gtpc3m16 | Concepts and Structures |
Magnetic tapes are used for online and offline processing and can be used for both input and output in the TPF system. Tapes are categorized as either real-time or general tapes.
Real-time tapes are used to log transactions and collect dynamic system information for use by TPF utility and performance reporting programs and to record main storage dumps resulting from system errors. Real-time tapes are available to any Entry in the system at any time, but ECB-controlled programs that use real-time tapes cannot depend on creating consecutive records. Offline programs that eventually process real-time tapes contain logic to select specific records. The TPF system uses real-time tapes only for output.
Like most operating systems, the TPF system provides the customary tape open, close, read, and write macros, which are used when accessing a general tape. These macros are adequate for low priority jobs using a sequential data structure. Some TPF online batch-oriented Entries with excessive running time are segmented into several shorter entries, called phases, through the use of create entry macro requests. General tapes are usually associated with an entire transaction (in this case, transaction very closely corresponds to a batch-oriented job). TPF tape reserve and assign macro requests permit a transaction to pass an open set of general tapes from one Entry to another Entry.
Symbolic addressing is used for both real-time and general tapes, thus permitting application programs to address tapes through symbolic tape names assigned to hardware addresses. For example, the primary real-time tape is designated as RTA while the logging real-time tape is designated as RTL.
The tape status table (TSTB) is used to control tape operations. This table contains hardware addresses, device status, symbolic name assignments, and queuing and chaining information.
Tape devices are, in general, unique to a single central processing complex (CPC) in a loosely coupled complex. Magnetic tape addresses are allocated to a particular CPC through the use of a shared pool of addresses assigned as a result of an operational message that manages tape configurations.