From: robert.desonia@hal9k.ann-arbor.mi.us (Robert Desonia) 
Subject: Re: I don't understand SI
Distribution: world
Organization: HAL 9000 BBS, W-NET HQ, Ann Arbor, Michigan, USA
Reply-To: robert.desonia@hal9k.ann-arbor.mi.us (Robert Desonia) 
Lines: 54

MG>joohwee students (neural@iss.nus.sg) wrote:
MG>> 	I went buying SIMMs recently, and the sales person told me that 
the
MG>> are 9-chip SIMMs and 3-chip SIMMs, and one cannot use them interchan
MG>> If you use one, you have to use all of the same type.

don't believe everything you are told.  I can tell you that mixing them 
between 'banks' ok, and I can't see why mixing in one bank is not unless 
they are of different speeds ( e.g. mixing of 60ns and 100ns SIMMs in one 
bank ).  The two only differ in the type of chips it uses.  Assuming that 
the SIMMS are 1Mx9 ( 9 bit wide ), here is the two equivalent 
configuration.  The 3-chip SIMM uses two 4-bit wide 4Mbit (1M of 4-bit 
nibbles ) and one 1-bit wide 1Mbit chip ( for a total of 9-bit wide 1Mbyte 
).  The 9-bit SIMM uses nine 1-bit wide 1Mbit chips.  These are equivalent 
because of the way that it is 'pinned' on the SIMM board.  At the SIMM 
interface, they both act as 9-bit wide 1MByte SIMMS ( 2*4+1=9*1 ).  [sorry 
if too techie for ya].


MG>> 	Similarly, one cannot plug in two 1MB SIMMs and one 4MB SIMMs to 
gi
MG>> the system a total of 6 MEG. Why is that so ?? If my system supports
MG>> of 8 MEG (it has 8 SIMM slots), can I plug in 4 4MB SIMMs to give my
MG>> 16MB ??

That sounds correct.  the problem is that if your computer takes 9-bit 
wide SIMMs, you can not mix different sizes in one bank.  Why you ask?  
Simple,  if you understand why there is banks.  Assuming you have a 32-bit 
CPU ( 386DX or 486 ), the data bus (e.g. the mechanism to retrieve data 
from memory ) is 32-bits wide, so the computer expects to see 32 bits when 
it asks for data.  To get that bandwidth ( 32-bit wide ), the motherboard 
links 4 1Mx9 ( one bit is not data, but parity, so I will ignore that in 
this simple explaination ) to get 32bits [ (9-1)*4=32 bits ].  That means 
that a SIMM in a bank stores only 1/4 of the 32 bit wide data.  If you 
have a 16-bit bus, two 1Mx9 SIMMs are linked together to get 16-bit wide 
data, which is the reason why 286 banks are 2 SIMMs wide, and 32-bit banks 
are 4 SIMMs wide.  If your computer required 1Mx36 ( e.g. 32-bit wide data 
with 4 parity bits, used in some PS/2s and ASTs ), you could upgrade by 
one SIMM at a time.

Hope that this message is not over your head, but the answer to your 
question was not simple.  I could of just said, 'because I said so.'

-rdd    

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