In IBM mainframe

IBM mainframe

IBM mainframes are large computer systems produced by IBM from 1952 to the present. During the 1960s and 1970s, the term mainframe computer was almost synonymous with IBM products due to their marketshare...

 operating system

Operating system

An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...

s, such as OS/360, MVS

MVS

Multiple Virtual Storage, more commonly called MVS, was the most commonly used operating system on the System/370 and System/390 IBM mainframe computers...

, z/OS

Z/OS

z/OS is a 64-bit operating system for mainframe computers, produced by IBM. It derives from and is the successor to OS/390, which in turn followed a string of MVS versions.Starting with earliest:*OS/VS2 Release 2 through Release 3.8...

, a Data Control Block (DCB) is a description of a dataset

Data set (IBM mainframe)

data set , dataset , is a computer file having a record organization. The term pertains to the IBM mainframe operating system line, starting with OS/360, and is still used by its successors, including the current z/OS. Those systems historically preferred this term over a file...

 in a program. A DCB is coded in Assembler

Assembly language

An assembly language is a low-level programming language for computers, microprocessors, microcontrollers, and other programmable devices. It implements a symbolic representation of the machine codes and other constants needed to program a given CPU architecture...

 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 method

Access method

An access method is a function of a mainframe operating system that enables access to data on disk, tape or other external devices. They were introduced in 1963 in IBM OS/360 operating system...

s.

The control block acted as the Application programming interface

Application programming interface

An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...

 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)http://publib.boulder.ibm.com/infocenter/zvm/v5r4/index.jsp?topic=/com.ibm.zvm.v54.dmsa6/hcsd3b00155.htm

Typical contents of a DCB

• symbolic file name (to match a JCL
  Job Control Language
  Job Control Language is a scripting language used on IBM mainframe operating systems to instruct the system on how to run a batch job or start a subsystem....
  
   statement for opening the file)
• type of access (eg. random, sequential, indexed)
• physical characteristics (blocksize
  Storage block
  A storage block is a physical sector on the surface of a disk or diskette. It is the smallest unit of transference between the main memory and a given disk drive....
  
  , logical record length)
• number of I/O
  I/O
  I/O may refer to:* Input/output, a system of communication for information processing systems* Input-output model, an economic model of flow prediction between sectors...
  
   buffers to allocate for processing to permit overlap of I/O
• address of I/O
  I/O
  I/O may refer to:* Input/output, a system of communication for information processing systems* Input-output model, an economic model of flow prediction between sectors...
  
   operating system
  Operating system
  An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
  
   library subroutines (eg. read/write)
• other variables as required by the subroutines according to type