Processes- The Process Model
Conceptually, every process has its own virtual CPU but in reality CPU switches back and forth from process to process, but to understand the system, it is much easier to think about a collection of processes running in (pseudo) parallel then to try to keep track of how the CPU switch is from program to program. This Rapid switching back and forth is called "Multi-Programming."
Mainly three Types of Process MOdels are here
In this there is 4 program in memory. We have single shared processor by all programs. There is only one program counter all programs in memory. In this program counter is initialized to execute the part of program A then with the help of process switch it transfer control to program B and execute some part of this. After that program counter for program C is active and execute some part of it and so onto program D. Then all the steps continuous may be randomly with the help of process switches until all program is not completed.
There is 4 processes, each with it own flow of control(that is it on logical program counter), and each when running independently of the other ones. There is only one physical program counter. So when each process runs, its logical program counter is loaded into the real program counter. When it is finished(for the time being), the physical program counter is saved in the process stored logical program counter in memory.
3) One Program At One Time
At any given instant only one process run and other process after some interval of time. In other point of view all processes progress but only process actually runs at given instant of time.