The other day, I removed a Pentium 4 from an old PC that I was dismantling. I was intrigued by the little brown components on its backside (sorry about the picture quality):
Here, there is a grid of gold contacts (in groups of 8). Some of the groups of 8 contacts have a tiny brown IC soldered1. Most don't. To me, it looks like those empty pads are for soldering on more brown things. (A lot of other boards have such spots where you can add "expansion packs"--for example, an avr development board I've worked with has spots for extra EEPROM, some LEDs, and a motor driver chip).
But I couldn't think of any use of an "expansion pack" on a microprocessor. The most I could think of was "something or the other to make branch prediction more efficient", but that's probably not it.
I found a pic of another Pentium (this one's LGA though), with brown thingies in all spots:
So, my question is, what is the function of these brown ICs, and what do I get by adding more to my Pentium, if it is possible to do so?
1. I know that "soldered" probably isn't the right word here -- but basically there is an IC attached to the group, with one IC pin per pad