Processes

Last updated Nov 8, 2022 Edit Source

A program in execution, also known as a job. One process can have multiple threads.

• Sizes of text and data sections are fixed
• Stack can grow and shrink dynamically: each time a function is called, an activation record is pushed to the stack and popped when the function returns
• Heap can grow and shrink dynamically: changes as memory is dynamically allocated in runtime Information related to each process is then stored in a Process Control Block (PCB)

1. New: The process is being created
2. Running: Instructions are being executed
3. Waiting: The process is waiting for I/O or event
4. Ready: The process is ready to run, but is waiting to be assigned to the CPU
5. Terminated: The process has completed

• Timer interrupt is used in multiprogramming systems to switch between ready processes
• Running -> waiting is due to some interrupt source from the currently running process
• Process always goes through the ready state before running (no direct from waiting to running)

Process scheduling

This creation process can have 2 types:

• Parent and child execute concurrently
• Parent waits for all children to terminate before continuing execution. This is done through system calls wait() aka join()

1. Exit: Process asks the OS to delete it
2. Abort: Parent terminates children processes

Can be faster than message passing as less system calls required. Only ones are to establish the shared memory regions after which all access is treated as routine memory access.

No use of shared variables. Works through 2 system calls:

1. `send(message)
2. `receive(message) Direct: processes must name each other explicitly Indirect: messages are sent and received through a mailbox or port

a. False. That is the ready state. The waiting state is for processes waiting for some I/O operation or event b. True c. False. It is used by the parent to wait for children to finish. d. False. To support message passing, extra kernel memory needs to be allocated for process mailbox

What are two main differences between the data and stack regions of a process memory?

1. Data region is used to store global variables while stack region is used to store the currently executing local functions and parameters.
2. Data region is fixed, while the stack can grow and shrink as the program executes. P0: Ready -> Running -> Waiting -> Ready P1: Running -> Ready -> Running

Context Switch A: Save state of P1 into PCB1 B: Load state of P0 from PCB0 C: Save state of P0 into PCB0 D: Reload state of P1 from PCB1