Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!ncar!noao!asuvax!behemoth!mph
From: mph@behemoth.phx.mcd.mot.com (Mark Huth)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Virtual Memory / doable 1.4 request
Message-ID: <10948@behemoth.phx.mcd.mot.com>
Date: 23 May 89 21:59:31 GMT
References: <8905200449.AA20664@postgres.Berkeley.EDU>
Reply-To: mph@behemoth.UUCP (Mark Huth)
Organization: Motorola Microcomputer Division, Tempe, Az.
Lines: 31

In article <8905200449.AA20664@postgres.Berkeley.EDU> dillon@POSTGRES.BERKELEY.EDU (Matt Dillon) writes:
>:Come on you guys, this is silly! Who cares whether VIRTUAL_MEM or
>:PHYSICAL_MEM is the default? Just have it be user selectable,
 ...
>	I think everyone is agreed that that would be even worse!  You can't
>use select something like that.  But I agree... which flag is used is just

Uh, I assume you me the royal everyone, meaning HM Dillon.  I certainly do
not agree that the user shouldn't be allowed to select the default memory
allocation type.  

I think that most programs will work either way, provided that all chip
memory allocations are presumed to be physical allocations.  It seems to me,
that data which the custom chips access directly must all be in chip
memory, and of course, it cannot move once the pointers have been specified.

Other DMA device drivers will require modifications to lock the virtual
buffer pages into memory and to translate the pointers to physical addresses
for the controller.  Once the DMA is finished, the virtual view of memory
may be used.  Careful selection of the transition between virtual and physical
space may eliminate additional state information requirements.  That is, have
the task context translate the address to physical via an OS trap somewhere
in the task side of the IO calls, rather than forcing the driver task to do it.

These problems arise from the fact that the 68851 is inserted to only allow the
68020 to be a logical master, all other masters are physical.  The '030
allows no choice in the matter, since the MMU is buried on the chip.  Logical
bus masters are nice, but the Amiga architecture would require some serious
mods to support it.  Of course, then you could have lots of chip ram!

Mark Huth