[Coco] Some may call this heresy

[CoCo Demus]

It's a problem because when I write to address DF76 it may or not issue a
reading before the write. And the 6545 CRTC has an auto-increment feature.
So writing to it woul jump(or not) one address each time making it
impossible to use.

more strangely i found out that STA $DF76 will likely not generate the read
but a STA (VAR),X where *VAR = DF76 will certainly do (not always again)

So using the ROM made it 100% reliable. I may sometimes get a garbage byte
at the boot but this is the same that causes the hang.



On Mon, May 28, 2018 at 9:45 PM, RETRO Innovations <go4retro at go4retro.com>
wrote:

> On 5/28/2018 11:27 PM, CoCo Demus wrote:
>
>> Nope, the 6510 generates a read before a write and I don't understand why.
>> I couldn't get a reliable read/write on same port so I messed around using
>> a ROM chip to decode:
>>
>> IO2|R/W|A7-A0
>>
> Maybe it's the RMW behavior of the 6502:
>
> http://forum.6502.org/viewtopic.php?t=2
>
> Why is the read before the write a problem?
>
>
>
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