What is System Call?

System call provides the services of operating system to the user programs via Application Program Interface (API). It provides an interface between a process and operating system to allow user level processes to request services of operating system.

WHAT IS PROCESS?

A process is an activity of executing a program. It is a program under execution. Every process needs certain resources to complete its task.

Process Control Block (PCB)

A process control block is a data structure maintained by the operating system for every process. The PCB is identified by an integer process ID (PID). A PCB keeps all the information needed to keep track of a process as listed below:

PROCESS STATE MODELS

Process state defines the current activity of the process. Process states are: new, running, ready, blocked, suspended (suspended ready, suspended blocked), etc.

SCHEDULING

Scheduling depends on three scheduler in the system

1. Long-term scheduler
2. Medium-term scheduler
3. Short-term scheduler

Long-term Scheduler

• It determines which process are to be admitted to the system for processing. It selects processes from the queue (from disk) and leads them into memory for execution.

Medium-term Scheduler

• The mid term scheduler involves in the decision to swap processes from main memory to secondary memory and vice-versa.

Short-term Scheduler

• It selects a process from the ready queue and allocates CPU to one of then, i.e., process state changes from ready to run.

Dispatcher

The dispatcher is the module that gives a process control over the CPU after it has been selected by the short term scheduler.

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