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MACROS

The macro processing facility (MPL) of ASM51 is a "string replacement" facility. Macros allow frequently used

sections of code be defined once using a simple mnemonic and used anywhere in the program by inserting the

mnemonic. Programming using macros is a powerful extension of the techniques described thus far. Macros can

be defined anywhere in a source program and subsequently used like any other instruction. The syntax for macro

definition is

%*DEFINE

(call_pattern)

(macro_body)

Once defined, the call pattern is like a mnemonic; it may be used like any assembly language instruction by

placing it in the mnemonic field of a program. Macros are made distinct from "real" instructions by preceding

them with a percent sign, "%". When the source program is assembled, everything within the macro-body, on

a character-by-character basis, is substituted for the call-pattern. The mystique of macros is largely unfounded.

They provide a simple means for replacing cumbersome instruction patterns with primitive, easy-to-remember

mnemonics. The substitution, we reiterate, is on a character-by-character basis—nothing more, nothing less.

For example, if the following macro definition appears at the beginning of a source file,

%*DEFINE

(PUSH_DPTR)

(PUSH

DPH

PUSH

DPL

)

then the statement

%PUSH_DPTR

will appear in the .LST file as

PUSH

DPH

PUSH

DPL

The example above is a typical macro. Since the 8051 stack instructions operate only on direct addresses,

pushing the data pointer requires two PUSH instructions. A similar macro can be created to POP the data pointer.

There are several distinct advantages in using macros:

A source program using macros is more readable, since the macro mnemonic is generally more indicative

of the intended operation than the equivalent assembler instructions.

The source program is shorter and requires less typing.

Using macros reduces bugs

Using macros frees the programmer from dealing with low-level details.

The last two points above are related. Once a macro is written and debugged, it is used freely without the worry

of bugs. In the PUSH_DPTR example above, if PUSH and POP instructions are used rather than push and pop

macros, the programmer may inadvertently reverse the order of the pushes or pops. (Was it the high-byte or low-

byte that was pushed first?) This would create a bug. Using macros, however, the details are worked out once—

when the macro is written—and the macro is used freely thereafter, without the worry of bugs.

Since the replacement is on a character-by-character basis, the macro definition should be carefully

constructed with carriage returns, tabs, ect., to ensure proper alignment of the macro statements with the rest of

the assembly language program. Some trial and error is required.

There are advanced features of ASM51's macro-processing facility that allow for parameter passing, local

labels, repeat operations, assembly flow control, and so on. These are discussed below.

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