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ICT 444 COMPILERS AND TRANSLATORSICT 444 COMPILERS AND TRANSLATORS

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Introduction to Compilers

Simply stated, a compiler is a programme thatreads a programme written in one language

the source language and translates it into anequivalent programme in another language the target language.

TargetSource

ProgrammeProgramme

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Messages

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A Language Processing System

PREPROCESSOR

Skeletal source programme

Source programme

COMPILER

Target assembly programme

ASSEMBLER

Re-locatable machine code

LOADER/LINKEDITOR Library, re-locatable objectfiles

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so u e mac ne co e

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Analysis of a source programme

n comp ng, ana ys s o e source programmeconsists of three phases:

1. Linear Analysis: This is the process inwhich the stream of characters making up thesource programme is read from left-to-right and

grouped into tokens which are sequences ofcharacters having a collective meaning.

2. Hierarchical Analysis: This is where the

stream of characters or tokens are groupedhierarchically into nested collections with

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3. Semantic Analysis: This is where certain

components of a programme fit together

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Symbol-Table Management

the identifiers used in the source programme

of each identifier.

,

information about storage allocation of the, .

In the case of procedure names, they provide

such things as the number and types of itsarguments, the method of argument supply,

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A Symbol Table is a data structure containing a

, wattributes of the identifier.

This data structure allows us to find the recordfor each identifier quickly and to store orretrieve data from that record quickly.

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Preprocessors: Preprocessors produce

following functions:

. allow a user to define macros that are

.

Macro processors deal with two kinds of: u .

Macro definition is normally indicated by some

unique character or keyword like define ormacro with formal parametersin it definition.

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The use of a macro on the other hand consists

parametersi.e. values for it formal parameters.

2. File Inclusion: This involves apreprocessor that includes header files in aprogramme text.

3. Rational processors: These areprocessors that augment older languages with

more modern flow-of-control and data-structuring facilities.

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4. Language extensions: These are

the language by what amounts to built-in.

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Assemblers

An assembly code is a mnemonic version ofmachine code, in which names are used

instead of binary codes for operations andnames are also given to memory addresses.

Some compilers produce assembly code, that is

On the other hand, other compilers perform the

o o e assem er us, pro uc ng re-locatable machine code that can be passed

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A typical sequence of assembly instruction:

MOV a, R1ADD #2, R1

MOV R1, b

address a into register 1, then adds the,

1 as a fixed-point number, and finally stores the

.Thus, it computes an expression like

RDAppiah= a + ; n a anguage e .

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A Pass

A pass refers to the process of reading an input fileonce and in some cases, writing an output file.

Two-Pass Assembly

e s mp es orm o assem er ma es wo

passes over the input.In the first pass, all the identifiers that denotestora e locations are found and stored in asymbol table, which is usually different from thatof the com iler.

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Here, identifiers are assigned storage locations

.

After reading the illustration above, and on the

assumption that a word consist of four bytes,the symbol table might contain the followingentries:

IDENTIFIER ADDRESS

b 4

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In the second pass, the assembler scans the

.This time, it translates each operation code into

representing that operation in machine.

After this, it translates each identifier

for that identifier in the symbol table.

-locatable machine code, which indicates that it

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memory.

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The following is a hypothetical machine code

translated:

0011 01 10 00000010

Here, we take it that the first four bits are theinstruction code, with 0001, 0010, and 0011standing for load, store, and add, respectively.

The next two bits designate a register, and o1refers to register 1 in each of the three

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The two bits after that represent a tag with 00

,the last eight bits refer to a memory address.

e ag s an s or e mme a e mo e,where the last eight bits are taken literally as

e operan as n e secon ns ruc on .

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