IMS DB Question:

PSBGEN control statements consists of Assembler Language macro statements. The macros are supplied by IBM in a library named IMSVS.MACLIB. That invokes a cataloged procedure usually called PSBGEN and stored in PSBLIB. Each PSB consists of one or more control blocks called Program Control Block (PCB). Each PCB within a PSB defines one logical structure. All the PCBs within a PSB are collectively knows as an Application Data Structure.

IMS allows a PSB to define more than one logical data structure because a program is allowed to access to more than one database. A program us also allowed to concurrently access different parts of the same database. IMS allows the DBA to define any number of logical data structure in a single PSB.