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From: shs@ji.Berkeley.EDU (Steve Schoettler)
Newsgroups: comp.sys.amiga
Subject: Re: Commodore 68020
Message-ID: <21872@ucbvax.BERKELEY.EDU>
Date: Fri, 20-Nov-87 01:01:07 EST
Article-I.D.: ucbvax.21872
Posted: Fri Nov 20 01:01:07 1987
Date-Received: Sun, 22-Nov-87 11:02:56 EST
References: <5338@cisunx.UUCP> <2801@cbmvax.UUCP> <177@vertigo.UUCP>
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Reply-To: shs@ji.Berkeley.EDU.UUCP (Steve Schoettler)
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Organization: University of California, Berkeley
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In article <177@vertigo.UUCP> rusty@vertigo.UUCP (91341-M.W.HADDOCK) writes:
>  The memory starts at address 0x7f000000,
>
>					-Rusty-
>

Oops, I was afraid of that.  
Does anyone know if CSA has a jumper or switch on the board to change
the base address of expansion memory for the 68020?

It turns out that the best Prolog implementations (to date) are on tagged
architectures.  The current method for implementing Prolog on 32-bit 
machines is to use the upper 3 or 4 bits for tag and leave the extra
28 bits for data.  The advantage is you get a tremendous speedup and
use much less memory than if you put the tags in another memory word.
The disadvantage is that pointers and integers can only be 28 bits long.

Thus, if a 68020 board has memory above 0x0fffffff, prolog won't be able
to access it.

To the fellow porting CProlog to the Amiga:  Looks like it may not work
  on a CSA board.  That's too bad because Prolog can use all the speedup
  it can get.

To others who know:
  Can the CSA board be jumpered to use another address space?
  Do other Amiga 68020 cards (like Finally, CBM, ASDG) put memory way
  up there?

I don't intend to start a flame on whether or not one should use extra
address bits for tags.  The implementors of Cprolog and similar systems
made some design decisions and tradeoffs.  I just want to know if they
can be made to work on an Amiga.

-Steve Schoettler

Disclaimer:
  I am not associated with any company currently marketing prolog or 
  68020 products.