---
title: "System call; monolithic and VM architecture"
subject: "Operating System"
chapter: 1
key: "semester_6/operating_system:system-call-architecture"
source: https://www.bctengineer.com/q/semester_6/operating_system/system-call-architecture
site: BCT Engineering
---

# System call; monolithic and VM architecture

Operating System · Chapter 1: Introduction to Operating System

- Asked in: 2073 Magh, 2+3 marks
- Video: [System Calls](https://www.youtube.com/watch?v=lhToWeuWWfw) — Neso Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/operating_system/system-call-architecture
- Whole chapter: https://www.bctengineer.com/md/semester_6/operating_system/chapter-1

## Question

What is a system call? Explain in brief the following architectures of operating system: i) Monolithic architecture ii) Virtual machine.

## Short answer

A system call is the interface for user programs to request kernel services via a trap instruction. Monolithic architecture runs the entire OS in kernel mode as a single binary with direct procedure calls. Virtual machine architecture uses a hypervisor to create VMs running guest OSs; system calls trap to guest OS, privileged instructions trap to hypervisor.

## Answer

A **system call** is the programming interface through which a user program requests a service from the operating system kernel (for example, to open a file or create a process). It switches the CPU from user mode to kernel mode using a TRAP instruction.

#### i) Monolithic architecture

The entire OS is a single large program running in kernel mode. All components (process, memory, file system, device drivers) are compiled into one binary and call each other directly. Only a minimal structure exists: a main procedure that handles the system call, a set of service procedures that carry it out, and utility procedures shared by the services.

```text
 User programs
 ------------- system call (trap)
 +------------------------+
 | main routine           |
 | service procedures     |
 | utility procedures     |
 +------------------------+
 | Hardware               |
```

It is fast (no layer overhead) but difficult to maintain, and a bug anywhere can crash the system. Examples: original UNIX, MS-DOS.

#### ii) Virtual machine architecture

A virtual machine monitor (hypervisor) runs on the bare hardware and creates several exact copies of the hardware (virtual machines). Each VM can run its own guest OS. A program's system call traps to the guest OS in its own VM; when the guest OS executes privileged instructions (such as I/O), these trap to the monitor, which emulates them. The design gives full isolation between VMs and allows several different OSs on one computer (examples: IBM VM/370, VMware, VirtualBox).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does a system call enter the kernel?
   A. A normal jump to a kernel address from user mode
   B. The kernel polls user programs
   C. It is copied to disk first
   D. A TRAP instruction switches the CPU from user mode to kernel mode
   Answer: D. TRAP changes mode and jumps to a fixed kernel address. (Key point: System call and TRAP)

2. What minimal structure does a monolithic OS have?
   A. A main procedure, service procedures and utility procedures, all in one kernel-mode binary
   B. Separate user-mode servers for each service
   C. Several exact hardware copies
   D. A strict hierarchy of numbered layers
   Answer: A. It is fast but hard to maintain and a bug anywhere can crash the system. (Key point: Monolithic structure)

3. In the virtual machine architecture, what handles a guest's system calls?
   A. The guest hardware directly
   B. The virtual machine monitor traps and emulates them
   C. A shell in user mode
   D. The BIOS
   Answer: B. The hypervisor traps guest system calls and emulates them, giving isolation between VMs. (Key point: Virtual machine architecture)

## Related in Operating System

- [OS abstraction, extended machine, system call](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Aos-abstraction-system-call) — Operating System, chapter 1
- [Extended machine, kernel, system call](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Aextended-machine-kernel-syscall) — Operating System, chapter 1
- [Virtual machine structure and benefits](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Avirtual-machine-structure) — Operating System, chapter 1, asked 2×
- [OS as virtual machine; functions](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Aos-virtual-machine-functions) — Operating System, chapter 1
- [System call; read() execution](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Asystem-call-read-example) — Operating System, chapter 1
- [File system vs virtual memory; contiguous allocation](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Afs-vs-virtual-memory-contiguous-tape) — Operating System, chapter 5

## Related in other subjects

- [Short note: Monolithic and microkernel](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Amonolithic-microkernel-note) — Distributed System, chapter 3, asked 4×
- [DOS; monolithic and microkernel](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Ados-monolithic-microkernel) — Distributed System, chapter 3
- [Features of a typical operating system](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Afeatures-of-operating-system) — Microprocessor, chapter 6, asked 3×
- [Operating system layers](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Aos-layers) — Distributed System, chapter 3