Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!husc6!bbn!uwmcsd1!ig!jade!ucbvax!ji.Berkeley.EDU!shs
From: shs@ji.Berkeley.EDU (Steve Schoettler)
Newsgroups: comp.sys.amiga
Subject: Re: Commodore 68020
Message-ID: <21922@ucbvax.BERKELEY.EDU>
Date: Mon, 23-Nov-87 22:18:45 EST
Article-I.D.: ucbvax.21922
Posted: Mon Nov 23 22:18:45 1987
Date-Received: Thu, 26-Nov-87 22:15:32 EST
References: <21872@ucbvax.BERKELEY.EDU> <2844@cbmvax.UUCP>
Sender: usenet@ucbvax.BERKELEY.EDU
Reply-To: shs@ji.Berkeley.EDU.UUCP (Steve Schoettler)
Distribution: na
Organization: University of California, Berkeley
Lines: 51

In article <2844@cbmvax.UUCP> daveh@cbmvax.UUCP (Dave Haynie) writes:
>in article <21872@ucbvax.BERKELEY.EDU>, shs@ji.Berkeley.EDU (Steve Schoettler) says:
>
>> Thus, if a 68020 board has memory above 0x0fffffff, prolog won't be able
>> to access it.
>
>It's a little more involved that that, Boso!  Anything that uses upper bits
>as tagged won't work on any 68020 system that does fully decoded addressing.
>So if my memory board is at, say, 0x01000000, it's not going to be at 
>0x11000000 or 0x21000000, etc. if I'm fully decoding the 68020 addresses.

It's exactly that involved, but you have to know something about the
implementation.  The scenerio goes:
    you fetch a 32 bit value.  The upper 4 bits tell you whether it's
    an integer, a pointer, a list, etc.
    If it's a pointer, you MASK OFF the 4 bits (and.l #0ffffffff, d0) and
    access that address.

Yes, its slower, but that's what you have to do to work in cases like you
mentioned.

>The Amiga 68020 card doesn't put memory "up there", but like I explained,
>it's not the location that matters.  The Amiga board DOES have an MMU.
>This would allow you to map "up there" memory "down here", and (what's
>more important for tagged systems), MAY let you program memory wrapping, so
>your tags don't affect anything (I haven't actually tried this, but I 
>suspect it would work).  

The addition of that hardware might make it possible to eliminate the extra
MASK step.
>
>What you really want is the TI LISP chip on a coprocessor card.
>
What I really want is the Berkeley PLM (Prolog Logic Machine) chip
 on a coprocessor card.

By the way, why do you guys keep calling it a COprocessor card?  Isn't it more
like an instead-of-the-main-processor card?  :~)

While we're on the subject (we weren't, but it seemed like a good place to
bring it up), I'm really hoping ASDG's SDP can run other tasks on its 68000
if it's not doing I/O.  It would be really fun to try to convert it to a
"remote" Videoscape spooler or something like that.

>-- 
>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!"


Steve Schoettler