Task Control Block

A manifestation of a Process control block in an IBM OS/360_and_successors operating system environment.

Because of the complexities of the OS/360 and successors operating system dispatcher, a TCB does not strictly perform the functions of a Process Control Block, although it may perform some or many of them.

Rather, a TCB provides the anchor for a linked list of other, related control blocks, the composite of which performs the function of a Process Control Block.

Program Status Words may be stored in one control block (usually a PRB), while general purpose registers may be stored in another control block (SVRB, IRB, etcetera).

With the introduction of MVS/370#MVS.2F370 and successor systems, a whole new environment was introduced: the Service Request Block, which generally has a higher priority than any Task Control Block, and, indeed, which itself has two distinct priorities: a Global SRB (priority over all local address space SRBs and TCBs) and a Local SRB (priority over only the designated address space TCBs); and the MVS dispatcher must manage all of these with absolute consistency.