Re: DD2 & 3

From: silverdr_at_wfmh.org.pl
Date: Tue, 18 Jun 2013 19:12:24 +0200
Message-Id: <FAE7257B-5CF1-4648-82BD-6AA3330EB1D1@wfmh.org.pl>
On 2013-06-18, at 17:29, brain@jbrain.com wrote:

> 
> > On June 18, 2013 at 7:02 AM silverdr@wfmh.org.pl wrote: 
> > 
> > 
> > I mostly completed both DolphinDOS 2 and 3 remake pages: 
> > 
> > http://e4aws.silverdr.com/hard_projects/dolphindos2/ 
> > http://e4aws.silverdr.com/hard_projects/dolphindos3/
>  
> As Julian notes, kudos for all the hard work.

Thanks, Jim.

> My question(s):
>  
> I don't understand the need for the 6821 on DD3.  I mean, yes, I understand the need for a parallel port, but would it have been that hard to put the bit on a buffer and just turn if off when DD was on and parallel access was in use?

As I wrote there - for the pure 1541 use, whichever version ever appeared on the market eventually - it seems over-engineered to me no matter how I look at it ;-) See the lower address lines for example. DD2 gets along fine without touching anything below A11. In DD3 we have four additional gates to get A1 to A4 tapped on, leaving A0 untouched... I bet it would be much cheaper if the missing bit was put on a buffer [*]. For real reasons we would have to ask the creators themselves (I haven't found them on facebook yet ;-) but my best guess is that they wanted to be farsighted and less vulnerable to the whims of CBM than they were with the original design. Therefore they might have asked themselves the question "what if tomorrow Commodore takes more bits out or something?" "It's quite unlikely that they get rid of 6502 though so let's depend only on this one" - my guesses and speculations. Another thing is that they probably wanted the design to be applicable to many target devices without redesigning. No matter how laid out the controller's PCB is and whether there is a free port (be it VIA or CIA or whatever else) or not.

* - I am wondering what would it be if head movement in a 1541C for example would be IRQ driven (using DOS routines) while data transfer would be running in the main "thread"...

-- 
SD!


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Received on 2013-06-18 18:00:08

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