Software Design Methodology

164 Chapter 6. Typical Architectural Styles

6.5 VIRTUAL MACHINE

A. Structural pattern

A software system of virtual machine architecture usually consists of 4 components

interconnected as follows (Figure 6.18).

Figure 6.18 Virtual machine architecture

B. Computational model and design vocabulary

Virtual machines are software systems that simulate a computer system, but with

some functionality that is not native to the hardware and/or software on which it is

implemented.

Similar to the hardware architecture of a computer, the structure of a virtual

machine consists of three passive data components and one active component as

shown in Figure 6.18. Their functions are as follows.

(a) The program component stores the ‘program’ being interpreted, which is

an abstract representation of the process of data. It is usually in the form of a

sequence of structured data elements. Each element represents an ‘instruction’ or

‘statement’ for how to process the data. It is analogous to the program instructions

run on the hardware computer system, hence called ‘program’.

(b) The data component stores the information to be processed. Such

information is analogous to the values assigned to variables in the execution of a

program. The data component is analogou to the memory of computer hardware.

(c) The interpretation engine is the active component in the system. It selects

an instruction from the ‘program’ being interpreted, updates the program’s data

Data

(Program State)

Program

Internal

State

Interpretation

Engine

Input

Output

State Data

Data

Updates

Selected Instruction

Selected Data

Program

Instruction

Software Design Methodology 165

according to the instruction, and updates its internal state. It is analogous to the

CPU in computer hardware.

(d) The internal state stores the current state of the interpretation engine. It is

analogous to the registers or the instruction count in computer hardware.

C. Properties derived from the architecture

Virtual machine architectures have the goal of achieving the quality of portability.

Executing a program via an interpreter also adds flexibility through the ability to

interrupt and query the program and introduce modifications at run-time, but there

is a performance cost.

This can be useful in a number of ways. It can allow one to simulate and test

platforms that have not yet been built such as new hardware, and it can simulate

‘disaster’ mode that would be too complex, costly or dangerous to test with the real

system.

D. Examples

Common examples of virtual machines are interpreters, rule-based systems,

syntactic shells, and command language processors. For example, the Java

language is built to run on top of the Java virtual machine, which allows the Java

programs to be platform independent. A Java program can therefore be executed on

all computer platforms that have an implementation of Java virtual machine.

A Java virtual machine is a software system that enables compiled Java

programs to be executed. However, the term Java virtual machine also refers to the

abstract specification of any specific implementation of such a system. This

specification defines the architecture of the software, its dynamic behaviour, format

of the object code generated from a compilation of Java programs, as well as

various issues that may affect the portability of the Java programs. There are a

number of implementations of the abstract specifications on various hardware and

software platforms. The term Java virtual machine may also refer to any run-time

instances of the software. The following gives a very brief introduction to the

architecture of Java virtual machine as specified in the abstract specification in [

3].

Figure 6.19 shows the components and their connections in a Java virtual machine.