Task Control Block
The Task Control Block (TCB) is a data structure which contains the state of a task in, e.g., OS/360 and successors on IBM System/360 architecture and successors. == The TCB in OS/360 and successors == In OS/360, OS/VS1, SVS, MVS/370, MVS/XA, MVS/ESA, OS/390 and z/OS, the TCB contains, among other data, non-dispatchability flags and the general and floating point registers for a task that is not currently assigned to a CPU. A TCB provides an anchor for a linked list of other, related request blocks (RBs); the top-linked RB for a TCB contains the Program status word (PSW) when the task is not assigned to a CPU. When the control program's dispatcher selects a TCB to be dispatched, the dispatcher loads registers from the TCB and loads the PSW from the top RB of the TCB, thereby dispatching the unit of work.