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* DEMO.ASM
* This simple program adds the contents of 
* address $0000 (labeled VAR0) to the contents of 
* address $0001 (labeled VAR1) and stores the resulting 
* sum at address $0002 (labeled SUM), provided 
* the addition process happens without overflow.

* If an overflow occurs during the addition process, 
* the overflow flag OVERFL (stored at address $0003) 
* is set to $FF; else, it stays $00. 

* Include definition of variables for MC68HC11
#INCLUDE	'A:\VAR_DEF.ASM'

* Define program variables
	ORG	DATA
VAR0	RMB	1	;reserve 1 byte for VAR0
VAR1	RMB	1	;reserve 1 byte for VAR1
SUM	RMB	1	;reserve 1 byte for sum
OVERFL	RMB	1	;reserve 1 byte for overflow flag

* Start main program
	ORG	PROGRAM
	LDAB	#00	;assume no overflow (optimistic!)
	LDAA	VAR0	;load VAR0 in accumulator A
	ADDA	VAR1	;add VAR1 to accumulator A
	BVC	LABEL1	;jump if no overflow
* We have overflow! 
	COMB		;Invert accumulator B ($00 to $FF)
LABEL1	STAA	SUM	;store result of addition
	STAB	OVERFL	;store accB into overflow flag
	SWI		;stop the microcontroller