1. Module 2.2 COP4600 – Operating Systems Richard Newman
Process Basics
Module 2.2 COP4600 – Operating Systems Richard Newman

2. Process States 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.

3. 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.

4. Process Implementation
Resources associated with a process
Process Management
Process table entry – state and attributes
Register values
Memory Management
Text (instructions), Data (Constants, Globals, Heap), Stack
File Management
Resource Segment – open files, pipes, devices, etc.
Process Control Block (PCB) – all the control info about process

5. Implementation of Processes (1)
Figure 2-4. Some of the fields of a typical process table entry.
Tanenbaum & Bos,Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.

6. Implementation of Processes (1.5)
Figure 2-4. Some of the fields of the MINIX 3 process table. The fields are distributed over the kernel, the process manager, and the file system.
Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall, Inc. All rights reserved

7. Basic memory layout of a process
Figure (a) A program as stored in a disk file. (b) Internal memory layout for a single process. In both parts of the figure the lowest disk or memory address is at the bottom and the highest address is at the top.
Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall, Inc. All rights reserved

8. Basic memory layout of a process
As procedures are called and return, stack grows and shrinks
Proc-1 called, activation frame pushed
Proc-2 called, activation frame pushed
Proc-2 returns, activation frame popped
Proc-3 called, activation frame pushed
Proc-3 returns, activation frame popped
Proc-1 returns, activation frame popped
Stack
AF-1
Free
AF-2
AF-3
Data
Text

9. Context switches Process may give up CPU
Block on I/O or sleep
Interrupt handling
Processor sharing – multi-programming
Save state of current process, set up state for process that is to be run next
Special purpose registers
General purpose registers
Memory locations – text, data, stack, etc.
Update process state – may no longer be running
If handling an interrupt, process may not change state per system

10. Non-sequential events
Nonsequential events happen
Interrupts
Asynchronous event – clock/device signals CPU in hardware
CPU controller checks for interrupts each FDE cycle (per mask)
Signals
Software interrupt – sent by another process
Exceptions
Software – explicitly raise exception when problem occurs
Hardware – trap when problem occurs (overflow, DBZ, etc.)

11. Summary Process State (in big sense) Process memory layout
Process state (in scheduling sense)
Memory – registers, memory segments
Resources – open files, devices
Process memory layout
Context switches
Asynchronous events