Each source statement may include up to four fields: a label (or "*" for a comment line), an operation (instruction mnemonic or assembler directive), an operand, and a comment.

Label FieldThe label field occurs as the first field of a source statement. The label field can take one of the following forms:

1. A white space character (blank or tab) as the first character indicates that the label field is empty. The line has no label and is not a comment.
2. A symbol character as the first character indicates that the line has a label. Symbol characters are the upper or lower case letters a- z, digits 0-9, and the special characters, period (.), dollar sign ($), and underscore (_). Symbols consist of one to 15 characters, the first of which must be alphabetic or the special characters period (.) or underscore (_). All characters are significant and upper and lower case letters are distinct.

Operation FieldUpper case characters in this field are converted to lower case before being checked as a legal mnemonic.

Operand Field^Operand Format^M6800/M6801 Addressing Mode^ |no operand|accumulator and inherent| |<expression>|direct, extended, or relative| |#<expression>|immediate| |<expression>,X|indexed|