gtpc3m15Concepts and Structures

File Storage (DASD) Accessing

Although several different file storage devices (DASD), each with their own unique characteristics, are supported by the TPF system, all file accessing is conceptually done in a similar fashion. The generic terms used for file accessing are find (read a record) and file (write a record).

There are many variations of find and file, such as file the record but do not release the working storage block, and find the record and hold (reserving updating exclusively for the holding Entry).

WAITC is an important macro used with I/O operations. WAITC is an application program request to delay further processing until all the pending I/O operations for the application program issuing the WAITC are completed. An I/O counter within the ECB is used for this control. WAITC can be implicitly used with find and file macro requests; for example, find a record and wait. While data transfer occurs, the TPF system is free to shift control to other Entries that are ready for processing.

A level is one of the 16 pairs of data fields in the ECB. These fields are:

The file address reference word (FARW) is used to pass a file address between the application and the control program. The core block reference word (CBRW) is used to pass an address of a main storage block used for storing data. A main storage block is automatically obtained by the control program for a find macro request. Generally, the main storage block is released upon completion of a file macro request. However, the EXITC macro, summarized in Entry Termination (EXIT Processing), releases all main storage blocks associated with an Entry.

The allocation of data to physical file storage and the use of find and file macros have implications that are described in more detail in Data Organization. The important aspects are:

The TPF system handles the details of obtaining the physical location of data, that is, addresses that are directly utilized by the hardware. This is done in stages:

The file address information is in a format for either pool or fixed data record references by the time a find or file macro request is issued. The file address used by an application program is sometimes called symbolic to emphasize that the translation required to obtain the physical location of the data is not complete.

The use of data within fixed records as indexes to pool records is a common technique used in the TPF system environment. The combination of fixed and pool structures provides the application designer with the tools to implement application-oriented data management subsystems.

The TPF Database Facility (TPFDF) product provides a high-level application program interface (API) for database organization and accessing. See TPF Database Facility (TPFDF) for additional information.