1. Module 2.1 COP4600 – Operating Systems Richard Newman
Process Basics
Module 2.1 COP4600 – Operating Systems Richard Newman

2. Process definition
Algorithm = finite set of unambiguous instructions with finite initial storage and state, a specific starting instruction, and unambiguously determined next instruction and state at any point, except if the last instruction was halt.
Often expressed with flow chart or pseudocode
Program = sequence of instructions that embody an algorithm
Source code → assembly code → machine code
Process = program in execution = program + state
May have many processes executing same program
Process state = current instruction and state of memory, resources

3. pseudoparallelism Each process is given CPU in turn
PC and other registers hold state of the process that is running
System switches from one program to another by suspending current program, swapping state of registers, then starting new program
Figure 2-1. (a) Multiprogramming of four programs.
Tanenbaum & Bos,Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.

4. The Process Model - sequential
Conceptually, each program has its own program counter
Conceptually, each program runs independently of the others
It is up to the system to see that the programs do not interfere with each other
Figure 2-1. (b) Conceptual model of
four independent, sequential processes.
Tanenbaum & Bos,Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.

5. The Process Model – processor sharingB A
Process using CPU
Time
Figure 2-1. (c) Only one program is active at once.
Tanenbaum & Bos,Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.

6. Process Creation
Four principal events that cause processes to be created:
System initialization.
Execution of a process creation system call by a running process.
A user request to create a new process.
Initiation of a batch job.
Tanenbaum & Bos,Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.

7. Process Termination Typical conditions which terminate a process:
Normal exit (voluntary).
Error exit (voluntary).
Fatal error (involuntary).
Killed by another process (involuntary).
Tanenbaum & Bos,Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.

8. Process States (1) States a process may be in:
Running (actually using the CPU at that instant).
Ready (runnable; temporarily stopped to let another process run).
Blocked (unable to run until some external event happens).
Initializing (system preparing process to run).
Exiting (system cleaning up after process terminates).
Swapped out & Blocked (image not in RAM, waiting for event)
Swapped out & Ready (image not in RAM, not waiting for event)
Tanenbaum & Bos, Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.

9. Process States (2) Exiting Ready Running Blocked Initializing Swapped
Select & dispatch
Initialized
Ready
Running
Halt
Swap in
Preempt
Swapped
Out-R
Swap
out
Unblock
Blocked
Block
Swap in
Swapped
Out-B
Unblock
Swap out

10. Process States (3)
Figure 2-3. The lowest layer of a process-structured operating system handles interrupts and scheduling. Above that layer are sequential processes.
Tanenbaum & Bos,Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.

11. Summary What is a process Processor sharing Process Life Cycle
Not the same as a program!
Processor sharing
Pseudoparallelism
Process Life Cycle
Process states and state transitions