/* Given 8-bit variables A and B, each holding
 * an 8-bit signed 2's complement number. Write
 * a program to find the average of A and B.
 * Place the result into variable C.
 */

.INCLUDE <m328pdef.inc> 

.DSEG
A:     .BYTE 1
B:     .BYTE 1
C:     .BYTE 2

.CSEG

; inputs: 8-bit variables A and B
; output: 16-bit register C
Avg8s:
    ; load registers A and B
	lds  r24,A
	lds  r26,B
	; find average C = A+B/2
	rcall Adder816s		; C=A+B
	;   divide by 2
	asr r25             ; least significant bit moved to carry bit C
	ror r24		    	; carry moves into most significant bit of r24
	; store the 8 bit result
	sts  C,r24     	
	clr  r25    
	sts C+1,r25	
	rjmp Avg8s

; Add two 8-bit signed 2's complement numbers,
; where sum of A and B may be 9 bits
; input: r24 and r26 are two 8-bit numbers
; output: register pair r25:r24 equals sum of r24 and r25
Adder816s:
	; make variables 16-bit
	clr  r25      	 	; guess r25 is positive 0x00:A
	sbrc r24,7          ; if number is positive guess is correct so skip next instruction
	ser  r25            ; guess incorrect, number is negative 0xFF:A
	clr  r27
	sbrc r26,7
	ser  r27
	;add
	add  r24,r26
	adc  r25,r27
	;store
	sts  C,r24     		; store the least significant byte
	sts  C+1,r25  		; store most significant bytes               
	ret