Operating System

Part 1: Core Concepts (2 Marks)

• Operating System: System software acting as an intermediary between user applications and computer hardware, functioning as an Extended Machine (resource abstraction) and Resource Manager (allocating CPU, memory, devices).
• System Call: The programmatic programmatic interface provided by the OS allowing user-level processes to request privileged services from the kernel (e.g., fork(), read(), write(), exec()).
• Kernel Architectures:
  • Monolithic Kernel: All OS services (VFS, IPC, drivers, scheduler) execute in a single privileged address space (Linux).
  • Microkernel: Only essential primitives (IPC, low-level scheduling, memory mapping) run in kernel space; drivers and file systems run as user-space servers (Minix, QNX).

Part 2: Structural Frameworks (10 Marks)

Dual-Mode Operation Transition

1. [User Process Execution] ---- User Mode (Mode bit = 1)
2. Executes System Call (e.g. read())
3. Software Interrupt (Trap / Syscall Instruction)
   • [Kernel System Call Handler] -- Kernel Mode (Mode bit = 0)
   • (Validates arguments, executes privileged hardware I/O)
4. Returns from Trap
   • [User Process Resumes] ---- User Mode (Mode bit = 1)

Part 3: Distinction Tables

Part 4: Worked Problems & Exam Traps

Process Creation with fork() System Call

#include <stdio.h>
#include <unistd.h>

int main() {
    pid_t pid = fork();

    if (pid < 0) {
        printf("Fork failed!\n");
    } else if (pid == 0) {
        // Child process receives return value 0
        printf("Hello from Child! (PID = %d)\n", getpid());
    } else {
        // Parent process receives Child's PID
        printf("Hello from Parent! (Child PID = %d)\n", pid);
    }
    return 0;
}

fork() duplicates the calling process. If you write fork(); fork(); fork();, total processes created is $2^3 = 8$ (1 parent + 7 children).