Consider the following code snippet for the syscall function in kernel/syscall.c (RISC-V xv6): void syscall(void) { int syscall_num; struct proc p = myproc(); syscall_num = p->trapframe->a7; // system call number if(syscall_num > 0 && syscall_num trapframe->a0 = syscallssyscall_num; } else { printf("PID %d (%s): Invalid syscall %d\n", p->pid, p->name, syscall_num); p->trapframe->a0 = -1; } } What does the a7 register in the trapframe represent in this code?

(D) The number identifying the requested system call.

(A) The base address of the process stack.

(B) The return value of the system call.

(C) The number of arguments passed to the system call.

Operating Systems · Quiz 1 · 26 Oct 2025 · September 2025 term

Question 12: Consider the following code snippet for the syscall func…

Question 12

+3 marksOne correct option

Consider the following code snippet for the syscall function in kernel/syscall.c (RISC-V xv6):

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}

What does the a7 register in the trapframe represent in this code?

A
The base address of the process stack.
B
The return value of the system call.
C
The number of arguments passed to the system call.
D
The number identifying the requested system call.

Show answer

Correct answer

D
The number identifying the requested system call.

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Question 12 of 18 in the IIT Madras BS Operating Systems (Operating Systems) Quiz 1 paper sat on 26 Oct 2025, in the September 2025 term (IIT M IMPROVEMENT AN EXAM QIB2 26 Oct 2025). It carries 3 marks.

Question 12: Consider the following code snippet for the syscall func…

Question 12

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