- LPU MCA Semester 1
- CAP366
- Unit 1
CAP366 · Unit 1: Introduction to operating systems
What an operating system is and what it does
11 practice questions · in the MTE and ETE syllabus
Key points
- OS = resource allocator + control program
- Mode bit separates user and kernel mode
- Privileged instructions run only in kernel mode
- Timer interrupt lets the OS regain the CPU
An operating system is the software layer that sits between users/programs and the hardware. It plays two roles: a resource manager/allocator (CPU, memory, files, devices are shared fairly and efficiently) and a control program that stops programs from damaging each other or the machine.
The key protection mechanism is dual-mode operation: a mode bit separates user mode from kernel mode, and privileged instructions (I/O, setting the timer, disabling interrupts, changing memory maps) run only in kernel mode. A timer guarantees the OS regains the CPU from a program that never yields.
| Mechanism | Prevents |
|---|---|
| Mode bit + privileged instructions | Programs touching devices or other programs directly |
| Timer interrupt | A program keeping the CPU forever |
| Memory protection | Reading/writing another program's memory |
Exam pitfall: the kernel is the always-resident core of the OS, not the whole OS; a compiler or GUI is not an OS service, and a privileged instruction in user mode causes a trap, not a silent skip.
Try two questions
Why must the instruction that disables timer interrupts be privileged?
- A program could otherwise keep the CPU forever and the OS could never regain control
- The timer hardware would otherwise lose its count and the system clock would run fast
- Disk controllers would otherwise stop delivering their completion interrupts to drivers
- The process would otherwise be unable to make system calls into the kernel at all
Show the answer
A. The timer lets the OS preempt a running program. If user code could disable it, one program could monopolise the CPU.
Why not B: Disabling interrupts does not corrupt the counter; the issue is loss of OS control.
Why not C: Timer interrupts are separate from device interrupts, and this is not the protection reason.
Why not D: System calls use a trap instruction and do not depend on the timer interrupt.
A student program running in user mode tries to execute a privileged I/O instruction directly. What does the hardware do?
- It executes the instruction because the OS granted the program a time slice
- It ignores the instruction silently and continues with the next one
- It sets the mode bit to kernel and runs the instruction for the program
- It raises a trap so the OS regains control and can terminate the program
Show the answer
D. Privileged instructions in user mode cause a trap into the kernel, which usually kills the offending process. This is how dual-mode operation protects the OS.
Why not A: A time slice grants CPU time, not permission to run privileged instructions.
Why not B: Silent skipping would hide the fault; the hardware signals an exception instead.
Why not C: Only a trap or system call into OS code changes the mode, never the program itself.