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Unit Control Block
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<p>In <a href="/facts/IBM_mainframe/0kPkgADx">IBM mainframe</a> <a href="/facts/Operating_system/8XTuvMh2">operating systems</a> <a href="/facts/OS/360_and_successors/nQbdNbx6">OS/360 and its successors</a>, a Unit Control Block (UCB) is a <a href="/facts/Object_composition/zWbRC9Jd">memory structure</a>, or a <i>control block</i>, that describes any single <a href="/facts/Input/output/WQTE8Jas">input/output</a> <a href="/facts/Peripheral_device/3lEBPE14">peripheral device</a> (<i>unit</i>), or an <i>exposure</i> (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. </p><p>Some other operating systems have similar structures. </p>