GATE CSE 2001 | Question: 1.13

Question

A CPU has two modes -- privileged and non-privileged. In order to change the mode from privileged to non-privileged

• A. a hardware interrupt is needed
• B. a software interrupt is needed
• C. a privileged instruction (which does not generate an interrupt) is needed
• D. a non-privileged instruction (which does not generate an interrupt) is needed

Comments

@ritiksri8 Can you please cite any resources for the point you made.

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answer to this is given as C in GO PYQ Book. However, based on discussions here, shouldn't it be D?

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1. The Kernel Must Dictate "Where" to Return

When the CPU switches to user mode, it doesn't just lower its privilege level; it also jumps to a specific line of code in a user program (using the Program Counter or Instruction Pointer register).

• The Danger: If a non-privileged instruction could change the mode, a malicious user program could execute that instruction to force the CPU into user mode while still inside kernel space.
• The Consequence: The CPU would suddenly start executing secure kernel code with user-mode permissions. While it couldn't execute privileged commands, it could trigger unexpected errors, bypass kernel state checks, or crash the entire operating system. The kernel must be the one to safely decide exactly when and where the execution drops back to user space.

2. Preventing "Privilege Leaks" (Cleaning Up the Mess)

Before the CPU enters user mode, the operating system kernel must perform critical security "housekeeping." The kernel has access to sensitive data (like cryptography keys, passwords, or data from other users) stored in the CPU registers.

A privileged instruction like RFE (Return from Exception) or IRET (Interrupt Return) is designed to act as an atomic (all-in-one) hardware switch. It simultaneously:

1. Clears or restores the registers to what the user program expects.
2. Restores the user memory boundaries (Page Tables).
3. Flips the CPU hardware mode bit back to non-privileged.

If a regular, non-privileged instruction could just flip the mode bit mid-way, a program could trick the CPU into dropping to user mode while sensitive kernel data is still sitting exposed in the CPU registers.

3. Architecture Definition: Modifying Control Registers is Always Privileged

At the hardware level, the CPU tracks its current mode using a bit inside a physical Control Register (like the EFLAGS register in x86 or CPSR in ARM).

Hardware engineers hardcode the CPU architecture so that any instruction capable of modifying these core control registers is privileged. If an instruction can change the mode bit, even if it is changing it from 1 (Kernel) to 0 (User), that instruction by definition requires privileged status to execute.

Answer 1

Answer should be (D). Changing from privileged to non-privileged doesn't require an interrupt unlike from non-privileged to privileged. Also, to loose a privilege we don't need a privileged instruction though a privileged instruction does no harm. 

http://web.cse.ohio-state.edu/~teodores/download/teaching/cse675.au08/CSE675.02_MIPS-ISA_part3.pdf

Comments

The question is asking for a necessary condition. Even one counter-example is enough to prove that it is not necessary to execute a privileged instruction to return from a trap (consider the JRST instruction).

Therefore, option C is a sufficient but not necessary condition, which is what Arjun Sir had also explained. You can refer to his answer.

Returning from trap in some cpus only involves udating a register value which can be a non-privileged instruction.

A necessary condition would simply be that a non-privileged instruction can return from a trap. Also, a non-privileged instruction can be executed in both user mode and kernel mode, so this makes it a more flexible way of returning to user mode.

Also in some cpu like x86 architecture, returning from trap is a privileged instruction.

Option D is correct in general view. 

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@Abhinav_Mishra see this  Even one counter-example is enough to prove that it is not necessary to execute a non-privileged instruction to return from a trap. Therefore, option D is a sufficient but not necessary condition

 

Can I say this? If this was a MSQ then what would be correct?

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@Tushar Rana If it was an msq then we can argue that both must be correct.

Answer 2

Answer is (c)

Transitions from kernel to user mode are performed explicitly by the
operating system, generally at the end of an interrupt handler or
kernel call, using a privileged RFE (return from exception) instruction.

Ref- http://www.cs.rochester.edu/courses/252/spring2014/notes/08_exceptions

Comments

Correct me if I am wrong

Answer 3

To change from non privileged to privileged we require an interrupt known as software interrupt which is generated by a system call which gets translated into a privileged instruction. Option C would be apt.

Also The instructions that get executed in privileged mode will be privileged and never non-privileged. (The earlier answer which is supporting option D is having an absolutely wrong explanation)

Comments

to change the mode from kernel mode to user mode ...only a bit(mode bit) is changed in register, nothing else is required

So no privileged instructions or interrupts are required.

Ans is D only

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whattttt

Answer 4

Answer is :-  C ("Scheduler process" does not interrupt a running process.)

1. Device: Devices can interrupt a running process when they need attention from the CPU, such as when data transfer is complete or when input is required.

2. Timer: A timer interrupt occurs when a predefined time period has elapsed. This interrupt can be used for various purposes, including preemptive multitasking, where the CPU switches between different processes.

3. Power failure: While power failure interrupts can occur, they are typically not considered part of normal program execution but rather an external event that may result in the abrupt termination of a running process.

4. Scheduler process:The scheduler process itself doesn't interrupt a running process. Instead, it is responsible for managing the execution of processes, including deciding when to allocate CPU time to different processes. However, the scheduler's decisions may result in a context switch, which effectively pauses one process and starts another, but this isn't an interrupt in the traditional sense.

Answer 5

User mode is called non-privileged mode and kernal mode is called privileged mode.

 

Instructions that run in kernel mode are called privileged instructions and instructions that run in user mode are called non-privileged instructions.

 

Whenever we write a program, we write instructions inside the program. These instructions are in user mode. If any of these instructions wants to communicate or manipulate the hardware, then it will call the system call.

 

So here the operating system will generate an interrupt and make the mode bit = 1. Now the CPU has come from user mode to kernel mode. Now here in the system call run hone ke bad the operating system will generate an interrupt and make the mode bit = 0. Now the CPU has come back to user mode.

 

 

So here, to send the CPU back from privileged mode i.e. kernel mode to non-privileged mode i.e. user mode, we need a software interrupt which will be generated by the operating system and along with it, non-privileged instruction is also required.

 

So option (C) is correct answer.

 

 

 

Answer 6

option c

it is puerlly decision of the operating system in which particular mode instruction is executed . and operating system is always executed in kernal mode(privileged mode).