Unit 1: OS fundamentals and process management

An operating system is system software that acts as an interface between the user and the hardware, and as a resource manager that allocates the CPU, memory, files and devices among programs. Examples: Windows, Linux, macOS, Android.

Goals: convenience (easy to use) and efficiency (good use of resources).

An operating system is system software that acts as an interface between the user and the hardware, and as a resource manager that allocates the CPU, memory, files and devices among programs. Examples: Windows, Linux, macOS, Android.

Goals: convenience (easy to use) and efficiency (good use of resources).

• Resource manager view: the OS multiplexes resources in time (CPU sharing) and in space (memory partitioning) among competing programs, keeping track of who uses what, granting requests and resolving conflicts.

Generations: 1st (1945–55) vacuum tubes, no OS; 2nd (1955–65) transistors and batch systems; 3rd (1965–80) ICs, multiprogramming and time-sharing; 4th (1980–present) personal computers, GUIs, networked and mobile OS.

A process is a program in execution, with its own code, data, stack and CPU state. A program is passive (a file); a process is active.

Transitions: admit (new → ready), dispatch (ready → running), interrupt/time-out (running → ready), I/O wait (running → waiting), I/O completion (waiting → ready), exit (running → terminated).

The Process Control Block (PCB) is the data structure in which the OS stores everything about a process: process ID, state, program counter, CPU registers, scheduling information (priority), memory information, accounting information and I/O status. Context switch: saving the PCB of the running process and loading the PCB of the next one. It is pure overhead.

The Process Control Block (PCB) is the data structure in which the OS stores everything about a process: process ID, state, program counter, CPU registers, scheduling information (priority), memory information, accounting information and I/O status. Context switch: saving the PCB of the running process and loading the PCB of the next one. It is pure overhead.

A thread is a lightweight unit of execution within a process. Threads of the same process share code, data and files but have their own stack and registers.

• Benefits: responsiveness, resource sharing, economy (cheaper than processes) and use of multiple cores.
• Multithreading models: many-to-one, one-to-one and many-to-many (mapping user threads to kernel threads).

• Multicore: several processing cores on one chip; concurrency means tasks make progress together, parallelism means they run at the same instant on different cores.

• Amdahl's law: speedup ≤ 1 ÷ (S + (1 − S) ÷ N), where S is the serial fraction and N the number of cores — with 25% serial code, 4 cores give at most 2.29 times speedup.
• OpenMP: a set of compiler directives and library routines for shared-memory parallel programming in C, C++ and Fortran; a parallel region is executed by a team of threads.

c#include <omp.h>
#include <stdio.h>
int main() {
    #pragma omp parallel                 /* create a team of threads */
    printf("Hello from thread %d\n", omp_get_thread_num());

    long sum = 0;
    #pragma omp parallel for reduction(+:sum)
    for (int i = 1; i <= 1000000; i++) sum += i;   /* loop split among threads */
    printf("Sum = %ld\n", sum);
    return 0;
}

• Compile with gcc -fopenmp program.c.