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.
x
1
// HW and OS work together to invoke this function2
// whenver a keyboard key is pressed3
// Regardless of where you are at your_program()4
// this function can be invoked ascynhronously 5
void interrupt_keyboard(void) {6
}7
8
// Thread that never exits9
// This calls all the sub-functions synchronously10
void your_program(void) {11
while (forever) {12
check_for_brake_pedal();13
actuate_brakes();14
}15
}Kernel vs. User Space
19
1
// OS use "SWI" or "Software Interrupt"2
// "Software Interrupt" really means a "Deliberate Interrupt request to the HW"3
void deliberate_interrupt(void) {4
}5
6
// 7
void your_program(void) {8
while (forever) {9
int file = open("file.txt");10
}11
}12
13
// Psuedo-code for open14
// This is what the open function looks like inside the OS15
void open(filename) {16
R4 = open_request_number;17
R5 = filename18
SWI19
}Virtual Memory
Virtual memory is not purely virtual memory, it is virtual memory addresses that maps to real and physical hardware memory.