In IBM mainframe operating systems, such as OS/360, MVS, z/OS, a Data Control Block (DCB) is a description of a dataset in a program.
A control block is a data area with a predefined structure, very similar to a C struct, but typically only related to system's functions.
The addresses of I/O subroutines would be resolved during a linkedit phase after compilation or else dynamically inserted at OPEN time.
The equivalent control block for IBM DOS/360, DOS/VSE and z/VSE operating systems is a "DTF" (Define the file) Many of the constants and variables contained within a DCB may be left blank (i.e., these default to zero).
The results of the merge are stored in the JFCB which may also be written into the DSCB during the CLOSE process, thereby making the dataset definition permanent.