Intel 8080 - B4_Page_17

Chapter S. Macros
SBMAC MACRO
SBMAC MACRO
CALL SUBR
;;REDEFINITION OF SBMAC
ENDM
CALL SUBR
LINK:
IMP
DUN
SUBR:
RET
DUN:
ENDM
Notice that both versions of SBMAC contain CALL SUBR instructions. This is necessary to provide a return
address at the end of the SUBR routine. The jump instruction labelled LINK is required to prevent the SUBR
subroutine from executing a return to itself. Notice that the return address for the second CALL SUBR
instruction would be SUBR if the jump instruction were omitted. The IMP DUN instruction simply transfers
control past the end of the subroutine.
NOTE
The assembler allows the use of a source line consisting
only of a label. Such a label is assigned to the next source
line for which code or data is generated. Notice that
neither code nor data is generated for an ENDM directive,
so the label DUN is assigned to whatever instruction follows
the
ENDM directive. This construct is required because the
ENDM directive itself may not be given a label.
Method #2: Conditional Assembly
The second method for altering the expansion of the SBMAC macro uses conditional assembly. I n this example,
a switch (FIRST) is set TRUE just before the first call for SBMAC. SBMAC is defined as follows:
TRUE
FALSE
FIRST
SBMAC
FIRST
LINK:
SUBR:
DUN:
EQU
EQU
SET
MACRO
CALL SUBR
IF
SET
JMP
RET
ENDIF
ENDM
OFFH
0
TRUE
FIRST
FALSE
DUN
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