Re: 6502 Delay Loops using Applesoft

[Allen Bong <allenbsf6502@gmail.com>]

On Dec 3, 1:12 am, sc...@alfter.DIESPAMMERSDIE.us (Scott Alfter)
wrote:
> In article <02b1803a-1471-40cb-a69f-37dbab581...@o23g2000prh.googlegroups.com>,
> Allen Bong  <allenbsf6...@gmail.com> wrote:
>
> >Has anyone written a gereral delay loop calculating program in
> >Applesoft, to calculate the constants required to gererate the
> >required time delays in 6502 codes?
>
> The handful of times I've needed timing-critical delays (in an audio player
> way back in the day, and more recently in some hardware bit-banging code),
> it was easy enough to do them manually.  The 6502 datasheet tells you how
> many cycles each instruction needs to execute:
>
> http://archive.6502.org/datasheets/rockwell_r650x_r651x.pdf
> (go to page 10; it's the number in the lower right corner for each
> instruction)
>
> The Apple II runs at approximately 1 MHz (it's actually a smidge faster than
> that), so one cycle takes about one microsecond.  The shortest run time for
> any instruction is two cycles, or about two microseconds.  For short delays
> (small multiples of 2 us), a block of NOPs will do.  For longer delays, the
> math to calculate how long a loop will take is fairly simple.  Consider
> this:
>
>         LDY #10
> ]1      DEY
>         BNE [1
>
> The first two instructions take 2 cycles each.  The branch takes 3 cycles if
> the branch is taken to the same page, 4 if the branch is taken to a
> different page, or 2 if the branch isn't taken.  Assuming that the code is
> all on the same page (which it will be most of the time, but you'd want to
> verify where the assembler decides to place your code), LDY executes once,
> DEY executes 10 times, and BNE is taken nine times and not taken once.
>
>     2 LDY
>  10*2 DEY
>   9*3 BNE taken
> +   2 BNE not taken
> =====
>    51 cycles total
>
> For nested loops, you'd want to calculate the runtime for the inner loop and
> plug that into your calculation for the outer loop.
>
> I suppose if I had a regular need for calculating delay loops, I might've
> knocked together an app to do that.  As shown above, though, they're simple
> enough to put together by hand as you need them.
>
>   _/_
>  / v \ Scott Alfter (remove the obvious to send mail)
> (IIGS(http://alfter.us/ ;          Top-posting!
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>
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Hi Scott,

Actually, I was thinking something of a more general purpose form of
delay-loop calculator.  Not just for use by Apple 2 only.  For example
you might design a 6502 SBC running at 1.5MHz or 2MHz using 6502A or
6502B or 65C02 (CMOS).

Something like this one for the PIC would be most helpful.  Just enter
the oscillator frequency used by 6502, how much delay time you wanted
in uS, mS or S, then just press "calculate", and a subroutine is
produced.  Very cool, isn't?

Like the ones here:

http://www.golovchenko.org/cgi-bin/delay

http://www.biltronix.com/picloops.html

I don't expect to have the convenient of that but at least one written
in Applesoft would be highly appreciated.  If no one had done that, I
might try to roll one out myself in Applesoft.

Allen