WANTED: 3b1 ram expansion

[Michael "Ford" Ditto]

In article <1525 at mcgp1.UUCP> fst at mcgp1.UUCP (Skip Tavakkolian) writes:
>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.

No, the only difference between the 7300 and 3b1 motherboards (aside from
some pcb relayout) is the amount of ram installed).  The other differences
are in power supply rating, case style and disk size.

>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.

The amount of physical RAM addressable by the MMU is 4MB.  The MMU is a
custom random-logic MMU based around a 1K by 16bit ram array.  The top
two 68010 address lines are used for selecting I/O space and other things
not related to RAM.  That leaves 22 bits for the virtual address, giving
a per-process virtual address space of 4 Meg.  Some of this address space
is used for shared memory, the shared library, and for overlaying the
kernel when it is running.  2.5 Meg (from 0x080000 to 0x3fffff) is available
for user code and data.

>Hope this is all true :-)

Well, it's mostly true with respect to 68000's in general, but the Unix PC
is a bit weird.
-- 
					-=] Ford [=-

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