Page 142 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

5

Advanced Assemblers,

Linkers, and Loaders

This chapter discusses the advanced assembler and the linker, which are tools needed to
assemble large programs; it is written for the reader who intends to write a lot of
assembly-language programs or large assembly-language programs. Whereas the last
chapter gave sufficient detail for the reader to understand the assembler output of a C
compiler and to embed a limited amount of assembly language code in a C procedure,
this chapter provides additional tools and greater depth to enable you to write large
assembly-language programs using a relocatable, conditional, or macro assembler, and to
join together separately assembled programs using a linker program.
This chapter is optional. Current economics leads to writing programs in a high-
level language like C or C++ rather than in assembly language. Most programmers will
not need to write a lot of programs in assembly language nor to write large programs in
assembly language. Such readers can skip this chapter without losing any background
needed for the rest of this book.
The first section introduces the complementary ideas of cross assembler and
downloader. The next section describes the pair of ideas of relocatable assembler and
linker program. Section 5.3 discusses how conditional assembly is used. The next
section shows the power of macros in assembly-language programs. A final section
recommends good documentation standards for programs written in assembly language.
Upon completion of this chapter, the reader should understand the tools needed for
writing a large number of assembly-language programs or large assembly-language
programs. He or she should have little difficulty writing assembly-language programs in
the order of a couple of hundred lines long.

5.1 Cross Assemblers and Downloaders

In this section we introduce a close cousin of the assembler, the cross-assembler, which,
like the assembler, converts sequences of (ASCII) characters into machine instructions.
For the most part, this section's material is descriptive, almost philosophical, rather than
precise and practical. It is important general knowledge, and it is included here because
the reader should understand what he or she is doing when using a personal computer to
assemble a program for a microcontroller.

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