GATE CSE 2023 | Question: 13

Question

Which one or more of the following options guarantee that a computer system will transition from user mode to kernel mode?

• A. Function Call
• B. malloc Call
• C. Page Fault
• D. System Call

Comments

According to GATE official answer key only option D is the Answer . Also its a MCQ not a MSQ

---

no, it’s question number 23 and official key is C and D [MSQ of course]

Answer 1

• A. function call: functions may be user defined so It also does not guarantee to switch from user to kernel mode.

• A. Malloc () call: It can be done in both user and kernel modes. Hence, It cannot guarantee switching.

100. Page fault : When a page fault occurs, required page must be loaded from secondary to main memory. It will be done in privileged mode and guarantee user mode to kernel mode switching.

500. System call : System calls are executed  in kernel mode only. It definitely guarantee switching.

Hence Ans is C,D.

Answer 2

System Call guarantees that a computer system will transition from user mode to kernel mode.

System calls are the mechanism by which user-level programs can request services from the operating system (OS) and transition from user mode to kernel mode. When a user-level program executes a system call, it causes an interrupt to occur, which transfers control to the OS kernel. The OS kernel then performs the requested service and returns control to the user-level program.

Function calls and malloc calls do not necessarily result in a transition to kernel mode. They are both operations that can be performed entirely within user mode.

A page fault can trigger a transition to kernel mode, but it is not guaranteed to do so. A page fault occurs when a program attempts to access a page of memory that is not currently in physical memory. In this case, the OS must handle the page fault and may need to allocate physical memory or swap pages in/out of memory. This requires kernel mode access, but some page faults can be handled entirely within user mode without kernel intervention.

Comments

which page faults can be handled without switching to kernel mode?

Answer 3

Options A, C, and D should be straightforward. (Comment if you have any doubts)

For option B, a good read is available here https://stackoverflow.com/a/5716525

The point 1 in above link is important here,

malloc() will search its managed pieces of memory to see if there's a piece of unused memory that satisfy the allocation requirements.

It implies that a system call is not required (therefore no switch to kernel mode) to satisfy the requirement.

Hence, answer is C, D.

Comments

Why option A) Function call is not the correct option? Since, function call involves function call stack to be maintained in the stack memory which is ultimately a part of memory management component of OS. Since OS is directly involved, so I think user to kernel mode transition is necessary here. Correct me if i am wrong.

---

@ayush_27 the stack that a program accesses is in its own user space. You don’t need anyone’s permission (system call) to access it. A system in which programs would need system call after every dozen operations would be heavily under utilized (as most of the time would go in switching from user to kernel mode and vice versa).

---

Thanks @akshaw for clearing my misunderstanding.

Answer 4

Kernal is the core of operating system,though which we access key functionalities of operating system(system calls).

Switch of mode(from user to kernal) is required when program wants to execute instructions that are not in user space,hence wants to access privileged memory.

A. Function call access only the user space => Not required to switch modes. => Not required to switch to kernal modes.

B. Malloc search for unallocated memory from its heap from Logical address space of process. => Not required to switch to kernal modes. {https://stackoverflow.com/a/5716525}

C. On page faults we will call pageFaultServiceRoutine which is a system call. => Required to switch to kernal modes.

D. All system calls/interrupts are only executed in kernal mode. => Required to switch to kernal modes.

ANS: C,D