Sistemas Operativos

In computing, a process is an instance of a computer program that is being executed. It contains the program code and its current activity. Depending on the operating system (OS), a process may be made up of multiple threads of execution that execute instructions concurrently.[1][2]
A computer program is a passive collection of instructions; a process is the actual execution of thoseinstructions. Several processes may be associated with the same program; for example, opening up several instances of the same program often means more than one process is being executed.
Multitasking is a method to allow multiple processes to share processors (CPUs) and other system resources. Each CPU executes a single task at a time. However, multitasking allows each processor to switch between tasksthat are being executed without having to wait for each task to finish. Depending on the operating system implementation, switches could be performed when tasks perform input/output operations, when a task indicates that it can be switched, or on hardware interrupts.
A common form of multitasking is time-sharing. Time-sharing is a method to allow fast response for interactive user applications. Intime-sharing systems, context switches are performed rapidly. This makes it seem like multiple processes are being executed simultaneously on the same processor. The execution of multiple processes seemingly simultaneously is called concurrency.
For security and reliability reasons most modern operating systems prevent direct communication between independent processes, providing strictly mediatedand controlled inter-process communication functionality.
Process states
An operating system kernel that allows multi-tasking needs processes to have certain states. Names for these states are not standardised, but they have similar functionality.[1]
* First, the process is "created" - it is loaded from a secondary storage device (hard disk or CD-ROM...) into main memory. After that theprocess scheduler assigns it the state "waiting".
* While the process is "waiting" it waits for the scheduler to do a so-called context switch and load the process into the processor. The process state then becomes "running", and the processor executes the process instructions.
* If a process needs to wait for a resource (wait for user input or file to open ...), it is assigned the "blocked"state. The process state is changed back to "waiting" when the process no longer needs to wait.
* Once the process finishes execution, or is terminated by the operating system, it is no longer needed. The process is removed instantly or is moved to the "terminated" state. When removed, it just waits to be removed from main memory.[1][4]
Process Control Block (PCB, also called Task ControllingBlock, Task Struct, or Switchframe) is a data structure in the operating system kernel containing the information needed to manage a particular process. The PCB is "the manifestation of a process in an operating system".[1]
Implementations differ, but in general a PCB will include, directly or indirectly:
* The identifier of the process (a process identifier, or PID)
* Register valuesfor the process including, notably, the program counter and stack pointer values for the process.
* The address space for the process
* Priority (in which higher priority process gets first preference. e.g., nice value on Unix operating systems)
* Process accounting information, such as when the process was last run, how much CPU time it has accumulated, etc.
* Pointer to the nextPCB i.e. pointer to the PCB of the next process to run
* I/O Information (i.e. I/O devices allocated to this process, list of opened files, etc.)
During 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...