A251 User's Guide

Macro Processor

A macro processor enables you to define and to use macros in your assembly programs. When you define a macro, you provide text (usually assembly code) that you want to associate with a macro name. Then, when you want to include the macro text in your assembly program, you provide the name of the macro. The assembler replaces the macro name with the text specified in the macro definition.

Macros provide several advantages when writing assembly programs.

• The frequent use of macros can reduce programmer-induced errors.
  A macro allows you to define instruction sequences that are used repetitively throughout your program. Subsequent use of the macro faithfully provides the same results each time. A macro reduces the likelihood of errors introduced in repetitive programming sequences. Of course, introduction of an error into a macro definition causes that error to propagate through the program wherever the macro is used.
• The scope of symbols used in a macro is limited to that macro. You need not be concerned about using a previously used symbol name.
• Macros are well-suited for creating simple code tables. Producing tables by hand is both tedious and error-prone.

A macro may be thought of as a subroutine call with the exception that the code in the subroutine is included in–line at the point of the macro call. However, macros should not be used to replace subroutines.

Each invocation of a subroutine requires only the code necessary to call the subroutine. Each invocation of a macro includes the assembly code associated with the macro in–line in the assembly program. This can cause a program's size to grow rapidly if a large macro is used frequently.

In a static environment, a subroutine is a better choice since program size can be considerably reduced. But in time-critical, dynamic programs, macros speed the execution of algorithms or other frequently-called statements without the penalty of the procedure calling overhead.

Use the following guidelines when deciding between macros or subroutines:

• Subroutines are best when certain procedures are frequently executed or when the use of memory must be kept to a minimum.
• Macros are best when maximum processor speed is required and when the amount of memory consumed is less important.
• Macros reduce the amount of typing required to enter short, repetitive blocks of assembly code.

The A251 Macro Assembler provides three different macro languages:

• Standard Assembler Macros are similar to many other macro assemblers. They allow you to define macros that look like standard assembler instructions.
• MPL Macros are compatible with the Intel ASM-251 Assembler. They allow you to retranslate existing source files that were initially written for this macro assembler.
• C Preprocessor Macros are compatible with the C preprocessor. They allow you to use the same include files in your C and assembler source code.

The following table lists the assembler directives that may be used to enable or disable macro processing.