Motorola M68000 User's Guide download pdf (Page 43)

CHAPTER 3

ASSEMBLER SOURCE FORMAT

There are many assemblers available for the 68000 family. They differ

from each other in minor ways. It would be virtually impossible to present

the details of every assembler on the market. Rather, I will present what

is a relatively standard core, based on the specifications provided by

Motorola for its assemblers. There are many assembler features that

are left out. A careful reading of your assembler manual will provide

these details. What is presented here is enough information to get you

programming in 68000 assembler. You should, however, verify that your

assembler is compatible with this core.

The assembler processes the source program line by line. A line of the

source program can be translated into a machine instruction, or generate

an element or elements of data to be placed in memory; or the line may

only provide information to the assembler itself. The lines of the source

program are sometimes referred to as source statements.

Regardless of the use of a particular line of the source program, the

format of each line is relatively standard. The general format of a source

line consists of four fields, as follows:

[<label>] <operation> [<operand>] [<comment>]

Not all of the four fields must appear on all lines; brackets [ ] have been

used to indicate fields that are optional. The comment field is always

optional, but the label and/or operand fields may be required depending

on the contents of the operation field. Unless a source line consists solely

of a comment, the operation field is required. If a line is to consist only

of a comment, the first Character on the line must be an asterisk (°). This

must appear in the leftmost position on the line, column 1. The remainder

of the line is ignored by the assembler.

A field consists of one or more tokens. A token is the smallest mean

ingful unit of information that the assembler uses. Tokens are identifiers

or numeric constants. The symbolic names of the machine instructions

are an example of identifiers. The fields of a source line are separated by

one or more spaces. All 68000 assemblers recognize the space character

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Motorola M68000 User's Guide Page 43