Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!apple!dlyons
From: dlyons@Apple.COM (David Lyons)
Newsgroups: comp.sys.apple
Subject: GS--running from ROM is faster
Message-ID: <34254@apple.Apple.COM>
Date: 24 Aug 89 04:54:41 GMT
References: <wagner@PUCC> <8908212244.aa15584@SMOKE.BRL.MIL>
Organization: Apple Computer Inc, Cupertino, CA
Lines: 34

In article <8908212244.aa15584@SMOKE.BRL.MIL> SEWALL@UCONNVM.BITNET (Murph Sewall) writes:
>[...] UNLESS (to improve performance) applications routinely copy the Rev 3
>ROM into the RAM space it would occupy in a Rev 1 machine.  That (if I
>grasp the concept I only read about last week) is the "shadow RAM"
>technique some PC clone vendors are using to improve performance.  In the
>MS-DOS World RAM is faster memory than ROM; is that true of the IIgs?

Nope.  In the GS, running from ROM is actually faster than running from
RAM.  (So copying the ROM into RAM wouldn't help, even if it were not a
no-no of mammoth proportions, which it is, and even if it would work,
which it won't.)

(Why?  About 8% of the time, RAM is getting refreshed.  If you're *running*
from RAM, you have to stop and wait for it.  If you're running from ROM,
you don't.  This explanation is missing some details, but the idea is there.)


>[...] the "shadow RAM" technique makes sense in a IIgs, would an application be
>able to tell the difference between a Rev 1 and Rev 3 machine?

Applications *can* tell the difference between the ROM versions, but hardly
any will actually bother.  We don't generally *want* applications to care
which machine they're on.  (For completeness, here's how to check: Use
the IDROUTINE routine at $FE1F to get the ROM version.  From the 16-bit
world, you can use the Misc. Toolset function FWEntry to call IDROUTINE.)

 --Dave Lyons, Apple Computer, Inc.          |   DAL Systems
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   My opinions are my own, not Apple's.