Data Control Block

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 DCB is coded in Assembler programs using the DCB macro instruction (which expands into a large number of "define constant" instructions). 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 (and resided 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" (Define the file)[1]

Typical contents of a DCB

• symbolic file name (to match a JCL statement for opening the file)
• type of access (eg. random, sequential, indexed)
• physical characteristics (blocksize, logical record length)
• number of I/O buffers to allocate for processing to permit overlap of I/O
• address of I/O operating system library subroutines (eg. read/write)
• other variables as required by the subroutines according to type

See also

• Data Set Control Block (DSCB), a part of VTOC