Devider_16bit_by_8bit

                              ; Div8 divides a 16-bit-number by a 8-bit-number
                              ; Test: 16-bit-number: 0xAAAA, 8-bit-number: 0x55
                              ;
.NOLIST
.INCLUDE "8515def.inc"
.LIST
                              ;
                              ; 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