Page 152 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

5,4 Macro Assemblers 129

ADD: MACRO
LDX \1
LDAB \2
CLRA
\@: ADDA 1,X+
DBNE B, \ @
ENDM
Figure 5.6. Loop Macro to Add Consecutive Values
letters to generate unique labels inside the macro itself. This capability is especially
useful if a macro expansion has two or more "goto" labels in it; for without it,
unambiguous labels could not be generated. Using the macro invocation number, each
macro expansion generates labels that are different from the labels generated from the
same macro that are expanded at a different time. For example, the macro in Figure 5.6,
when implemented by ADD #M,N, adds the contents of the N bytes beginning in
location M, putting the result in accumulator A.
A macro definition can use a macro defined earlier, or even itself (recursively). For
macro assemblers that have conditional assembly, conditional directives within their
definition can be used to control the expansion. The actual parameters of the macro can
then be tested with these directives to determine how the macro is expanded. In particular,
the IFC and IFNC directives can be used to compare a macro parameter, considered as a
string, against any constant string. If a parameter is missing, it is a null string "\0." We
can compare a given parameter, such as the second parameter, considered as a string
denoted "\2," to a null string "\0." When the strings are equal, the second parameter is
missing, so this condition can terminate the macro expansion.
The example in Figure 5.7 illustrates the use of recursion, conditional assembly,
and early exiting of macros using MEXIT. You might want to use the ADDA instruction
with a series of arguments, to add several bytes to accumulator A. If you wish, you can
use a text editor to make copies of the ADDA instruction with different arguments.
However, you can define a macro whose name is ADDQ, in which the body of the macro
expands into one or more ADDA directives to implement the same effect. This ADDQ
macro uses recursion to add one parameter at a time, up to eight parameters in this
simple example, stopping when a parameter is missing (null string). When a null
(missing) parameter is encountered, the macro "exits" by executing MEXIT, thereby not
generating any more expansion or code.
ADDQ: MACRO
IFNC "\1",""
ADDA \1
ENDC
IFC "\2",""
MEXIT
ENDC
ADDQ \2,\3,\4,\5,\6,\7,\8
ENDM
Figure 5.7. Recursive Macro to Add up to Eight Values