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2 Assembly Language Programming for the 68000 Family

form of text editor. It really doesn’t matter which editor you use as long

as you can create a source file for input to the assembler. An assembler is

similar to a compiler in that it “translates” a source language into machine

language.

The output from an assembler is called the object code. Normally

this is machine language put into a special format that combines it with

other object modules into an executable image. This file is essentially a

picture of what goes into memory. When you finally run the program,

this file is brought into memory so that the microprocessor may execute

the instructions.

The operation of combining object modules is called linking. A special

program called a linker is used to perform this function. Figure 1 shows

the steps used to produce an executable program. The details will differ

from computer to computer. Your system may have a program similar

to a linker that converts the output of the assembler into an executable

form, but does not allow combining object modules. You should have no

trouble in learning the commands that perform these steps on a particular

machine.

There are quite a few 68000 assemblers available for a range of com

puter systems. It is not possible to present all the variations in assemblers

in this book. Motorola, as the designer of the 68000 microprocessor fam

ily, originated its assembly language. The most important task of the

assembly language designer is to devise a set of symbolic names for each

instruction the microprocessor can execute. These symbolic names are

known as mnemonics. For example, an instruction to move data from

one place to another has the mnemonic MOVE.

In order to allow the greatest flexibility, this book will use the standard

Motorola assembler syntax and mnemonics. There will probably be some

minor variations with the assembler you use. However, most of the pieces

of an assembly language program will be identical regardless of the

assembler used, and you should not find it difficult in relating the material

to your particular assembler. If you don’t presently have an assembler

and linker for your computer system, check with the manufacturer, who

probably sells a “developer’s package” that contains an assembler, a

linker, and the system documentation you will need. Many independent

software houses also supply development packages. Go to your local

computer store and compare these for compatability with the Motorola

standard. If the syntax or mnemonics of the assembler are very far from the

standard, you should probably consider another one. Other items that

are sometimes provided are an editor (a must if you don’t have one),

an interactive debugger, and other utilities to assist in rapid program

development. This book does not assume any specific development aids

or utilities.

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