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
Interrupt
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) {
check_for_brake_pedal();
actuate_brakes();
}
}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
SWI
}
Virtual Memory
Virtual memory is not purely virtual memory, it is virtual memory addresses that maps to real and physical hardware memory.