Operating Systems Quiz 1: 26 October 2025 (September 2025 term)

1#define GPIO_PORTA_BASE 0x50000000
2
3void write_gpio(){
4    uint16_t *data_reg = (uint16_t*)(GPIO_PORTA_BASE + 0x10);
5    uint32_t *dir_reg  = (uint32_t*)(GPIO_PORTA_BASE + 0x14);
6
7    *dir_reg = 0xFF;      // Configure lower 8 pins as output
8    *data_reg = 0xAA55;   // Write pattern to port
9}

1int pid;
2
3pid = fork();
4if (pid == 0) {
5    printf(1, "Child process\n");
6    exit(0);
7} else if (pid > 0) {
8    printf(1, "Parent process\n");
9    wait(0);
10}

1void
2syscall(void)
3{
4  int syscall_num;
5  struct proc *p = myproc();
6
7  syscall_num = p->trapframe->a7;   // system call number
8  if(syscall_num > 0 && syscall_num < MAX_SYSCALLS && syscalls[syscall_num]) {
9    p->trapframe->a0 = syscalls[syscall_num]();
10  } else {
11    printf("PID %d (%s): Invalid syscall %d\n",
12            p->pid, p->name, syscall_num);
13    p->trapframe->a0 = -1;
14  }
15}

1#include <stdio.h>
2#include <stdlib.h>
3#include <unistd.h>
4
5int main() {
6    int x = 5, pid;
7    pid = fork();
8    if (pid == 0) {
9        x += 3;
10        printf("Child 1: %d\n", x);
11    } else {
12        x += 7;
13        printf("Parent 1: %d\n", x);
14        pid = fork();
15        if (pid == 0) {
16            x += 10;
17            printf("Child 2: %d\n", x);
18        } else {
19            x += 15;
20            printf("Parent 2: %d\n", x);
21        }
22    }
23    return 0;
24}

1void userinit(void)
2{
3  struct proc *p;
4
5  p = allocproc();
6  initproc = p;
7
8  // allocate one user page and copy initcode's instructions
9  // and data into it.
10  uvmfirst(p->pagetable, initcode, sizeof(initcode));
11  p->sz = PGSIZE;
12
13  // prepare for the very first "return" from kernel to user.
14  p->trapframe->epc = 0;      // user program counter
15  p->trapframe->sp = PGSIZE;  // user stack pointer
16
17  safestrcpy(p->name, "initcode", sizeof(p->name));
18  p->cwd = namei("/");
19
20  p->state = RUNNABLE;
21
22  release(&p->lock);
23}