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From: leb@merk.UUCP (Larry Bakst)
Newsgroups: comp.sys.mac.hardware
Subject: Re: New Mac Frustrations
Message-ID: <4085@merk.UUCP>
Date: 16 Nov 90 08:24:11 GMT
References: <1990Nov9.172342.7363@umiami.ir.miami.edu> <1990Nov14.225709.13353@silvlis.com> <9236@latcs1.oz.au>
Reply-To: leb@merk.UUCP (Larry Bakst)
Organization: InsideOut
Lines: 47

In article <9236@latcs1.oz.au> jtc@latcs1.oz.au (John Catsoulis) writes:
>Since WHEN did a Mac EVER have an 8 bit data bus? It makes very little sense
>to give the 020 a 16 bit bus. Ok, so you don't need to add as much memory when
>you upgrade (16 bits wide as opposed to 32), but is this really warranted? Your
>going to buy as much memory as you want/need anyway, so why restrict it to
>being 16 bits wide. Sorry, but I don't like this AT ALL.

The effects of going from a 32 bit to 16 bit internal buses can be subtle.
The V8 gate array chip which is at the heart of the LC only has a 16 bit data
bus and that reduces the cost of the chip. If the design was fully 32 bit than
you would be forced to use 8 instead of 4 vrams chips which would again add
to the cost of the design. Other secondary factors is going from 16 to 32 bits
would be 2 more 74LS245's as well as more board space, power, and heat. The
data path to the ROM and the 96 expansion connector is still 32 bits wide.

>On another note, why did they use a '000 in the Classic rather than an 010?
>The difference in price is negligable (on the order of a few dollars), yet the
>010 can support (without getting into an argument over it) virtual memory and
>can therefore support this aspect of sys7. I'd be particularly interested in 
>comments from Apple people on this last one.

While it is true that the both 68010 and 68012 can rerun instructions neither
chip contains has an MMU.  In order to support the virtual memory features of 
System 7.0 Apple would have had to put an MMU into the classic. If you assume
that to use existing ROMS the MMU would have to be 68851 (PMMU) or 68030 (PMMU 
subset) compatible, then a custom MMU chip would have to be designed.
Having been involved in doing this when I worked at Alliant Computer I can
tell you that it's no easy task. Again, it would have raised the cost
(by adding another custom chip) and increased time to market.

Also, my understanding is that Motorolla has always had low yields on the 
6801x series of parts.  At one point they were giving even small buyers
of the 6801x series of chips better pricing on the 68020 and almost
begging them to switch.

In my opinion virtual memory as it currently is implemented in
System 7.0 is the least important feature of 7.0. Memory is very cheap
these days and VM just makes system response time variable, and quick
reponse time is one of the things I like best about the MAC.

I believe that the LC will be one of Apple's best selling products ever.

Larry Bakst
leb@merk.com
-- 
larry bakst
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