From: msi@ESD.3Com.COM (Mark Isfeld)
Newsgroups: comp.sys.mac.hardware
Subject: Re: Info on the New Macs coming Feg 10th
Date: 29 Jan 93 00:08:09 GMT
Distribution: usa

andrem@pyrtech.mis.pyramid.com (Andre Molyneux) writes:

>oueichek@gram (Ibaa Oueichek) wrote:

>>Michael J McNaughty (mjm4@kimbark.uchicago.edu) wrote:
>[some stuff deleted]
>>: I have a question. If you have a Mac LC, and Apple offers an upgrade path
>>: to the LCIII (Please!!!) will this allow the upgraded mac to have the
>>: same RAM expansion option to 36MB? Only having a 10MB max thanks to those
>>: wonderful ROMs is really disappointing.
>>  The 10 MB's limit isn't imposed by the ROMs, it's a limit of the motherboard
>>  designe.

>The 10MB limit *is* imposed by the ROMs.  On the original LC you wouldn't
>be able to exceed the limit (2 MB motherboard RAM + 2 * 4 MB SIMMS = 10 MB).
>However, on the LCII you should be able to get to 12 MB, since it has 4 MB
>soldered to the motherboard - but try it and you'll find the machine can
>only access 10 MB.

You are assuming that because the memory is there that the hardware is
capable of accessing it, therefore it must be the firmware that is the 
limiting factor.  This is probably wrong.  (I don't have any inside info
but as a hardware designer, the following makes sense) 
The explanation goes like this:

The DRAM controller buried in some custom chips in the LC probably
supports two banks of RAM.  Each bank can be 2 megabytes or 8 megabytes
depending on the size (config) of the RAM chips.  The Original LC provided one
bank on the motherboard of 2Mb, and you could add 2MB or 8MB for a total
of 10MB max.  Now the newer version comes with 4MB, which MUST be two banks
of 2MB (since the bus is 16 bits).  Now when you plug in new SIMM's you
physically have three banks of Memory.  But the memory controller cannot
handle 3 banks, only two.  So the Hardware disables the second bank of
2MB of memory soldered onto the card, and uses the SIMM memory as bank
two.  Now you have 10 MB still.

It Comes down to a decision on Apple`s part on how to provide 4MB minimum
memory.  If they had provided the second 2MB bank in SIMM modules, you 
would not be surprised to learn that 10MB is the limit(since you removed
the second 2MB bank leaving only 10MB in the system).  However, it costs
more to manufacture it that way.  Since sales are largely determined by 
entry level costs, they decided to solder the 2nd bank on the motherboard, 
and therefore save money on SIMM's (About $10-20$ MFG cost, maybe higher, 
including labor to install them.).  Furthermore, 1MB SIM's are becoming
less and less useful as a by-product of the upgrade, so the user doesn't 
really lose anything. (If the SIMM's removed are going to sit in a drawer, 
then getting them out didn't do you any good, might as well leave them 
in the machine.)

This could happen again!  The LCIII is supposed to support 36 MB of memory,
which must be three banks, 4MB on motherbaord, and 2 banks of 16MB in SIM's.
If so, and they decide to provide 8MB as minimum, then they may solder 8MB on
the motherbaord, but the max will not go up to 8 + 32 = 40, but remain
at 36.  (Of course if the config is actually 4MB plus 1 bank of 32 meg (using
funny 8MB simm's then this analysis may or may not apply)

I hope this clears up what seems like a silly thing.  It is actually a
sensible decision, given a low cost design which only supports two 
banks of memory.
--
-------------------------------------------------------------------------
| Mark Isfeld   
| Mark_Isfeld@3Com.com                     
| 408-764-5167