[vcf-midatlantic] Original Macintosh Architecture Questions

John Heritage john.heritage at gmail.com
Fri Jun 25 13:01:50 UTC 2021

Wow!  Good articles and thanks David!  Since you mentioned you loved the
original hardware design of the Mac, I hope you won't mind a few more

Interesting on the IWM making it to the Mac,  I knew that was a really
innovative chip on the Apple II, but didn't realize it also was used in
early Macs.  What was the reason the PC used 9 sectors per track while the
Mac and Amiga used 10/11 sectors per track each on the 3.5" floppies?  (I
assume the ST defaulted to 9 sectors per track to stay compatible with DOS
disks..)  Was it because the PC technically predated the original Mac and
Amiga/ST and the early drives/disks were risky with higher densities?  Did
Apple trade anything off for 400KB floppies in the Mac?

Hardware wise, is it fair to say that the Mac probably cost less to
manufacture in ~ 1985 than say the Amiga 1000 or Atari 520ST?    The Amiga
had several custom chips, and even the ST seemed to have more ICs on board,
though monitors were optional for both.  (I think in 1984 RAM was still
very expensive so I can see why the 128KB Mac would be high, especially as
an early adopter premium).   I always assumed the Mac premium was due to
it's software library and 'brand' (at least in the US) at the time, but
curious if there were any hardware reasons for the cost.

Last question -- were there 'fast ram' upgrades for the original Macs (say
pre-1990) that would allow the 68000 access to local memory for faster
execution than RAM on the shared bus?  or did all RAM have to be shared?
 It looks like the ram size limitations on the early Macs were relatively
close to what the 68000 could do..


On Thu, Jun 24, 2021 at 10:38 PM David Riley <fraveydank at gmail.com> wrote:

> On Jun 24, 2021, at 10:31 PM, David Riley <fraveydank at gmail.com> wrote:
> >
> > On Jun 24, 2021, at 4:05 PM, John Heritage <john.heritage at gmail.com>
> wrote:
> >> Last question on the Mac Audio --
> >>
> >> Is it fair to say that if you wanted audio on the Mac, you basically
> wanted to digitally sample a sound/take a sound sample and then have the
> CPU shape it so it would output correctly?  (i.e. costing some CPU cycles)
> >
> > You could say that, but it would probably be more accurate to say that
> when the Mac came around, not many other machines had real sampled audio
> out, so the sound quality was pretty good for the time already.  But it
> wasn't perfect, as related in this excellent story from Andy Hertzfeld's
> recollections of the development of the original Mac:
> https://www.folklore.org/StoryView.py?project=Macintosh&story=Boot_Beep.txt
> >
> > In any case, any application with high quality audio for the time (here
> I'm thinking something like Dark Castle, which actually has really great
> sound effects) probably either just lived with the iffy quality or
> pre-equalized the audio, but given that Dark Castle sounds pretty similar
> on a machine with a real sound chip like something from the II series, I
> doubt it makes that much of a difference. Keep in mind that a PWM through a
> properly tuned RC filter isn't going to sound a whole lot different from an
> actual DAC (again, delta-sigma DACs work basically on this principle, just
> a whole lot faster).
> >
> > That site, by the way, is replete with stories you won't find anywhere
> else (aside from Andy's book, which is the contents of the site plus a few
> more stories and lots of pictures).  I really loved reading through it.
> Also, speaking of, this is another great story from the same source about
> the development of the sound capabilities on the original Mac, including
> some details about the audio fetch during horizontal blanking:
> https://www.folklore.org/StoryView.py?project=Macintosh&story=Sound_By_Monday.txt
> - Dave

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