Intel 8080 - B3_Page_32

Chapter 4. Assembler Directives
Subdividing data as shown in the example simplifies data access and provides useful documentation throughout
your program. Notice that these EQU directives can be inserted anywhere within the program as you need them,
but coding them as shown in the example provides a more useful record description.
CONDITIONAL ASSEMBLY
The IF, ELSE, and ENDIF directives enable you to assemble portions of your program conditionally, that is,
only if certain cond itions th,lt you specify are sa tisfied.
Conditional assembly
is especially useful when your application requires custom programs for a number of com-
mon options.
As an example, assume that a basic control program requires customizing to accept input from
one
of six different sensing devices and to drive one of five different control devices. Rather than code some
thirty separate programs
to account for all the possibilities, you can code a single program. The code for the in-
dividual sensors and drivers must be enclosed by the conditional directives. When you need to generate a custom
program,
you can insert SET directives near the beginning of the source program to select the desired sensor and
driver routines.
IF, !ELSE, ENDIF Directives
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Because these directives are used in conjunction, they are described together here.
Label Opcode
Operand
optional: IF
expression
optional:
ELSE
optional:
ENDIF
The assembler evaluates the expression
in the operand field of the IF directive. If bit 0 of the resulting value is
one (TRUE), all instruction'; between the IF directive and the next ELSE or ENDIF directive are assembled.
When bit
0 is zero (FALSE) these instructions are ignored. (A TRUE expression evaluates to OFFFFH and
FALSE to
OH; only bit zero need be tested.)
All statements included between an IF directive and its required associated ENDIF directive are defined as an
IF·ENDIF block. The ELSE directive is optional, and only one ELSE directive may appear in an IF-ENDIF
block. When included, ELSE
is the converse of IF. When bit 0 of the expression in the IF directive is zero, all
statements between ELSE and the next ENDIF are assembled. If bit 0 is one, these statements are ignored.
Operands are not allowed with the ELSE and ENDIF directives.
An IF-ENDIF block may appear within another IF·ENDIF block. These blocks can be nested to eight levels.
Macro definitions (explained
in the next chapter) may appear within an IF·ENDIF block. Conversely, IF-ENDIF
blocks may appear within macro definitions.
In either case, you must be certain to terminate the macro definition