Blown Southbridge sorta still working

My inspirion 1150's southbridge chip, the 82801DB, was blown in a power surge recently.  The laptop was unplugged from the outlet, but I left my external USB HD plugged into the outlet and USB port, and it took out the 82801DB chip.  The chip literally has a hole on it and burned physically, and now the USB ports are completely dead, having me rely on using a CardBus adapter now.

However, I thought the fact that the other functions of the chip working, is completely odd.  For example, the AC97 audio controller portion of it is still working very well, and so is the SMBus controller, which appears to be integrated into the 82801DB chip.

So, how can the USB function be fried along with the physical part of the chip, yet the audio controller on it still be working perfectly fine?  I thought once the "smoke" gets out it's all over.

EDIT:  I thought it was also interesting, that with the Cardbus adapter, while using the laptop with my M-Audio MIDI controller, it was able to handle Reason, and Native Instrument FM7/8 while on stage yesterday, and the audio performed extremely well, without any bugs or breaks in the audio.