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From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.amiga
Subject: Re: Memory Management
Message-ID: <2868@cbmvax.UUCP>
Date: Wed, 25-Nov-87 13:39:01 EST
Article-I.D.: cbmvax.2868
Posted: Wed Nov 25 13:39:01 1987
Date-Received: Sun, 29-Nov-87 01:20:53 EST
References: <7958@steinmetz.steinmetz.UUCP>
Organization: Commodore Technology, West Chester, PA
Lines: 52

in article <7958@steinmetz.steinmetz.UUCP>, oconnor@sunray.steinmetz (Dennis Oconnor) says:
> In article <...> klm@atkins.UUCP (Kevin [...] McBride) writes:
>>In article <...> dillon@CORY.BERKELEY.EDU (Matt Dillon) writes:
>>>    Virtual memory != EachTaskGetsItsOwnAddressSpace
>>>    As far as the Amiga goes, the only compatible solution is a Linear
>>>virtual address space over the entire system.
>>
>>This seems like a perfectly rational and compatible way of doing VM on
>>the Amiga. 
>>I like it!  I like it!  I wish I had thought of it!
> 
> Well, on the one hand, this means you are still limited to 8.5MB of
> memory. Doesn't it? 

Of course not!  The limits you see are based on the 68000 memory map.  There's
8.5M - 64K of autoconfigure space; leaving the .5M - 64K for I/O, that gives
you the popular 8 megs.  Now add the basic 512K of CHIP RAM, and you're up to
your 8.5 meg.  The A2000 and A500 have an additional 512K of RAM at $C00000,
for a total of 9 megs of real memory.  

Now if you wanted to talk in theoretical maximums for the 68000 based machine,
there's another 1.5 meg of space allocated to CHIP RAM, for a total of 10.5
meg given an as-yet non-existant Agnus to address that 2 meg.  Then there's
an addition 1.25 meg in the $C00000 area, for a total of 11.75 meg that can
be addressed by a 68000 based machine.

But that's for a 68000 based machine.  Anything with an MMU probably has a
68020 attached.  That adds an additional 4 gigabytes - 16 megs of address
space.  I don't forsee anyone hooking up a gig or two of real memory in the
next year.

> So why bother? Just buy the 8MB and have real speed.
> What does 8MB cost, anyway? $1500 ?

Given the above linear address model, which is certainly the one that makes
sense under AmigaOS, you have a virtual memory handler.  If you don't want
to use it, don't.  If you do, you probably have something in your startup
sequence that does:

	AddVMem 20M SWAP:

Or something along those lines.  That adds 20M of virtual memory to the free
memory list, links a SWAP: device like a hard disk to your page fault
server, and you're all done.  If you have 8 megs of real memory on top of that,
and only use 6meg, you never get a fault, and you never page.  If you use
15 megs, though, instead of dying in flames, you get to run, only slower.

> 	Dennis O'Connor 	oconnor@sungoddess.steinmetz.UUCP ??
-- 
Dave Haynie     Commodore-Amiga    Usenet: {ihnp4|uunet|rutgers}!cbmvax!daveh
   "The B2000 Guy"              PLINK : D-DAVE H             BIX   : hazy
		"I can't relax, 'cause I'm a Boinger!"