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52 Assembly Language Programming for the 68000 Family

A symbol in the label field can be made to equal a numeric constant.

Anywhere this symbol appears in your program it is interpreted as if you

wrote the constant itself. For example, you could define the symbol MAX

to represent the constant 1000.

A symbol in the label field can also be used to specify the memory

location of an instruction. This is a true label. Although the first field is

called the label field, only the symbols that are present in the label field

of a source line that translates into a memory location are the true labels

of a 68000 assembly language program. Unless a label starts in column

1, it must be delimited with a semicolon. In the latter case, the label can

start in any column as long as it is the first thing on the line.

The Operation Field

The operation field contains either a machine instruction or an as

sembler directive. Each machine instruction has a special symbol or

mnemonic associated with it. If a particular machine instruction is de

sired, the proper mnemonic must be placed in the operation field. As

sembler directives have symbolic names that are different from the ma

chine instructions. The assembler is thus able to differentiate between a

machine instruction and a directive.

If a machine instruction is placed in the operation field, the assembler

will generate the appropriate words to be placed in memory correspond

ing to the translation of the source statement. Assembler directives may or

may not generate bytes to be stored in memory. Some directives merely

control the format of the assembly listing, or provide other information

about the program. Directives are also used to define symbols.

The Operand Field

Many machine instructions as well as assembler directives require one

or more operands. The operand field is used to provide these operands.

Individual operands can consist of constants, variables, or special sym

bols. Expressions made up of constants, variables, and special symbols

are also permitted. The rules for making up expressions vary slightly from

assembler to assembler. Standard arithmetic expressions such as

COUNT+5

1 2 ... 41 42 43 44 45 46 47 48 49 50 51 ... 255 256

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Motorola M68000 Guida Utente Pagina 46