Motorola M68000 Uživatelská příručka stáhnout pdf (Strana 155)

CHAPTER 11

ADVANCED ARITHMETIC

In Chapters 4 and 5 you learned the ADD, SUB, ADDQ, and SUBQ

instructions. These instructions, along with the condition code register,

give you the capability to perform other arithmetic operations such as

multiplication and division. For example, you can perform multiplication

by repeated addition, and division by repeated subtraction. These are not

the best methods to use, but they are simple. By using the shift instructions

shown in Chapter 10, more efficient algorithms can be implemented.

Fortunately, the 68000 has a set of powerful arithmetic instructions that

include multiplication and division as well as some other rather interesting

instructions. In this chapter we will take a look at the complete set of

arithmetic instructions and introduce some new concepts such as decimal

arithmetic. Yes, I did say “decimal” arithmetic. Up to this point we have

been dealing strictly with binary arithmetic operations.

Multiple Precision Addition and Subtraction

Before discussing the multiplication and division instructions, I want to

introduce the concept of arithmetic precision. Basically, the precision of

a calculation is proportional to the number of bits used in the calculation.

The more bits used, the greater the precision. Even though we are

confined to integer arithmetic, we can use a larger number of bits to

represent larger numbers which, in turn, can represent scaled values. For

example, we can represent time intervals in thousandths of seconds rather

than seconds. This would require the ability to represent numbers 1,000

times larger for the equivalent times in seconds. However, the precision

would be greater, since times would be stored with accuracy down to a

thousandth of a second.

The 68000 performs additions and subtractions on bytes, words, or

longwords. Therefore, there are actually three built-in precisions avail

able. But what if 32 bits is not a great enough precision for our calcu

lations? Is there anything we can do? The answer is yes, but not with a

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Motorola M68000 Uživatelská příručka Strana 155