to executing user thread; if AST was posted, call context switch code