gtpc3m0wConcepts and Structures

Data Event Control Block Overview

Before the addition of TPF data event control block (DECB) support, the TPF 4.1 system restricted the number of entry control block (ECB) data levels (D0-DF) that were available for use to 16 (the number of data levels defined in the ECB). A DECB can be used in place of data levels for FIND/FILE-type I/O requests by applications. Although a DECB does not physically reside in an ECB, it contains the same information as standard data levels: a core block reference word (CBRW), a file address reference word (FARW), file address extension words (FAXWs), and a detailed error indicator.

TPF programs can dynamically create and release DECBs by using C++ functions tpf_decb_create and tpf_decb_release, or the DECBC macro. The number of DECBs that can be created is only limited to the storage that is available in the ECB private area (EPA).

DECB frames are connected to page 2 of the ECB from address field CE2DECBPT. The DECB frames contain multiple DECBs that hold the CBRW and FARW. A DECB may contain the address of a working storage core block. This address (or pointer) is maintained in the CBRW section of the DECB. Even though an address is in the CBRW, this does not mean that the core block is attached. The core block type indicator will not be X'0001' when attached. Figure 20 shows the format of a DECB:

Figure 20. Format of a DECB


For more information about DECBs, see TPF Application Programming.