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From: genius@cs.mcgill.ca (Michel NGUYEN)
Subject: Re: The Amiga's Future
Message-ID: <1991Jun11.134256.9465@cs.mcgill.ca>
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Organization: SOCS, McGill University, Montreal, Canada
References: <#g1H3+$o@cs.psu.edu> <12901@uwm.edu> <22308@cbmvax.commodore.com>
Date: Tue, 11 Jun 91 13:42:56 GMT


>You're confusing the issues here.  Sure, the '030 and the '386 can each
>physically address 4GB of memory.  Most systems built around these processors
>don't let you get anywhere close to that much memory.  A '386 with only an
>ISA expansion bus typically limits you to 16MB of memory, total.  That's
>a board design limit, not the '386's limit.  Some '386 boards let you drop in
>some extra memory in a private memory slot, still not that close to 4GB; the
>board limit might be 16MB or 128MB.

>No.  The 68000 can address 16MB of RAM, the 68030 can address 4GB of RAM.  That
>says nothing about what a Mac can do.  In theory, a Mac can support 256MB of
>RAM per NuBus slot, so a full sized Mac II with display card can support an
>additional 1.25MB of RAM, over and above what's on the motherboard (actually,
>in theory, around 1.33MB of RAM, though only 1.25MB is contiguous, due to the
>strange memory architecture of NuBus).  NuBus RAM is, however, slow.

>You're wrong.  There's no standard for motherboard RAM amoung IBM compatibles.  
>You can figure it easily enough.  The ISA bus supports a maximum address range
>of 16MB, though a small amount of that is always taken up by the motherboard.
>Some PClones are expanded via custom daughterboards, for which you'd have to
>examine the specifications.  If they're expanded instead by banks of SIMM 
>modules, keep in mind that for PC SIMMs, the largest available with the 
>standard parity support is 4MB.  If you axe the parity like Apple did on all
>but the IIci, you get 16MB.  That's a total of either 16MB or 64MB per 32 bit
>wide bank.

All (that I know of) of 386 ISA based have 32 bit memory bus. The ISA bus
is used for peripherals (Modem, HD, Floppy, Graphic Cards,...), otherwise
the bus between the CPU and the memory is 32 bits. Now the EISA boards 
begin to replace the ISA boards, so EVERYTHING is on the 32 bits bus.
Many PC clones manufacturers (Compaq, ALR, ...) have systems with 
128MB on the motherboard ALONE. Of course extra-memory can be added on
daughterbards. Limit....I don't know if there is one, except for the
internal space. I am talking about EISA systems. If you want info about
those systems, check PC Magazine, there is always systems reviews.
Those systems are NOT new. They are been out for a couple of months 
already. (I saw the review on January-Feb)

Theorically, the 386 and 486 can have more than 16000 segments of
4GB each segment (48 bits). This feature is ideal for multiuser
systems ---> memory protection. So far, no OS (that I know of) has
taken that advantage. Not even Unix, maybe OS/2 4.0 ?? :-)

>-- 
>Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
>   {uunet|pyramid|rutgers}!cbmvax!daveh      PLINK: hazy     BIX: hazy
>	"This is my mistake.  Let me make it good." -R.E.M.


