8-bit Divide: Difference between revisions
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(when the rol instruction itself produces a carry, subtract immediately; ring counter to save 15 deys; remainder) |
(Fix a couple bugs in the code (A wasn't zeroed before entering loop, and was missing a SEC instruction)) |
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| Line 9: | Line 9: | ||
sta Dvd ;Stores dividend | sta Dvd ;Stores dividend | ||
sty Dvs ;Stores divisor | sty Dvs ;Stores divisor | ||
lda #$00 ;A must be zero before entering the loop below. | |||
sta ResHi ;Clear result, setting up a ring counter | |||
ldy #$01 ;by initializing the result to $0001. | |||
sty Res ;When the 1 gets shifted out, we're done | sty Res ;When the 1 gets shifted out, we're done | ||
- asl Dvd | - asl Dvd | ||
rol A ;Shift | rol A ;Shift in 1 bit of dividend | ||
bcs + ;If carry is set, the dividend is already greater | bcs + ;If carry is set, the dividend is already greater | ||
cmp Dvs ;Check if fractional dividend is greater than divisor | cmp Dvs ;Check if fractional dividend is greater than divisor | ||
bcc ++ | bcc ++ | ||
+ sbc Dvs ;Subtract (C is always set) | + sbc Dvs ;Subtract (C is always set) | ||
sec ;Necessary if we reached here via the `bcs +` above | |||
++ rol Res ;Shift result (1 if subtraction was done, 0 otherwise) | ++ rol Res ;Shift result (1 if subtraction was done, 0 otherwise) | ||
rol ResHi | rol ResHi | ||
Latest revision as of 20:27, 21 January 2024
The 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
lda #$00 ;A must be zero before entering the loop below.
sta ResHi ;Clear result, setting up a ring counter
ldy #$01 ;by initializing the result to $0001.
sty Res ;When the 1 gets shifted out, we're done
- asl Dvd
rol A ;Shift in 1 bit of dividend
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)
sec ;Necessary if we reached here via the `bcs +` above
++ rol Res ;Shift result (1 if subtraction was done, 0 otherwise)
rol ResHi
bcc -
ldx Res
ldy ResHi
rts
