There was a time when the only limit on how much memory could be installed in your computer was down to the size of your salary. The top module is of the 30 pin type and you could get silver or gold contacts. It could make a difference.
The 72 pin module below it was not always a straight 72. In some cases it was 70 + 2. IBM MCA architecture machines used the + 2 pins to communicate the type and size to the computer, and they would not work in other makes which required the more standard 72 pin. They looked exactly the same.
SIMMs could run singly or in pairs. More often that not memory slots were paired, so one saw either two slots or four slots. The first bank could have one or two modules installed, but the second bank if used had to have both slots filled. Oh, and they had to be matched pairs.
The price of memory doesn’t just determine how much you can install. It can also force an upgrade. Why? Well, if you have four slots but only two are filled, there is a temptation to buy more. A bit of searching on the Internet reveals that your memory is four times the price of memory for the latest motherboards, and you quickly work out that it would almost be as cheap to upgrade memory, CPU and motherboard than it would cost to buy matched pairs of older technology memory. Ouch!!
That is why you bought these. The image shows a 256mb module. The great thing about 168 pin modules was that motherboards often had only two or three slots, so you could use one, two or three sticks and size didn’t matter. Unfortunately, the maximum memory capacity of three slot motherboards was 768mb, great for Windows 2000 or even XP. Motherboards designed with harder work in mind sported four slots, but that involved pairing again. The major advantage was that the four slots would accommodate 512mb and sometimes even 1gb modules.
You knew that the party wouldn’t last though, eh. Just when you thought it was safe to fill all four slots, along came DDR memory and 64bit. Faster, more pins, less notches, and yes, you needed a new motherboard and CPU. It got worse. DDR2 followed in hot pursuit. You already guessed that DDR and DDR2 are not interchangeable. Neither is DDR3.
While none of the above are slot compatible, it doesn’t end there. Some machines do not like double sided modules. Memory speed may not be accepted. In four slot machines, positioning of memory if different speeds, make and size can make a difference.
When I added two 1gb modules to my machine, I thought that it would never make it to the desktop again. It did finally but then wouldn’t shut down on the same day as the instruction was given. Both pairs were of different make but same speed, and both pairs worked if installed without the other. So I swapped the memory in the paired slots and hey presto, it worked like a dream.
It is a little disconcerting that, in the year 2009 AD, you still can’t buy memory from your favorite store and be reasonably sure that it will work in your computer, even though it is ostensibly the exact same type as specified in the computer manual. Even memory bought as matched pairs does not always work well, despite the claim that matched pairs is the way to go.