server thread context: (1) all point to single PCB for Sprite - don't have info in proc table for all server threads, don't have much debugging info (2) nil (3) point to per-thread full PCB - lots of unused fields in PCB o need back pointer in the PCB to thread (4) point to per-thread "server" PCB - have to be careful about accessing the union type correctly o need back pointer in the PCB to thread