Re: 6502 illegal opcodes questions

["Michael J. Mahon" <mjmahon@aol.com>]

John Selck wrote:
> Am 18.05.2006, 16:14 Uhr, schrieb Paul Schlyter <pausch@saaf.se>:
>
> > Could you point at one single case where use of undocumented opcodes did
> > significantly speed up an application on the 6502 platform?
>
>
> Bresenham interpolation.
>
> Without illegal opcodes:
>
> STA $12
> TXA
> STA $FE00,Y
> TAX
> LDA $12
> SBC $10
> BCS .skip
> ADC $11
> INX
> ..skip
>
> With illegal opcodes:
>
> SHX $FE00,Y
> SBC $10
> BCS .skip
> ADC $11
> INX
> ..skip
>
> Best case: 21 vs 11 clock cycles (almost 2x speed with illegal)
> Worst case: 25 vs 15 clock cycles (still 1.67x speed)

I note that the TAX in the first case is unnecessary, trimming the
best case advantage to 19 vs 11 cycles (1.7x) and the worst case
advantage to 23 vs 15 cycles (1.5x).

-michael

Parallel computing for 8-bit Apple II's!
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