Path: utzoo!attcan!uunet!world!decwrl!sun-barr!newstop!sun!imagen!atari!apratt
From: apratt@atari.UUCP (Allan Pratt)
Newsgroups: comp.sys.atari.st
Subject: Re: TT RAM
Message-ID: <2304@atari.UUCP>
Date: 19 Sep 90 00:21:46 GMT
References: <3107@medusa.informatik.uni-erlangen.de>
Organization: Atari Corp., Sunnyvale CA
Lines: 56

csbrod@medusa.informatik.uni-erlangen.de (Claus Brod ) writes:

>Is the TT's memory mapped by the MMU so that it appears
>contiguous?

No.  There is no address translation going on in the MMU.  If there
were, every program that wrote an address into a chip (video, DMA,
etc.) would have to be able to do the logical-to-physical translation.
This is impractical, and would result in nearly 100% incompatibility
with existing programs.  The MMU is used, however, so don't mess with
it.  It's used to get around a bug in the 68030's design; I may explain
in more detail later.

>I need this info for my hard disk driver which has to
>decide if some transfer is to be made into fast or slow RAM.

Right you are.  That's what the _FRB cookie is for.  But you are
looking at the problem in the wrong terms: rather than deciding what
kind of RAM the transfer is destined for, your program should decide
only what it has to: in this case, decide if it's being made to/from a
place that the ACSI DMA chip can access.  This prevents you from
hard-coding the location or size of different pieces of RAM.  This
saves you from having to re-release your driver  when somebody
complained that he couldn't access VME RAM with it, because you didn't
think of that.

The ACSI DMA chip can access memory only in the low 24 bits of the
address space, so that's your answer: you check the high byte of the
transfer address. If it's zero, you can use ACSI directly.  If it's
not, you must use the FRB.  You never examine any part of the address
other than the high byte, and you only test that against zero. 
Minimizing the information you require in this way leads to maximum
portability and future compatibility.

As an aside to SCSI driver writers, you have less to worry about. As
mentioned above, the definition of accessibility on the ACSI bus is
"the high byte must be zero."  The definition of accessibility on the
SCSI bus is "the bus must be 32 bits wide."  This means that A24/D16
VME memory is not accessible to SCSI, and you should use the buffer
pointed to by the _FRB cookie.

Finally, to defeat nit-pickers before they strike, I *know* that the
ACSI DMA chip can't really talk to everything in the 24-bit ST address
space: it can't see ROM, for example.  But it *can* access any real RAM
in that space, and if you're trying to DMA to something else (like the
palette) you deserve what you get.

>----------------------------------------------------------------------
>Claus Brod, Am Felsenkeller 2,			Things. Take. Time.
>D-8772 Marktheidenfeld, West Germany		(Piet Hein)
>csbrod@medusa.informatik.uni-erlangen.de
>----------------------------------------------------------------------

============================================
Opinions expressed above do not necessarily	-- Allan Pratt, Atari Corp.
reflect those of Atari Corp. or anyone else.	  ...ames!atari!apratt