View Full Version : SIMM 72 paring

January 27th, 2012, 05:59 AM
Ah yes, i recall that a SIMM30 machine with a 286 or up needed to Pair it's SIMM30 modules.
The Pentium was the revival of that technique which was thought to been banned for life after release of the SIMM72 modules.

This was slightly before i got interested in computers somewhere in 1998 the year in which i got my very first machine but that is a story for a different time folks. ;)

January 27th, 2012, 08:52 AM
Heck, folks were still arguing about this when the 80386 came out, insisting that you had to install 72 pin SIMMs in matched pairs. I never did.

January 27th, 2012, 12:27 PM
I know my 486 actually takes them as single chips, not pairs and still boots. I did play with a Pentium once that wanted ONLY matched pairs though, was a pain for me as 2 chips of the same size wouldn't work together, but not as much of a pain as some of the old servers that want 4 chips of exactly identical Reg ECC DDR1 ram...

January 28th, 2012, 02:04 AM
30-pin SIMMs are only 8-bits wide. That's why you need to fit them in pairs on a 16-bit system like a 286 or 386SX, and in fours on a 32-bit 386DX or 486.

72-pin SIMMs are 32-bits wide. So you only need one on a 32-bit system, and the Pentium needs two because it has a 64-bit bus.

January 28th, 2012, 08:48 AM
When someone said "pair", I recall the argument over "matched pairs". There were a lot of people who insisted on the 30 pin SIMMS be matched (i.e. same manufacturer, same chips) or things wouldn't work. And this carried over to 72-pin SIMMs for the same idiotic reasons. I ignored this "common sense" wisdom with no problems at all.

January 28th, 2012, 09:49 AM
I did the same. Manufacturer, and even number/type of chips had no actual bearing on whether two (or four) SIMMs would work together.

I think my 386 has a matching pair and two odd ones, and it works perfectly.

January 28th, 2012, 06:01 PM
I also have a few pentium era mobos that don't need 2 identical rams to work, some take 1 and some 2.

February 27th, 2012, 06:41 AM
There is a lot of miss knowlege here with 30 pin simms they are 8 data bits wide so a 286,386sx requires 2 a 386dx cpu is 32 bit width so it takes 4 modules now come the 72 pin modules 2 variants fast page memory or edo memory the 72 pin is 32bits wide not 64bit width early 486 boards were fast page ram with parity 9 chips later boards where designed not to require parity chips thereby saving money the edo 72 pin are 32 bits wide but in order to get many to operate correctly they needed to be installed in matche pairs for relyability that is why some people think they are 72 bits wide