Data Control Block


In IBM mainframe operating systems, such as OS/360, MVS, z/OS, a Data Control Block is a description of a dataset in a program. A DCB is coded in Assembler programs using the DCB macro instruction. High level language programmers use library routines containing DCBs.
A DCB is one of the many control blocks used in these operating systems. 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. A DCB may be compared to a FILE structure in C, but it is much more complex, offering many more options for various access methods.
The control block acted as the Application programming interface between Logical IOCS and the application program and usually was defined within the application program itself. 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"

Typical contents of a DCB

Many of the constants and variables contained within a DCB may be left blank.
The OPEN process results in a merge of the constants and variables specified in the DD JCL statement, and the dataset label for existing magnetic tape and direct-access datasets, into the DCB, replacing the zero values with actual, non-zero values.
A control block called the JFCB initially holds the information extracted from the DD statement for the dataset. 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.
An example is the BLKSIZE= variable, which may be specified in the DCB as zero. In the DD statement, the BLKSIZE is specified as a non-zero value and this, then, results in a program-specified LRECL and a JCL-specified BLKSIZE, with the merge of the two becoming the permanent definition of the dataset.