[Coco] CoCoFEST! Challenge: CoCo DMA

[RETRO Innovations]

On 3/8/2020 7:24 PM, James Ross wrote:
> RETRO Innovations wrote:
>>>> http://www.go4retro.com/2020/02/26/direct-memory-access-possibilities-on-the-tandy-color-computer/
>>> http://www.go4retro.com/2020/02/28/coco-dma-early-efforts/
>> http://www.go4retro.com/2020/03/05/coco-dma-fighting-on-the-bus/
> What a fantastic project and series of articles. I scanned over them and read bits and pieces. It is quite a bit over my head still, but as time permits; I want to re-read these word for word and see if I can learn more.
>
> I was under the impression that from the CoCo’s cartridge slot only the CoCo1/2 allow DMA to be performed, not a stock CoCo3? I got that from the time that John Kowalski was on CoCoTalk and he was talking about a cart he had designed (or was designing) which was quite interesting.
>
> - JR
>
You understand correctly.  The CC3 includes a buffer IC between the CPU 
and the data bus.  Normally, this would be of no concern, since we don't 
care about the CPU during a DMA.  However, the two way buffer IC puts 
*itself* on the data bus during a write (thinking that the CPU needs to 
write some data to memory or I/O.  But, in a DMA situation, the CPU (and 
all buffers associated with it) need to be off the bus. John considered 
a way to put the data on the data bus, and then strobe the R/W to !Write 
at the very end of the bus cycle, in hopes of the RAM seeing the DATA 
and the R/W line being low, but not the data coming from the buffer IC 
dropping itself on the bus.  I see the point, but I doubtful (as is 
John) such an idea is workable.  That said, lots of folks are modifying 
their CC3, so a hardware mod to enable this functionality might not be a 
huge deal to folks.

Jim

-- 
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