Introduction to task control blocks (TCBs)

A task control block (TCB) represents a z/OS® task. A running CICS® system is a z/OS address space containing multiple TCBs and hence multiple z/OS tasks. Within CICS, a CICS task itself runs on a TCB. Before the open transaction environment (OTE) was implemented in CICS, CICS used a single TCB to run all user applications, this TCB is known as the quasi-reentrant TCB (QR TCB).

A TCB is a dispatchable unit of work in z/OS. The z/OS dispatcher dispatches TCBs. The CICS dispatcher dispatches CICS tasks to run on the QR TCB and other TCBs.