8-bit Divide
From Nesdev wiki
Jump to navigationJump to searchThe following code divide two 8-bit integers (range 0...255) and outputs a 16-bit result in fixed point 8.8 format. It is only using real calculations, no lookup table, thus the size of the code is very small.
;8-bit divide
;by Bregalad
;Enter with A = Dividend, Y=Divisor
;Output with YX = (A/Y) << 8, A = remainder
Division
sta Dvd ;Stores dividend
sty Dvs ;Stores divisor
ldy #$00
sty ResHi ;Clear result, setting up a ring counter
iny ;by initializing the result to $0001.
sty Res ;When the 1 gets shifted out, we're done
ldy #$10 ;The loop is for 16-bit result
- asl Dvd
rol A ;Shift divisor in 1 bit
bcs + ;If carry is set, the dividend is already greater
cmp Dvs ;Check if fractional dividend is greater than divisor
bcc ++
+ sbc Dvs ;Subtract (C is always set)
++ rol Res ;Shift result (1 if subtraction was done, 0 otherwise)
rol ResHi
bcc -
ldx Res
ldy ResHi
rts