CAIE 9618 · Paper 3 · Topic 3.3.2

Virtual
Machines

Hypervisors · Type 1 & 2 · Benefits · Testing · JVM · Intermediate Code

CSZone Cambridge International AS & A Level Computer Science 9618

What is a Virtual Machine?

A Software Emulation of a Computer

A virtual machine (VM) is a software emulation of a complete computer system. It runs on physical hardware (the host) and presents a virtual hardware environment to an operating system (the guest). Multiple VMs can run simultaneously on one physical machine.

KEY TERMS

Host machine — the physical computer running the hypervisor

Guest OS — the operating system running inside the VM

Hypervisor — software layer that manages and runs VMs

Snapshot — saved state of a VM at a point in time

BENEFITS

Server consolidation — run many VMs on one physical server

Isolation — if one VM crashes, others unaffected

Easy to back up, restore, and clone

Test different OS/software without dedicated hardware

Run legacy software on modern hardware

Type 1 vs Type 2 Hypervisors

Bare-Metal vs Hosted Hypervisors

TYPE 1 — BARE-METAL HYPERVISOR

Guest OS

Hypervisor (runs on hardware directly)

Physical Hardware

Runs directly on hardware — no host OS. Better performance, more secure. Used in enterprise data centres.

Examples: VMware ESXi, Microsoft Hyper-V, Xen

TYPE 2 — HOSTED HYPERVISOR

Guest OS

Hypervisor (app)

Host OS

Physical Hardware

Runs as an application on a host OS. Easier to set up, slower. Used for development and testing.

Examples: VirtualBox, VMware Workstation, Parallels

VMs for Testing & the JVM

Safe Testing & Write Once, Run Anywhere

VMs FOR SOFTWARE TESTING

Test software in a clean, isolated environment without affecting host

Use snapshots to restore to a known-good state after each test

Test on multiple OS (Windows, Linux, macOS) on one machine

Safely test malware in sandboxed VM without risk to host

Cloud providers (AWS, Azure) deliver scalable VMs on demand

JVM — JAVA VIRTUAL MACHINE

Java source code compiled to bytecode (intermediate code)

JVM interprets bytecode on the host platform at runtime

"Write once, run anywhere" — same bytecode runs on Windows, Linux, macOS

Each platform has its own JVM implementation

JIT (Just-In-Time) compiler: frequently used bytecode compiled to native machine code for speed

Exam Practice

Cambridge-style questions

Question 1

A software developer wants to test their application on three different operating systems before release. Explain how a virtual machine could be used to carry out this testing, and state two benefits of this approach. [4]

2

The developer installs a Type 2 hypervisor on their workstation and creates three separate VMs — one running each target operating system (e.g. Windows 10, Ubuntu Linux, macOS). The application can be installed and tested in each VM without needing three separate physical machines.

1

Benefit 1: Snapshots can be taken before each test run — if testing causes a crash or corrupts the environment, the VM can be instantly restored to the clean state, saving time.

1

Benefit 2: Testing is isolated — if the software contains a bug that crashes an OS, it only crashes the VM, leaving the host machine and other VMs unaffected.

Common Mistakes

Don't lose easy marks

1

Saying VMs "run on the cloud" — this conflates VM technology with cloud computing. VMs run on hypervisors, which can run on your own hardware (on-premise) or in a cloud data centre. The cloud uses VMs, but VMs are not exclusive to the cloud.

2

Confusing the JVM with a general virtual machine — the JVM is a specific VM for running Java bytecode. It's not a hypervisor and doesn't run guest operating systems. A general VM virtualises hardware; the JVM virtualises an instruction set for bytecode execution.

3

Saying bytecode is "compiled directly to machine code" — bytecode is an intermediate representation (not machine code for any specific CPU). It's interpreted by the JVM at runtime. JIT compilation may convert hot paths to native code, but the initial compilation is to bytecode, not machine code.

Topic Summary — 3.3.2

What You Need to Know

VIRTUAL MACHINES

Software emulation of hardware. Type 1 hypervisor: bare-metal, enterprise use (ESXi, Hyper-V). Type 2 hypervisor: runs on host OS, for development (VirtualBox). Benefits: isolation, snapshots, server consolidation, multi-OS testing.

JVM & INTERMEDIATE CODE

Java source → bytecode (platform-independent intermediate code). JVM interprets bytecode on each target platform. "Write once, run anywhere." JIT compiler converts frequently-run bytecode to native machine code for performance.

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