z/OS® is made up of programming instructions that control the operation of the computer system.

These instructions ensure that the computer hardware is being used efficiently and is allowing application programs to run. z/OS includes sets of instructions that, for example, accept work, convert work to a form that the computer can recognize, keep track of work, allocate resources for work, execute work, monitor work, and handle output. A group of related instructions is called a routine or module. A set of related modules that make a particular system function possible is called a system component. The workload management (WLM) component of z/OS, for instance, controls system resources, while the recovery termination manager (RTM) handles system recovery.

Sequences of instructions that perform frequently used system functions can be invoked with executable macro instructions, or macros. z/OS macros exist for functions such as opening and closing data files, loading and deleting programs, and sending messages to the computer operator.

As programs execute the work of a z/OS system, they keep track of this work in storage areas known as control block. In general, there are four types of z/OS control blocks:

• System-related control blocks
• Resource-related control blocks
• Job-related control blocks
• Task-related control blocks

Each system-related control block represents one z/OS system and contains system-wide information, such as how many processors are in use. Each resource-related control block represents one resource, such as a processor or storage device. Each job-related control block represents one job executing on the system. Each task-related control block represents one unit of work.

Control blocks serve as vehicles for communication throughout z/OS. Such communication is possible because the structure of a control block is known to the programs that use it, and thus these programs can find needed information about the unit of work or resource. Control blocks representing many units of the same type may be chained together on queues, with each control block pointing to the next one in the chain. The operating system can search the queue to find information about a particular unit of work or resource, which might be:

• An address of a control block or a required routine
• Actual data, such as a value, a quantity, a parameter, or a name
• Status flags (usually single bits in a byte, where each bit has a specific meaning)

z/OS uses a huge variety of control blocks, many with very specialized purposes. The three most commonly used control blocks are:

• Task control block (TCB), which represents a unit of work or task
• Service request block (SRB), which represents a request for a system service
• Address space control block (ASCB), which represents an address space