8-bit Divide: Difference between revisions
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(Fix a couple bugs in the code (A wasn't zeroed before entering loop, and was missing a SEC instruction)) |
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The following code divide two 8-bit integers (range 0...255) and outputs a 16-bit result using | 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. | ||
<pre> | <pre> | ||
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;by Bregalad | ;by Bregalad | ||
;Enter with A = Dividend, Y=Divisor | ;Enter with A = Dividend, Y=Divisor | ||
;Output with | ;Output with YX = (A/Y) << 8, A = remainder | ||
Division | Division | ||
sta Dvd ;Stores dividend | |||
sty Dvs | sty Dvs ;Stores divisor | ||
lda #$00 | lda #$00 ;A must be zero before entering the loop below. | ||
sta | 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 | - asl Dvd | ||
rol A ;Shift | rol A ;Shift in 1 bit of dividend | ||
cmp Dvs | bcs + ;If carry is set, the dividend is already greater | ||
bcc + | cmp Dvs ;Check if fractional dividend is greater than divisor | ||
bcc ++ | |||
+ rol Res | + sbc Dvs ;Subtract (C is always set) | ||
rol | sec ;Necessary if we reached here via the `bcs +` above | ||
++ rol Res ;Shift result (1 if subtraction was done, 0 otherwise) | |||
rol ResHi | |||
bcc - | |||
ldy | ldx Res | ||
ldy ResHi | |||
rts | rts | ||
</pre> | |||
[[Category:Arithmetic]] | |||
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
