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From: info-mac@uw-beaver
Newsgroups: fa.info-mac
Subject: Re: am I missing something?
Message-ID: <493@uw-beaver>
Date: Thu, 24-Jan-85 00:33:18 EST
Article-I.D.: uw-beave.493
Posted: Thu Jan 24 00:33:18 1985
Date-Received: Fri, 25-Jan-85 05:19:35 EST
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Organization: U of Washington Computer Science
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From: Thomas.Newton@cmu-cs-spice.arpa

The display on the Mac does indeed slow the main processor down, though I
think it's more on the order of 45-50% when accessing only RAM, and about
25% overall if you use the Toolbox routines a lot.

Note that there is only one "row" of memory chips in either a 128K Mac (64K
bits/chip by 16 chips/word) or a 512K Mac (256K bits/chip by 16 chips/word).
Since these are not dual-ported RAMs, the processor and the display hardware
cannot access them simultaneously.  Accessing "display memory" ties up ALL of
memory because "main memory" is on the same chips as "display memory".

The ROMs have their own private bus to the 68000, which is why the "average
speed" of the processor is higher if you use the Toolbox routines a lot.  If
someone added extra RAM beyond 512K (or implemented the 512K as four "rows"
of 64K chips), it would probably be desirable to hook the extra memory up in
the same fashion as the ROMs.

                                        -- Thomas Newton
                                           Thomas.Newton@cmu-cs-spice.ARPA