; 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