In z/OS®, dispatchable units of work are represented by two kinds of control blocks: Task and service request blocks.

Task control blocks (TCBs)

These control blocks represent tasks executing within an address space, such as user programs and system programs that support the user programs.

A TCB contains information about the running task, such as the address of any storage areas it has created. Do not confuse the z/OS term TCB with the UNIX® data structure called a process control block or PCB.

TCBs are created in response to an ATTACH macro. By issuing the ATTACH macro, a user program or system routine begins the execution of the program specified on the ATTACH macro, as a subtask of the attacher's task. As a subtask, the specified program can compete for processor time and can use certain resources already allocated to the attacher's task.

The region control task (RCT), which is responsible for preparing an address space for swap-in and swap-out, is the highest priority task in an address space. All tasks within an address space are subtasks of the RCT.

Service request blocks (SRBs)

These control blocks represent requests to execute a system service routine. SRBs are typically created when one address space detects an event that affects a different address space; they provide one mechanism for communication between address spaces.

The routine that performs the function or service is called the SRB routine ; initiating the process is called scheduling an SRB; the SRB routine runs in the operating mode known as SRB mode.

An SRB is similar to a TCB in that it identifies a unit of work to the system. Unlike a TCB, an SRB cannot "own" storage areas. SRB routines can obtain, reference, use, and free storage areas, but the areas must be owned by a TCB. In a multiprocessor environment, the SRB routine, after being scheduled, can be dispatched on another processor and can run concurrently with the scheduling program. The scheduling program can continue to do other processing in parallel with the SRB routine. As mentioned earlier, an SRB provides a means of asynchronous inter-address space communication for programs running on z/OS.

Only programs running in a mode of higher authority called supervisor state can create an SRB. These authorized programs obtain storage and initialize the control block with things such as the identity of the target address space and pointers to the code that will process the request. The program creating the SRB then issues the SCHEDULE macro and indicates whether the SRB has global (system-wide) or local (address space-wide) priority. The system places the SRB on the appropriate dispatching queue where it will remain until it becomes the highest priority work on the queue.

SRBs with a global priority have a higher priority than that of any address space, regardless of the actual address space in which they will be executed. SRBs with a local priority have a priority equal to that of the address space in which they will be executed, but higher than any TCB within that address space. The assignment of global or local priority depends on the "importance" of the request; for example, SRBs for I/O interrupts are scheduled at a global priority, to minimize I/O delays.