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Path: utzoo!utgpu!water!watmath!clyde!rutgers!cbmvax!daveh
From: daveh@cbmvax.UUCP
Newsgroups: comp.sys.amiga
Subject: Re: Commodore 68020
Message-ID: <2828@cbmvax.UUCP>
Date: Thu, 19-Nov-87 13:21:45 EST
Article-I.D.: cbmvax.2828
Posted: Thu Nov 19 13:21:45 1987
Date-Received: Sat, 21-Nov-87 16:30:41 EST
References: <1235@omepd>
Distribution: na
Organization: Commodore Technology, West Chester, PA
Lines: 34

in article <1235@omepd>, hah@mipon3.intel.com (Hans Hansen) says:
> 
> 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.

You could size the bus for 8520s if you were starting out from scratch, but
in reality this isn't done.  The main reason is that the 8520s are set up
for 16 bit reads and writes.  One is connected to 68000 data lines 0-7,
the other to 68000 data lines 8-15.  The main reason is that the 68000 
doesn't do dynamic bus sizing; it only sizes relative to instruction
width, not device port size.  In fact, it would be impossible on the Amiga
to have the 68020 dynamically size the 8520s to 8 bits.  The reason is the
way the 68020 port devices go up the data bus.  A 32 bit device is connected
to bits 0-31.  A 16 bit device connects to bits 16-31.  And an 8 bit device
connects to bits 24-31.  The entire Amiga system looks like a 16 bit port
to the 68020.

CSA RAM goes outside the normal 16 bit address space of the 68000.  The 
Commodore-Amiga 68020 card has fast 32 bit memory that's put in the 
normal 68000 address space (it looks just like a normal 2 meg expansion
card to the system).  This has the disadvantage of not permitting more
than the standard amount of RAM under normal conditions.  But it also
allows that memory to be normal FAST RAM (so it gets allocated first),
and it allows 16 bit DMA devices to work with it.  

> Hans   hah@inteloa.intel.com
-- 
Dave Haynie     Commodore-Amiga    Usenet: {ihnp4|caip|rutgers}!cbmvax!daveh
   "The B2000 Guy"              PLINK : D-DAVE H             BIX   : hazy
		"I can't relax, 'cause I'm a Boinger!"