Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!cbmvax!valentin
From: valentin@cbmvax.commodore.com (Valentin Pepelea)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: MMU games with Motorola chips
Message-ID: <14588@cbmvax.commodore.com>
Date: 21 Sep 90 18:16:14 GMT
References: <1990Sep14.185419.28320@ecst.csuchico.edu>
Reply-To: valentin@cbmvax.commodore.com (Valentin Pepelea)
Organization: Commodore, West Chester, PA
Lines: 46

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 :-)

Yes, it is possible to do that. The virtual address space on the 68000 is
not just a huge 4 Gigabyte plane. While indeed the processor is limited to
access 4 Gigabytes of physical memory, virtual memory is divided into seven
address spaces. Four of them are

	- user data space (001)
	- user program space (010)
	- supervisor data space (101)
	- supervisor program space (110)

A separate translation table can be made for each of these cases. Therefore
when CPU is in supervisor mode, the supervisor mode translation table will
be used.

But this strategy would not work on the Amiga, where I/O space is often
accessed from user mode.

>	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?

Definitely not. The MMU resides between the CPU and external memory. The
graphic chip processors are part of the external world.

>	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???

Notice that there is only one Dave Haynie.

Valentin
-- 
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