Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 71)

CHAPTER 5

CONDITIONAL AND

ARITHMETIC INSTRUCTIONS

In Chapter 4 you learned a small number of 68000 instructions. These

were enough to write some very simple programs. I hope that you have

taken the time to actually run some of the programs presented in the

chapter, as well as a few programs of your own design. Nothing gives you

more confidence that you are successfully mastering assembly language

programming than the joy you experience when a program actually

performs as it should. By now you should have become familiar with

your own computer’s procedures for editing, assembling, and executing

assembly language programs. If you are having trouble, you should

carefully review your computer system's manuals before going on with

this chapter.

The remainder of this book is designed to expand your vocabulary

of 68000 instructions. Rather than present all the remaining instructions

at one time, we will present groups of instructions and specific examples

of their use. You should not attempt to learn all of the 68000 instructions

before practicing with each group. Take the time to actually experiment

with the instructions presented in each chapter. Don’t limit yourself to

the exercises. Be creative.

In this chapter you will learn the very powerful set of conditional

instructions. These instructions allow the control flow of your program

to vary depending on the results of certain instructions. Among these

are the arithmetic instructions you have already learned as well as several

others. Crucial to understanding the conditional instructions is a thorough

knowledge of certain bits in the condition code register and how they are

affected by arithmetic instructions.

Arithmetic and the Condition Code Register

As you learned in Chapter 2, the 68000 has a special register known as

the condition code register (CCR), This register is not used in the same

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Motorola M68000 Betriebsanweisung Seite 71