Path: utzoo!attcan!uunet!cbmvax!daveh
From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: MMU games with Motorola chips
Message-ID: <14489@cbmvax.commodore.com>
Date: 17 Sep 90 18:28:03 GMT
References: <1990Sep14.185419.28320@ecst.csuchico.edu>
Reply-To: daveh@cbmvax.commodore.com (Dave Haynie)
Organization: Commodore, West Chester, PA
Lines: 42

In article <1990Sep14.185419.28320@ecst.csuchico.edu> mrush@csuchico.edu writes:

>	I have a question about an MMU on a 680x0 based computer:

>	On a system with Memory Mapped I/O, is it possible to set up the
>MMU so that USER programs can have ALL addresses go to RAM, yet have the OS
>still be able to access the magic I/O address?  (sort of make ALL I/O through
>OS calls, and give the USER a completely RAM address space; regardless of
>whether or not this is PRACTICAL :-)

Well, you really wouldn't want non-RAM accesses to go to RAM.  However, the
68030/68851 MMU gives you the option of having either unified or separate
tables for each kind of memory space (the important ones being User Data,
User Instruction, Supervisor Data, and Supervisor Instruction).  So, for 
example, it would be simple to allow only Supervisor Data space accesses to
talk to I/O locations.  Of course, you would have to modify the Amiga OS
somewhat to do that -- lots of I/O is currently done in user space.  But I'm
sure things like UNIX already behave this way on Amiga hardware.

>	On a more Amiga-oriented note:  There's been talk that we may get
>Virtual Memory in later versions of the OS; will it be possible to have
>Virtual CHIP Memory?

Virtual chip memory can't be done.  Virtual memory is possible as long as you
have control over that virtual memory exclusively by one CPU.  That's because
only the 68030, for instance, see it's virtual addresses -- everything else
in the system sees physical addresses.  As Chip memory is addressed by both
68030 and Agnus, you have two problems.  First of all, Agnus has no way to 
detect or generate a page fault, and secondly, no way to speak in terms of
the 68030's virtual addresses.

>	And finally, how do all you ENGINEER-types (like Dave H.) manage to
>design NEW hardware, AND keep up on everybody ELSES (Intel, IBM, Apple, etc.)
>new hardware AT THE SAME TIME???

We don't sleep much.  And we never rest.

>	-- Matt
-- 
Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
   {uunet|pyramid|rutgers}!cbmvax!daveh      PLINK: hazy     BIX: hazy
      Get that coffee outta my face, put a Margarita in its place!