Path: utzoo!news-server.csri.toronto.edu!rutgers!cbmvax!daveh
From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.amiga.programmer
Subject: Re: Virtual Memory Program?
Message-ID: <19748@cbmvax.commodore.com>
Date: 11 Mar 91 19:53:34 GMT
References: <1991Feb14.193510.13772@bradley.bradley.edu> <%1+-SC%@irie.ais.org> <1991Feb20.175103.24611@jato.jpl.nasa.gov> <1986@public.BTR.COM>
Reply-To: daveh@cbmvax.commodore.com (Dave Haynie)
Organization: Commodore, West Chester, PA
Lines: 36

In article <1986@public.BTR.COM> valentin@public.BTR.COM (Valentin Pepelea) writes:
>In article <1991Feb20.175103.24611@jato.jpl.nasa.gov>
>jdickson@jato.Jpl.Nasa.Gov (Jeff Dickson) writes:

>>	To support a virtual memory application (more like a virtual disk),
>>is it safe to assume that the application will only use AmigaDOS to access
>>the file's contents? Without an MMU I don't see any other way. Comments?

>You are indeed right. The problem with accessing a disk directly through
>its device driver is that the device driver does not know when it is not
>allowed to DMA into a portion of memory. 

Of course, "the way it often is" is not the same thing as "the way it should
be".  Only a device driver knows the details of its underlying hardware.  
Because of this, a device driver is responsible for knowing, for example, which 
memory it has DMA access to and which memory it doesn't have DMA access to.
This, of course, counts on the memory systems in the computer behaving 
according to a consistent set of rules.

>As for the AmigaDOS filing system, the MASK value in a mount list identified 
>the region of memory into which DMAing is allowed. 

The MASK parameter, however, is an acknowledged kludge.  It is a way to let the
user often force a piece of hardware to work properly, despite hardware or
firmware bugs.  So MASK can be used to support a 24-bit DMA device that doesn't
know it only has 24 bit access to memory.  And it can support hacked-in memory
devices that don't support DMA.  It is useful, but it is not the correct way of
handling these problems.  It is just the one tweakable parameter available to
the user.  

>"An operating system without virtual memory      Name:      Valentin Pepelea

-- 
Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
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	"What works for me might work for you"	-Jimmy Buffett