Concepts of Programming Languages

1.7 Implementation Methods 25

implementation and has the advantage of very fast program execution, once

the translation process is complete. Most production implementations of lan-

guages, such as C, COBOL, C++, and Ada, are by compilers.

The language that a compiler translates is called the source language. The

process of compilation and program execution takes place in several phases, the

most important of which are shown in Figure 1.3.

The lexical analyzer gathers the characters of the source program into lexi-

cal units. The lexical units of a program are identifiers, special words, operators,

and punctuation symbols. The lexical analyzer ignores comments in the source

program because the compiler has no use for them.

The syntax analyzer takes the lexical units from the lexical analyzer and uses

them to construct hierarchical structures called parse trees. These parse trees

represent the syntactic structure of the program. In many cases, no actual parse

tree structure is constructed; rather, the information that would be required to

build a tree is generated and used directly. Both lexical units and parse trees are

further discussed in Chapter 3. Lexical analysis and syntax analysis, or parsing,

are discussed in Chapter 4.

Operating

system

command

interpreter

Scheme

interpreter

C

compiler

Virtual

C

computer

Virtual

Ada

computer

Ada

compiler

...
...

Assembler

Virtual

assembly

language

computer

Java Virtual

Machine

Java

compiler

.NET

common

language

run time

VB.NET

compiler

C#

compiler

Virtual

VB .NET

computer

Virtual C#

computer

Bare

machine

Macroinstruction

interpreter

Operating system

Virtual Java

computer

Virtual

Scheme

computer

Figure 1.2

Layered interface of
virtual computers,
provided by a typical
computer system