Learning Objectives

At the end of this lecture, you should be able to:

• Define a computer process and distinguish it from a static program.
• Identify the content of a process’s memory address space.
• Explain how the operating system keeps track of running and sleeping processes.

Topics

In this lecture, we will cover the following topics:

• Processes and programs.
• Memory address spaces.
• The process control block.

Notes

• Definition of a Process
• Multiple Processes in the System (Context Switching)
• The Process Control Block
• Threads and Multi-Threading

Definition of a Process

• A process is nothing but a program in execution
• And what is a program?
  • A program is a bunch of machine instructions.
• When you run a program, like doing ./glue as we did in the previous case, what happens in that case?
• Well the shell (or your GUI server) is going to call on the operating system and ask it to fork a new process, which is then going to execute the program glue
• Each process will have a memory address region allocated for it, referred to as the process address space.
• In each process address space, we have sections of memory that contain different things:
  • The code region that contains the program’s code as machine instructions
  • The global region that contains global variables
  • The heap which is a region for dynamic memory allocation
  • The stack which,
    • Grows downward
    • Hosts local variable and statically allocated variables

Multiple Processes in the System (Context Switching)

• The operating system is in charge of asking the CPU to execute multiple different processes, including itself
• so it must have a way to switch between different processes
• how do you know where the process is executing from?
  • The program counter register
• So the OS will save the process’s program counter, its registers along with all its information before switching it out and then load another one.

The Process Control Block

• The PCB contains all of the information necessary to know about a process, its ID, its state, its memory regions, page tables, children, etc.
• Show the task_struct in Linux

Threads and Multi-Threading

• Threads are lightweight processes, they share the data, code, and open files
• But they have each their own registers and stack
• Switching between processes is easier than switching between threads
• In Linux, everything is a task!