Re: C64 Expansion Port IO1 and IO2 and PEEK and POKE

From: Leif Bloomquist <leif_at_schemafactor.com>
Date: Mon, 9 May 2016 15:57:43 -0400
Message-ID: <CAOTCz+HdKHKYCUa184Utmq_FO2+kRbOmCSfCsT2jujY_-mwAEA@mail.gmail.com>
Thanks.  This is a new topic for me - what's a dummy read?  Something
that happens when doing an STA on a referenced address?

The VICE monitor has come a long way since I last used it!   Usually
on real hardware.  :-P

-Leif


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On Mon, May 9, 2016 at 1:51 PM, Kajtár Zsolt <soci@c64.rulez.org> wrote:
>> you can use VICE to examine what happens:
>>
>> go to monitor and put a tracepoint on $df00:
>>
>> (C:$e5cf) trace df00
>> TRACE: 2  C:$df00  (Trace load store)
>> (C:$e5cf) x
>>
>> ...leave monitor, do poke 57088,123 and enter the monitor again, you see this:
>>
>> #2 (Trace  load df00)  213 049
>> .C:b82a  91 14       STA ($14),Y    - A:37 X:37 Y:00 SP:f9 ..-...ZC   82195204
>> #2 (Trace store df00)  213 049
>> .C:b82a  91 14       STA ($14),Y    - A:37 X:37 Y:00 SP:f9 ..-...ZC   82195204
>>
>> so, what you are seeing is the dummy load that happens before the actual store
>> :)
>
> Apart from the wrong cycle numbers it's just the regular dummy read for fixing
> up the high part of address after indexing. It's expected as it's not optional
> for a write.
>
> --
>                                                     -soci-
>

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Received on 2016-05-09 20:00:02

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