A process is an instance of a program in execution. A set of processes combined together make a complete program.
There are two categories of processes in Unix, namely
- User processes: They are operated in user mode.
- Kernel processes: They are operated in kernel mode.
The states that a Process enters in working from start till end are known as Process states. These are listed below as:
- Created-Process is newly created by system call, is not ready to run
- User running-Process is running in user mode which means it is a user process.
- Kernel Running-Indicates process is a kernel process running in kernel mode.
- Zombie- Process does not exist/ is terminated.
- Preempted- When process runs from kernel to user mode, it is said to be preempted.
- Ready to run in memory- It indicated that process has reached a state where it is ready to run in memory and is waiting for kernel to schedule it.
- Ready to run, swapped– Process is ready to run but no empty main memory is present
- Sleep, swapped- Process has been swapped to secondary storage and is at a blocked state.
- Asleep in memory- Process is in memory(not swapped to secondary storage) but is in blocked state.
The working of Process is explained in following steps:
- User-running: Process is in user-running.
- Kernel-running: Process is allocated to kernel and hence, is in kernel mode.
- Ready to run in memory: Further, after processing in main memory process is rescheduled to the Kernel.i.e.The process is not executing but is ready to run as soon as the kernel schedules it.
- Asleep in memory: Process is sleeping but resides in main memory. It is waiting for the task to begin.
- Ready to run, swapped: Process is ready to run and be swapped by the processor into main memory, thereby allowing kernel to schedule it for execution.
- Sleep, Swapped: Process is in sleep state in secondary memory, making space for execution of other processes in main memory. It may resume once the task is fulfilled.
- Pre-empted: Kernel preempts an on-going process for allocation of another process, while the first process is moving from kernel to user mode.
- Created: Process is newly created but not running. This is the start state for all processes.
- Zombie: Process has been executed thoroughly and exit call has been enabled.
The process, thereby, no longer exists. But, it stores a statistical record for the process.
This is the final state of all processes.