Task Control Block

An instance 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 control program dispatcher, a TCB does not strictly perform the functions of a Process Control Block, although it may perform many of these.

Rather, a TCB provides the anchor for a linked list of other, related request control blocks (RBs), the composite of which (i.e., the TCB plus the top-linked RB) performs the function of a Process Control Block.

Program Status Words may be stored in one control block (usually a PRB, a Program Request Block), while general purpose registers may be stored in another control block (an SVRB, Supervisor Call Request Block, an IRB, Interruption Request Block, and others).

With the introduction of MVS/370 and successor systems, a whole new environment was introduced: the Service Request Block (SRB), 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 local address space TCBs); and MVS's dispatcher must manage all of these with absolute consistency across as many as two processors (MVS/370) and as many as sixteen processors (successor systems).