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From: hah@mipon3.intel.com (Hans Hansen)
Newsgroups: comp.sys.amiga
Subject: Re: Commodore 68020
Message-ID: <1235@omepd>
Date: Wed, 18-Nov-87 03:03:29 EST
Article-I.D.: omepd.1235
Posted: Wed Nov 18 03:03:29 1987
Date-Received: Sat, 21-Nov-87 04:57:43 EST
References: <2720@cbmvax.UUCP> <5338@cisunx.UUCP>
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Reply-To: hah@mipon3.UUCP (Hans Hansen)
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Organization: Intel Corp., Hillsboro
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In article <5338@cisunx.UUCP> ejkst@cisunx.UUCP (Eric J. Kennedy) writes:

$Can this 32 bit memory be used along with 16 bit memory expansions?
$If so, how does it work?  I.e. what determines which type of memory
$is used if both types are available to the system.  

The 68020 and 68030 have two input pins that are used to perform dynamic
bus sizing.  What this means is that the hardware used to decode the
addresses of RAM, ROM, and misc., I/O controllers should/must also
configure these two input pins.  As the 68020 addresses various segments
of the Amiga memory map the size bits are changed to reflect the width
of the memory of controller.  With this arrangement the 68020 can even
use 8 bit wide RAM and ROMs.  The programming model does not change as
the 68020 will adjust its reads and writes depending on the target
address width.

$ 
$Also, I seem to remember reading that CSA's Turbo-Amiga has a 12 Meg
$limit of 32 bit memory.  How does that work?  Just what are the limits
$with a 32 bit system?  

This is only a limit of their RAM boards.  The 68020 can address a full
32 bit address space in each of four modes, (however this feature has
not been used to my knowlege, as 4 GigaBytes of linear 'real' memory
has proven to be enough for now.)

What the 68020 boards do is to treat the Amiga as a 16 MegaByte 16 bit
wide slot within the total address space.  The 32 BitWide 68020 RAM
expansion is made outside of the Amiga 16 MegaByte slot.  So for the
most part while addressing the Amiga slot the dynamic bus sizing is set
to 16 bits, however there are a few I/O ports that most be decoded to
8 bits, i.e., the 8520s are 8 bit devices.

For more info get a copy of the "MC68020 32-Bit Microprocessor User's 
Manual" from Motorola.

$
$Thanks,
$
$Eric Kennedy

Your welcome

Hans   hah@inteloa.intel.com