High Quality Content by WIKIPEDIA articles! 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. High level language programmers use library routines containing DCBs.