COBOL BL

 DMAP compiler option create the data division map in the compile listing. BL, base locator, etc just the base address for a 4K block of virtual memory. BL is an index (start from 1) to a table of base address kept in TGT (target global table). The value is to be loaded into a GPR as base register. A field in data division can be access with a displacement. The compile listing will show the BL and DISP clay for each field in the program. 

BLL (base link locater) is similar to BL but for field defined in the linkage section of the program. BLL actual storage valus are also kept in TGT

Address space TCB

 When an address space is created, the first tcb is handcrafted into the address space to kick start the processing required later. The first tcb is RCT (region control task) which initialixe the address space and prepare the address space for swap out and swap in. It will attached the dump task used to dump the address space. It will then start the STC (started task control) task which analyze the command used to start this address space (START, MOUNT, LOGIN) and start the next task accordingly. 

In case of START INIT command (batch), STC will start the initiator task which will start the job step task. 

LOGIN will start the TMP (terminal monitor program) and MOUNT will start the mount processor instead.