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From: cs4w+@andrew.cmu.edu (Charles William Swiger)
Newsgroups: comp.sys.apple2
Subject: Re: Apple // series speedup
Message-ID: <8aG5xmi00WAI81PVQt@andrew.cmu.edu>
Date: 9 May 90 18:20:34 GMT
References: <9005091436.AA18965@apple.com>
Organization: Carnegie Mellon, Pittsburgh, PA
Lines: 20
In-Reply-To: <9005091436.AA18965@apple.com>

The shadowing technique used to speed up the ROM for IBM's would be
counterproductive for the Apple //gs for the simple reason that the
//gs' ROM runs faster than its RAM currently does.  Besides which, if
Apple is going to redesign the motherboard in order to speed up its ROM
chip then it had damn well _better_ speed up the rest of the computer
too!

The shadowing that the //gs does use is from RAM banks $00 and $01 to
banks $E0 and $E1, for write and read-modify-write instructions only,
and only when shadowing is enabled (via the "Quagmire" pseudo-register,
I believe.)  The purpose of this is to allow old 8-bit programs to run
in the faster (2.7 MHz) bank $00-01 locations as much as possible,
rather than the slow (1.0 MHz) bank $E0-E1 locations.  (The $E0-$E1
banks contain the memory locations that the screen memory, I/O cards,
and other hardware are actually mapped to.



					-- Charles William Swiger
					cs4w+@andrew.cmu.edu