NotesAtHKU

Process

A process is an instance of a program running on a computer.

OS needs to keep track of information to represent a process:

Memory Program code, data, etc.
Resources in use I/O in use, physical memory, etc.
Execution state Register set, current process state, etc.

Address space

The process's view of it's memory, a range of memory locations:

\Vert\underset{\uparrow}{0}\Vert\text{[Text segment]} \quad \text{[Data segment]} \quad \text{[Heap]}\Rightarrow \quad \Leftarrow\text{[Stack]}\Vert\underset{\uparrow}{max}\Vert

Text segment Stores program code
Data segment Stores global / static vars
Heap Stores dynamically allocated memory
Stack Stores local vars and function call info

Process states

In the execution life cycle, the process moves through a series of states:

Process control

Process control block (PCB)

The PCB (or Process descriptor) is a data structure used by the OS to represent a process. It contains:

Process ID (PID)
Process state
Program counter (address of next instruction)
Register context (snapshot of register contents when process was last in running state)
Scheduling information (e.g. process priority, pointers etc.)
etc.

Process Table & Process List

Table where OS keeps pointers to each process's PCB
When a process is completely terminated, OS removes process from process table and frees up all used resources
OS maintains a Ready list and a Blocked list that stores references to the PCBs of processes that are not running

Process lifecycle

Create process

How to create a new process:

A process spawns a new process
Forming a tree of processes with parent and child relationship

Actions taken:

Assign unique PID
Allocate memory for process
Initialize PCB
Add process to Ready list

System call: fork() creates a new process by duplicating the calling process. The child process gets a unique PID and a copy of the parent's address space.

exec() replaces the current process's address space with a new program. It is often used after fork() to run a different program in the child process.

Terminate process

Self termination: exit() or return from main
Process may terminate in response to a signal, error or fault
Parent can obtain information from terminated child via wait(), waitid() or waitpid()
- Zombie process: Any terminated process in this state is a zombie process
- Child resources & PCB is kept until only after parent has collected information about the child
Any child processes will normally be kept alive and not terminated

Suspend process

Temporarily deactivate the process so it is not being considered for processor scheduling.
Suspension only happes externally:
- Request by parent process
- OS can suspend a blocked process to free up memory for another ready process
Must be resumed externally

Process abstraction