How does vista talk to the DVD drive during setup

[p4ntb0y]

I have spanned the installation media accross 4 CD's and would like to know how Winpe asks for each CD for the install.wim which has been split accross 4 CD's, at the file copy phase it ask's for each CD in turn, it then ask's for the first CD once it has copied all the SWM files to the drive.

Depending on what partions you have etc can change the cd rom letter so my question is how does winpe vista handle this?