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From: noah@Apple.COM (Noah Price)
Newsgroups: comp.sys.mac.hardware
Subject: Re: Max RAM on SE/30 and II's?
Summary: What to do with 128 MBytes?!!
Keywords: memory RAM Mac SIMM
Message-ID: <41392@apple.Apple.COM>
Date: 28 May 90 01:02:19 GMT
References: <16786@phoenix.Princeton.EDU>
Organization: Apple Computer Inc, Cupertino, CA
Lines: 62


In article <16786@phoenix.Princeton.EDU> clye@phoenix.Princeton.EDU (Christopher Lye) writes:
>[The SE/30 manual] says in the hardware specs that as denser memory becomes
>available the SE/30 will be capable of supporting 128MB of RAM.

This is true -- it really just means that the SIMM has the address lines
wired to it for that much DRAM.

[Techno-greek, for those who care]  The standard SIMM allows 24 bits of
address (12 bit row address, 12 bit column address) for a total of 16
MBytes per SIMM.  This means, on an SE/30 or any Mac II family
computer, you have four times 16 MBytes, or 64 MBytes maximum per
bank.  Two banks give you 128 MBytes.

For a "normal" SIMM (that is, one with no more than eight parts, or
nine for parity), this requires 16 Mbit DRAM which is still a
generation of memory away.  The 4 Mbit parts that are available now
will get you a 4 MByte SIMM, for a total of 16 MBytes per bank on an
SE/30 or Mac II family computer, or a maximum of 32 MBytes total for
both banks.  1 Mbit parts are still the cheapest $/bit, and
conveniently a system full of 1 Mbit SIMMs gives you 8 MBytes -- the
most physical (or "real" as you call it below) memory today's Mac OS
will take advantage of.

So now you're calling me a liar, saying "but Noah, so and so will sell
me a (4, 16, 32, whatever) MByte SIMM today!"  Yes, I've seen some
vendors making "oversize" SIMMs by putting more DRAM chips on each
SIMM, often with some additional logic, to get larger memory
configurations using today's memory technology.  Today, A/UX can take
advantage of this, but all you can use it for under Mac OS is a RAM
disk.  You'll probably see more of it once Mac OS can take advantage of
more than 8 MBytes, though if you're doing very disk intensive work you
may find you prefer having a RAM disk to having more system memory...

>I've heard that the virtual memory limit ... also applies to "real"
>memory.
>
>Is this so?

In short, no it isn't.  But Macs today won't use more than the first
8 MBytes.  There's plenty of discussion on virtual memory going on in
comp.sys.mac.misc, so I won't rehash it all here.

> ... why would Apple make the SE/30 capable of supporting 128MB?

Because that's how many address lines there were on the SIMM.  :-)

>Is this a software or a hardware limitation?

128 MBytes is a hardware limitation.  8 MBytes (of physical memory) is
a software limitation under today's system software.

>Would the extra memory only be useful in terms of a RAM disk? 

Today, more than 8 MBytes of physical memory is only useful under A/UX
or as a RAM disk.

noah

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
noah@apple.com                                    Mac IIci Hardware Design Team
...!{sun,decwrl}!apple!noah          (not the opinions of) Apple Computer, Inc.