Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!cs.utexas.edu!uunet!microsoft!brianw
From: brianw@microsoft.UUCP (Brian WILLOUGHBY)
Newsgroups: comp.sys.apple
Subject: Re: Really small question (a really long explanation)
Message-ID: <10142@microsoft.UUCP>
Date: 4 Jan 90 02:56:29 GMT
References: <kZTLjCG00WB7Q=4bJa@andrew.cmu.edu> <9542@microsoft.UUCP> <1989Dec15.200302.8233@ncsuvx.ncsu.edu> <10041@microsoft.UUCP> <37569@apple.Apple.COM>
Reply-To: brianw@microsoft.UUCP (Brian WILLOUGHBY)
Organization: Microsoft Corp., Redmond WA
Lines: 34

In article <37569@apple.Apple.COM> dlyons@Apple.COM (David A. Lyons) writes:
>The first part of that is the key:  reads from 0 and 1 are fast.  Consider
>scrolling the text screen, for example:  the reads and writes are all to
>banks 0 and 1, so the scrolling is *faster* than if the reads and writes
>were to banks $E0 and $E1--so shadowing *is* partially a speed issue.

Quite true.  In fact, the TransWarp on my II Plus takes advantage of similar
shadowing because it only slows to 1 MHz when *writing* to video memory and
then also shadows the data to the 48K RAM to update the screen.  Reads (except
to slot memory) are always at 3.58 MHz.

>brianw@microsoft.UUCP (Brian Willoughby) writes:
>>Fortunately, shadowing doesn't cause writes OUTSIDE of the video areas to be
>>slowed.
>
>I read that wrong on my first try--to clarify, access to banks $E0 and
>$E1 is always slow, but access to non-shadowed areas of banks 0 and 1
>is fast, and all reads from 0 and 1 are fast.

I seem to have trouble with my wording.  What I should have said was that a
write to Bank $00 or $01 at any address that is not in a video area is not
slowed, because these writes do not need to be synched to the real video
memory.  I was trying to make the point that shadowing slows down some, but
not all, writes to the first two Banks.  If you didn't need video (don't ask
me why), you could treat the entire first two banks as contiguous RAM memory.
Then, with shadowing turned *off*, *all* of the accesses would be full speed.
Shadowing, therefore, reduces performance in certain cases (but admittedly,
these are rare cases).

Brian Willoughby
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