[Coco] Some may call this heresy

[CoCo Demus]

I meant it generates a read to DF76 before the write to DF76. I know it
needs to read from VAR, but VAR is zero page.

On Mon, May 28, 2018 at 11:48 PM, CoCo Demus <retrocanada76 at gmail.com>
wrote:

> 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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>>
>
>