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Start Input/Output
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<p>In <a href="/facts/MVS%2f370/VPSmB9ML">MVS/370</a> and successor versions of <a href="/facts/IBM_mainframe/0kPkgADx">IBM mainframe</a> <a href="/facts/Operating_system/8XTuvMh2">operating systems</a>, Start Input/Output (STARTIO) is a macro instruction and a "branch entry" for low-level device access, where the programmer is responsible for providing a list of device-specific <a href="/facts/Channel_command_word/vDBtN2wQ">CCWs</a>, that is, a <a href="/facts/Channel_program/vDBtN2wQ">channel program</a>, to be executed by I/O channels, control units and devices and a number of "exits", several of which may be immediate returns to the <a href="/facts/Input%2fOutput_Supervisor/zdRyMSTl">Input/Output Supervisor</a> (IOS). Invokers of STARTIO must be in supervisor mode and key 0. STARTIO interfaces directly with the IOS component of MVS. </p>