identifying PC Simms

Jeff Walther trag at io.com
Thu Sep 29 21:19:49 CDT 2005


>Date: Wed, 28 Sep 2005 23:53:00 -0400
>From: "Joe Stevenson" <ikvsabre at comcast.net>

>I have 16 30-pin simms left over from various past incarnations of 
>my PCs, and I'm
>  trying to figure out what I've got.

>  I no longer have a motherboard to test them, so I have no idea what is what. 
>
>Is there any not-to-painless way to figure out what I've got?

Not all that painless, but the only way I know that works...

Take a SIMM.  Count the number of chips.  Find the model markings on 
one of the chips.  There are usually two or three lines of writing on 
a chip.  One of these will be a date or batch code and is irrelevant. 
The line you want will start with a one, two or three (usually two) 
character manufacturer code (e.g., K or KM for Samsung, TC for 
Toshiba, M(numeral)M for Mitsubishi, HM or HN for Hitachi, etc.), 
followed by some longish, about four to eight, alphanumeric code 
which is mostly numerals, then a dash or space and a speed number in 
nanoseconds, which may or may not have the trailing zero truncated.

For example: HM5116400BS-8, MSM511000C-7,  KM44C16100B-5, TC514400AJ-6.

Then go to a datasheet archive such as 
<http://www.datasheetarchive.com/> and enter the part number in the 
search field.  It often helps to truncate the trailing characters 
back to the first number in the body.  E.g. HM5116400, MSM511000, 
KM44C16100, etc.

The datasheet will tell you the capacity and organization of the 
chip.  For example, a 1 MB 30 pin SIMM with eight chips on it will be 
composed of 1M X 1 chips.  These have one million addresses with 1 
bit at each address.  Eight of them working in parallel provide 1 
million addresses with eight bits at each address or 1 megabyte.

Multiply the total capacity of the chip by the number of chips on the 
SIMM.  Remember that you're working with bits here, not bytes. 
Divide by 8 and you've got the capacity in megabytes--except...

Some SIMMs are parity SIMM and they are based on 9 bits of data 
rather than 8 bits of data, so you'll need to divide that capacity by 
9, not by eight for a parity SIMM.  A 30 pin parity SIMM will have 
nine or three chips instead of eight or two, so they're fairly easy 
to identify.

However, a three chip 30 pin SIMM will have two chips with a certain 
capacity and a third chip with 1/4 the capacity or either of the 
other two.  In this case, calculate the total capacity of the two 
larger chips and divide by eight.  Or find the capacity in bits of 
one big chip and divide by four.

In most cases, if the SIMM has eight or nine chips, then the capacity 
in bytes is equal to the number of addresses any of the chips 
supports (see the datasheet).  If the SIMM has three chips, then the 
capacity in bytes is still equal to the number of address which any 
of the three chips supports.

For example, you find a three chip SIMM with two 4M X 4 chips and one 
4M X 1 chip on board.  The capacity of this SIMM is 4MB or 4 
Megabytes.  You find a SIMM with eight or nine 4M X 1 chips on board, 
its capacity is also 4MB.

The real trick is figureing out the capacity of the chips from the 
markings on them.  Google searches sometimes help, but often (almost 
always) just lead you to chip distributers spamming the search engine 
space with part numbers to lead part searches to their sites.  They 
often don't even have the chip in question, and rarely have any 
useful information available on their website.

SIMMs that can steer you wrong are composite SIMMs where groups of 
smaller capacity chips are used to build a higher capacity SIMM.  For 
example, building a 16 MB 30 pin SIMM out of eight 4M X 4 chips. 
These are rare and should be easily identified because there should 
be a non-memory chip on board to handle the address translations.

Jeff Walther





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