Path: utzoo!censor!geac!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!hsdndev!husc6!encore!wcarroll
From: wcarroll@encore.com (Mr. New Dad)
Newsgroups: comp.sys.mac.misc
Subject: Re: The New Macs: Greedy Compromises?
Message-ID: <130024@jake.encore.com>
Date: 6 Dec 90 21:45:25 GMT
References: <1142@duteca.UUCP>, <1990Dec2.084149.25494@world.std.com>
Organization: Encore Computer Corp, Fort Lauderdale, FL
Lines: 60

From article <1142@duteca.UUCP>, by reinoud@duteca (Reinoud Lamberts):
> The 68020 has a feature called 'dynamic bus sizing', allowing it to
> work with 32, 16 or 8 bit busses at any time, something a 68030 can't
> do. 

According to the 68030 User's Manual (MC68030UM/AD REV 1), in section
7.2.1 _Dynamic Bus Sizing_, "The MC68030 dynamically interprets the 
port size of the addressed device during each bus cycle."

> A narrower memory bus greatly reduces system cost: fewer memory
> chips necessary, fewer bus buffers, cheaper printed circuit board
> (smaller, fewer traces, maybe fewer layers).

We'll get back to this.

> 
> What do you mean by 'incompatible architecture'? The 68020 instruction
> set is actually a SUPERSET of the 68030!
> 

Back to the 68030 User's Manual, the '020 has two instructions (CALLM
and RTM) not in the '030, and the '030 has four MMU instructions (PFLUSH,
PLOAD, PMOVE, and PTEST) not in the '020. The only other differences
listed are: 128 byte data cache, on-chip MMU, and 6 new control registers.



by boris@world.std.com (Boris Levitin):
> das@Apple.COM (David Shayer) writes:
>>The CPU spends a large percentage of time executing code out of ROM,
>>so it has a wider path to the ROM.  It has a 16 bit path to RAM to
>>save a little money.  Believe it or not, you run out of room for
>>traces on a PC board, and either you have to make a bigger board
>>(which costs more for the board material and the bigger case to hold
>>it) or you cut out some traces.  
> 
> Are you saying the extra inch of board space and the extra inch of case
> required to accomodate the extra traces would significantly add to the cost
> of making the LC?

The extra 16 bits are more than an extra inch of board space. You have
twice as many memory chips, twice as many memory buffers, and the control
logic for all of that. It is not a trivial thing.

> 
> Regarding your assertion that the average user doesn't do anything that
> uses an FPU: besides spreadsheets, there are Adobe Type Manager and TrueType
> (I don't know for a fact that TrueType makes use of the FPU, but if it 
> doesn't, it must be really slow).  Using quadratic (TrueType) or cubic 
> (ATM) splines to display text on the screen is a very floating-point-
> math-intensive process.

Does the average user really do these things? If they did, don't you think
computer companies would sell more models with FPUs included?


-- 
William R. Carroll  (Encore Computer Corp., Ft. Lauderdale FL)
wcarroll@encore.com         uunet!gould!wcarroll
"The brain-dead should not be allowed to operate motor vehicles!" - Me