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Undid GF edit by Ira Leviton Those weren't typos - EXCP is the name of the macro and the service is normally referred to by the macro name rather than the long name
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{{Short description|Computer science macro}}
{{refimprove|date=August 2019}}
In [[IBM mainframe]] [[operating system]]s, '''Execute Channel Program in Real Storage''' ('''EXCPVR''') is a supervisor[[Macro (computer science)|macro]] generating a [[system call]], implemented as a [[Supervisor Call instruction]], for low-level device access, where the programmer is responsible for providing a list of device-specific [[Channel_command_word#Channel_command_words|CCWs]], that is, a [[Channel_program#Channel_Program|channel program]], to be executed by I/O channels, control units and devices.<ref name="Murphy1995">{{cite book|author=Hank A. Murphy|title=MVS control blocks|year=1995|publisher=McGraw-Hill Ryerson, Limited|isbn=978-0-07-044309-9|pages=121, 134–135}}</ref> Additionally, the programmer is expected and required to ensure that the input/output data areas are in real storage and are "fixed", either prior to invoking EXCPVR or via the page fix appendage. EXCPVR can in some situations provide improved performance compared to EXCP.<ref name="Sacks1994">{{cite book|author=David J. Sacks|title=MVS Answer Book|date=16 March 1994|publisher=Wiley|isbn=978-0-471-60821-9|page=74}}</ref>
 
==Relationship to Execute Channel Program (EXCP)==
EXCPVR may be viewed, historically, as a V=R (i.e., Virtual=Real) version of [[Execute Channel Program|EXCP]]. However, EXCPVR is not restricted to V=R applications. Indeed, EXCPVR may refer to non-V=R data areas provided such data areas are "fixed" and the channel command words which reference such data areas have been translated from virtual to real addresses by the programmer using the LRA [ Load Real Address ] privileged instruction, as the data channels deal only with real addresses{{efn|Except for 4300 machines equipped with ECPS:VSE}}, not virtual addresses. EXCPVR was first introduced in SVS and was continued in MVS/370.
 
==Support for Format 1 CCWs==
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[[Category:IBM mainframe operating systems]]
[[Category:System calls]]
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