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AARM User's Guide
AARM Introduction Assembling Programs Operators Control Statements 
Macro Processor Standard Macro Processor 
Standard Macro Controls Defining Standard Macros Calling Standard Macros Local Symbols Exiting A Macro Built-in Macros REPT IRP IRPC Standard Macro Operators NUL Operator & Operator % Operator ;; Operator ! Operator C Preprocessor Exceptions Side-Effects Predefined Macros Examples Error MessagesNUL Operator
When a formal parameter is omitted from a macro invocation, it is assigned a value of NULL. You can check for NULL parameters by using the NUL operator in an IF control statement in the macro.
The NUL operator requires an argument. If no argument is found, NUL returns a value of 0.
For example:
EXAMPLE MACRO X
IF NUL X
EXITM
ENDIF
NOP
ENDM
EXAMPLE ; Invoke the macro with no parameters
;Nothing is output since the macro exits (because the x parameter is NULL)
Note
- A blank character (' ') has an ASCII value of 20h and is not equivalent to a NULL.
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