[development] Upgrade system

Hi, Last night I converted E-Commerces old update system to the very cool new one. However I did run over 1 issue. When I am loading a fresh version of the schema, how do I tell the system which revisions have already been loaded. I know that the update system takes this from the system table, but I couldn't even find where Drupal does this. If anyone can enlighten me, I will be much appreciated. Thanks in advance. Gordon.