Atmel AVR Microcontroller Basic Math Code Library

Divide 8 by 8 in software by http://www.avr-asm-tutorial.net

;
; Registers
;
.DEF rd1l = R0 ; LSB 16-bit-number to be divided
.DEF rd1h = R1 ; MSB 16-bit-number to be divided
.DEF rd1u = R2 ; interim register
.DEF rd2  = R3 ; 8-bit-number to divide with
.DEF rel  = R4 ; LSB result
.DEF reh  = R5 ; MSB result
.DEF rmp  = R16; multipurpose register for loading
;
.CSEG
.ORG 0
;
	rjmp start
;
start:
;
; Load the test numbers to the appropriate registers
;
	ldi rmp,0xAA ; 0xAAAA to be divided
	mov rd1h,rmp
	mov rd1l,rmp
	ldi rmp,0x55 ; 0x55 to be divided with
	mov rd2,rmp
;
; Divide rd1h:rd1l by rd2
;
div8:
	clr rd1u ; clear interim register
	clr reh  ; clear result (the result registers
	clr rel  ; are also used to count to 16 for the
	inc rel  ; division steps, is set to 1 at start)
;
; Here the division loop starts
;
div8a:
	clc      ; clear carry-bit
	rol rd1l ; rotate the next-upper bit of the number
	rol rd1h ; to the interim register (multiply by 2)
	rol rd1u
	brcs div8b ; a one has rolled left, so subtract
	cp rd1u,rd2 ; Division result 1 or 0?
	brcs div8c  ; jump over subtraction, if smaller
div8b:
	sub rd1u,rd2; 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 rel  ; rotate carry-bit into result registers
	rol reh
	brcc div8a  ; as long as zero rotate out of the result
	            ; registers: go on with the division loop
; End of the division reached
stop:
	rjmp stop   ; endless loop

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