Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 47)

Assembler Source Format 33

are allowed by all assemblers. The characters +, —, and / are inter

preted as addition, subtraction, multiplication, and division respectively.

Most assemblers allow the use of parentheses in arithmetic expressions.

Consult your assembler manual for details on expression evaluation.

If more than one operand is required with an instruction or assembler

directive, the operands are separated by commas. These commas are

delimeters, and you must not insert a space before or after their use. For

example,

ADD.L D2,D3

results in the two registers, D2 and D3, being added together, with the

result placed in register D3.

The 68000 microprocessor uses a variety of addressing modes. The

addressing mode is the method the CPU uses to locate its operands in

memory. In order to specify the particular addressing mode desired, the

operands are formed with the use of special delimiters. For example,

MOVE. L DO,(AO)

indicates the register indirect mode of addressing used with the AO

to indicate this type of addressing. You will learn more about the 68000

addressing modes in Chapter 6.

The Comment Field

The comment field is used to provide information for the programmer

and others who may have occasion to examine the program. Assembly

language is not self-documenting. Often, even the programmer may have

difficulty in remembering exactly how her program works if she has been

away from it for some time. Comments are best used to provide a running

description of the program’s operation. Comments help those who may

have to maintain the program in the future. Comments can also be used

to provide information as to how to use a particular program.

A comment can be used on every line of the program. The first space

after the operand field starts the comment. The remainder of the line is

ignored by the assembler. This is why it is very important not to include

any spaces in the operand field. Comments are not interpreted or used in

any way by the assembler. When a comment is the only thing on a source

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Motorola M68000 Betriebsanweisung Seite 47