Fundamentals of an OS

What is an OS?

A good example can be reference at section 6.5 of Structured Computer Organization.

Fundamentally, an OS provides services:

  • File services (open, read, write, close etc.)
  • Networking services (open, read, write, close internet socket)
    • Berkeley socket interface
  • Multi-tasking services
    • Creating multiple threads
    • Dictating priorities for threads


An interrupt is an asynchronous function call that can interrupt normal flow of a program.

// HW and OS work together to invoke this function
// whenver a keyboard key is pressed
// Regardless of where you are at your_program()
// this function can be invoked ascynhronously 
void interrupt_keyboard(void) {

// Thread that never exits
// This calls all the sub-functions synchronously
void your_program(void) {
  while (forever) {

Kernel vs. User Space

// OS use "SWI" or "Software Interrupt"
// "Software Interrupt" really means a "Deliberate Interrupt request to the HW"
void deliberate_interrupt(void) {

void your_program(void) {
  while (forever) {
    int file = open("file.txt");

// Psuedo-code for open
// This is what the open function looks like inside the OS
void open(filename) {
  R4 = open_request_number;
  R5 = filename

Virtual Memory

Virtual memory is not purely virtual memory, it is virtual memory addresses that maps to real and physical hardware memory.

POSIX Interface

Revision #6
Created 5 months ago by Preet Kang
Updated 5 months ago by Preet Kang