In IBM mainframe operating systems from the OS/360 and successors line, a Unit Control Block (UCB) is a memory structure, or a control block, that describes any single input/output peripheral device (unit), or an exposure (alias), to the operating system. Certain data within the UCB also instructs the Input/Output Supervisor (IOS) to use certain closed subroutines in addition to normal IOS processing for additional physical device control.
A similar concept in Unix-like systems is a kernel's devinfo structure, addressed by a combination of major and minor number through a device node.
Read more about Unit Control Block: Overview, Handling Parallel I/O Operations
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