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34 Assembly Language Programming for the 68000 Family

line, you must use an asterisk in column 1. You can see the comments in

Figure 6.

On Choosing Symbols

When you need to select a new symbol for use as a constant, variable,

or label for an instruction, you are free to create arbitrary symbols as

long as you adhere to the rules for creating an identifier. However, some

assemblers do not allow you to create symbols that are the same as the

instruction mnemonics or assembler directives. These reserved symbols

are known as the keywords of the assembler. Although it may seem clear

when a symbol is used as an instruction rather than a variable, some

assemblers are not that smart. Even if your assembler can make this

distinction, it is a good practice to avoid using keywords. Consult your

assembler manual. You can usually find a table of all the keywords that

the assembler recognizes.

It is good programming practice to choose symbols that have a mean

ing related to their use in the program. For example, if you use a vari

able to keep track of a count, why not name it COUNT? Short symbols

like I, J, or N can be used, but don’t tell us much. Labels for instruc

tions can indicate the function of a particular portion of the program.

The label RE ADD AT A clearly indicates the reading of some data. The

label L23 does not convey any meaning. Although many assemblers al

low’ extremely long identifiers, keeping them to eight characters or less

is standard practice. Most programmers line up the source line fields on

tab stops set at every eight columns, and long identifiers make lining up

the fields difficult unless a lot of extra space is used to accommodate the

longest symbols.

Constants

A constant is a value that doesn’t change during program assembly

or execution. Two types of constants can be used: integers and character

strings.

Integer constants are numeric quantities that can be represented by

32 bits or less. You will remember from Chapter 1 that numbers can be

represented in various number bases. If a constant is specified without

indicating this base or radix, it is assumed to be in the decimal number

base. To indicate that a constant is written in a number base other than

10, we can prefix the number with a radix indicator. The radix indicators

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Motorola M68000 Betriebsanweisung Seite 48