In general, I/O operations does not make use of the CPU but are handled by external devices. Compared to the CPU, I/O operations are very slow and while waiting for I/O the CPU is idle doing nothing. To overcome this problem multiprogramming was invented.
In systems using multiprogramming a program loaded to memory and ready to execute is called a job.
The simple idea is to always have one job execute on the CPU by changing job when an I/O request is made.
In a multiprogramming system, a job can be in one of three states.
In a multiprogramming system, a the following state transitions are possible.
|Running||Waiting||When a running job requests I/O, the job changes state from running to waiting.|
|Running||Ready||When an I/O requests completes, the running job changes state from running to ready.|
|Waiting||Ready||When an I/O requests completes, the job waiting for the request to complete changes state from waiting to ready.|
|Ready||Running||When an I/O requests completes, one of the ready jobs are selected to run on the CPU and changes state from ready to running.|
The problem is, how will the system know when an I/O request is completed?
To implement mulitprogramming interrupts are used to notify the system of important events such as the completion of an I/O request.
In a multiprogramming system, several jobs are kept in memory at the same time. Initially, all jobs are int the ready state.
One of the ready jobs is selected to execute on the CPU and changes state from ready to running. In this example, job 1 is selected to execute.
Eventually, the running job makes a request for I/O and the state changes from running to waiting.
Instead of idle waiting for the I/O request to complete, one of the ready jobs is selected to execute on the CPU and have its state change from ready to running. In this example job 3 is selected to execute.
Eventually the the I/O request job 1 is waiting for will complete and the CPU will be notified by an interrupt. In this example, job 1 was waiting for a keypress on the keyboard.
The state of the waiting job (job 1) will change from waiting to ready.