Virtualizing the CPU

• Time sharing: Running one process, then stopping it and running another
• What is a process?: Running program
  • Memory (address space): Instructions and data
  • Registers: Program counter, stack pointer
  • I/O info: list of files open

Process API

Process creation in words

• Load a program code into memory, into the address space of the process
  • Programs initially reside on disk in executable format
• Eager load: load the entire program all at once before running
• Lazy load: load pieces of code or data only as they are needed during program execution

some more words

OS has to do the following as part of process creation:

Allocate memory for the program’s run-time stack (local variables, function parameters, and return address)

• Initialize the stack with argc and argv array

Allocate memory for the program’s heap

• Used for dynamically allocated data: See malloc() and free()

Setup I/O

• Each process by default has three open file descriptors: Standard input, output and error

Start the program running at main()

• OS transfers control of the CPU

Process creation in 1000 words worth

Process creation by loading a program

A picture showing process states

Process states and transitions

If you didn’t get the picture, read this

• Running: a process is running on a processor. This means it is executing instructions.

• Ready: a process is ready to run but for some reason the OS has chosen not to run it

• Blocked: a process has performed some kind of operation (e.g. I/O) that makes it not ready to run until some other event takes place.

Data structures

• Process list
  • Ready processes
  • Blocked processes
  • Current running process
• Register context

PCB (Process Control Block)

• A C-structure that contains information about each process