OPERATING SYSTEM NOTES

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Processes

• A process is a program that is currently running.

• It includes the program code, data, and execution status.

• A program is just a set of instructions, while a process is the program in action.

Files

• A file is a collection of data stored on a computer with a specific name (filename).

• Almost everything stored on a computer is in a file.

• Types of files include:

  • Text files – store written content

  • Program files – store software code

  • Data files, directory files, etc. – store other kinds of data

System Calls

• A system call is how a program asks the operating system to do something.

• Examples of what system calls do:

  • Run a process

  • Use hardware (like a printer or hard drive)

  • Communicate with the OS kernel

Shell and Kernel

• Shell:

  • A user interface (usually a command line) to interact with the OS.

  • Examples: command.com, sh, bash, csh.

• Kernel:

  • The core of the OS.

  • Manages the system's memory, processes, files, and devices.

  • Loads first when the OS starts and stays protected in memory.

Virtual Machines (VMs)

• A virtual machine is software that acts like a real computer.

• It can run programs and operating systems as if it were a separate device.

• Multiple VMs can run on a single physical machine (host).

• Each VM is called a guest.