WANTED: 3b1 ram expansion

[Skip Tavakkolian]

In article <8118 at alice.UUCP>, wilber at alice.UUCP writes:
><In article <591 at proxftl.UUCP>, aaron at proxftl.UUCP writes:
><> I have a 3b1 (2mb ram, 67mb HD), and I really need another 2 megs or so.
><> Does anyone know where I can get this or have some for sale?
><the MMU in the 3b1 could only handle 2.5 megs of real memory.
><Anyone know anything about this?
><--------
><	Christopher J. Calabrese
><	AT&T Bell Laboratories
><	ulysses!cjc
> You can have up to 4Mb of real memory on a 3b1.  In practice the limit is
> usually 3.5 Mb, since most people would rather get a combo board with 1.5 Mb
> and two serial ports than a RAM board with 2 Mb and no serial ports.  Because
> of a limitation of the MMU, the maximum user memory *per process* is 2.5 Mb.
> At least that is how I understand the situation.
> Bob Wilber


There is an old BYTE review you may want to look for (this was for the original
7300 several years back).  Here is what I recollect. The MMU in 3b1/7300 has
24 address lines (I am not sure if it is 68451s).  The difference in 7300 and
the 3b1 (mother board designs) is in the number of address lines which are
ignored (out of the possible 2^24 = 16777216).  That, I believe, is the only
difference in 7300 and 3b1. So that 7300 MMU can address no more than 2 MB
and the 3b1 no more that 4 MB.

This however should not make any difference in the per *user* process address
space in a virtual memory system (demand page allows sizes smaller than a
whole process to be swapped in and out of memory).

Maximum per user process address limit is what the 68010 can address.

Hope this is all true :-)

Sincerely
-- 
Fariborz ``Skip'' Tavakkolian
UUCP	...!uw-beaver!tikal!mcgp1!fst

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