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Execute Channel Program in Real Storage

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In IBM mainframe operating systems, Execute Channel Program in Real Storage (EXCPVR) is a supervisor call for low-level device access, where the programmer is responsible for providing a list of device-specific channel commands to be executed by I/O channels, control units and devices. Additionally, the programmer is expected and required to ensure that the input/output data areas are in real storage and are "fixed".

EXCPVR may be viewed, in the abstract, as a V=R version of EXCP. The user of EXCPVR must be authorized and must remain in supervisor mode and key throughout the EXCPVR process.

EXCPVR's front-end is always in TCB mode, as EXCPVR is a Type 1 SVC. In MVS and subsequent instances of the OS, EXCPVR's back-end is always in SRB mode, but the back-end contains emulation code which allows the exits developed for earlier instances of the OS to function largely as before.

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