I had my first opportunity to take a 7100 apart today. A user put it on her car and drove off with it and it smashed. It was totally dead but when disassembled it would light the display and make GPRS noises when you were near a speaker. She ended up with a 7290 loaner.

RIM hid the screws for the phone under the metal strip across the back so you have to peel it off and basically exposing yourself as a warranty-breaker.

The interesting part was the hidden battery inside that I found. There is a very small 4 or 5 cm watch style battery soldered to the main board in the phone. I'm not sure as to its purpose and/or what will happen to the phone when that bettery eventually dies in a few years time. Most likely by then you will have changed phones but you never know things can die early.

Maybe because the G is the generic version, the battery holds some carrier specific information in static RAM or perhaps the PIN is held there. I could see a situation like in SUN workstations, when the battery dies, the Serial (or in the phone's case, the PIN) becomes FFFFFFFF

In case anyone is interested, here's the major parts inside the 7100g

ANALOG DEVICES AD6529BABC 0505 608550.1(biggest chip) Probably GSM/GPRS chipset
ANALOG DEVICES AD6521ACA (second biggest) Probably GSM/GPRS chipset
Spansion 70PL254J00BAWA2 0509MVM A (I think this is the ARM cpu)
SEC 516 FI70 K1S321611C (Samsung uTRAM)
Unknown brand chip L2870-A01 449A05
Possible TI DSP chip (very small) near the hidden battery marked AMZ TI 52K AEXJ

It probably a lithium battery that maintains the clock when the main battery is dead or out. It is probably kept topped up by the main battery when it is in and charged.

Well for most phones the clock is syncronized to either the GSM tower or in the case of the blackberry, the clock syncs to the PC.

Would you like to show its photo?

Something got to preserve the volatile memory while the main battery is out of the case.

I don't know that any of the memory in the 7100 is volatile. I made my guess based on experience. I have two batteries, one in the phone and one charged up on deck. When I swap, everything comes back fine -- the dead battery goes to an external charger. I did let the battery die and remain dead for quite some time just the other day. Everything came back fine when I swapped batteries, but the clock seemed to have stopped many hours earlier -- I was then prompted to update the date and time.

So, I would guess, the memory is non-volatile and the battery maintains the clock. The alternative is the battery maintains volatile memory for an extended period of time and some super-cap maintains the clock for a shorter period. One way or the other, they are maintained seperately.