/*
 * Write a subroutine named Div8 to divide a 16-bit number by an 8-bit number.  
 * Next, write a program named Div8_test to test the subroutine Div8
 * by dividing the 16-bit-number: 0xAAAA by the 8-bit-number 0x55
 *
 * Q = N/D Divide a 16-bit-number NH:NL  by an 8-bit-number Q
 *
 * Source: 
 * 1. Binary division in AVR Assembler
 *    http://www.avr-asm-tutorial.net/avr_en/calc/DIVISION.html
 * 2. Integer division (unsigned) with remainder
 *    http://en.wikipedia.org/wiki/Division_algorithm
 */

.DEF NL = r0  ; LSB 16-bit-number to be divided
.DEF NH = r1  ; MSB 16-bit-number to be divided
.DEF DIV = r3 ; 8-bit-number to divide with
.DEF QL = r4  ; LSB result
.DEF QH = r5  ; MSB result

Div8_test:
	ldi r16,0xAA ; 0xAAAA to be divided
	mov NH,r16
	mov NL,r16
	ldi r16,0x55 ; 0x55 to be divided with
	mov DIV,r16
	rcall Div8	
	rjmp Div8_test
 
 /* Div8
  * Q = N/D Divide a 16-bit-number NH:NL  by an 8-bit-number Q
  * input
  *   N = Numerator (dividend)
  *   D = Denominator (divisor)
  * output
  *   Q = Quotient 
  */
Div8:
    push r0
	push r1
	push r2

	clr r2 ; clear interim register
	clr QH ; QH:QL = 0b0000 0000 0000 0001
	clr QL
	inc QL

div8a:         ; start of the division loop
	clc        ; clear carry-bit
	rol NL     ; rotate the next-upper bit of the numerator
	rol NH     ; to the interim register (multiply by 2)
	rol r2
	brcs div8b ; a one has rolled left, so subtract
	cp r2,DIV  ; Division result 1 or 0?
	brcs div8c ; jump over subtraction, if smaller
div8b:
	sub r2,DIV ; subtract number to divide with
	sec        ; set carry-bit, result is a 1
	rjmp div8d ; jump to shift of the result bit
div8c:
	clc        ; clear carry-bit, resulting bit is a 0
div8d:
	rol QL     ; rotate carry-bit into result registers
	rol QH
	brcc div8a ; as long as zero rotate out of the result
	           ; registers QH:QL go on with the division loop
	pop r2
	pop r1
	pop r0
	ret        ; End of the division reached

program8: