Browse Prior Art Database

Cancel Subchannel Enhancement

IP.com Disclosure Number: IPCOM000104813D
Original Publication Date: 1993-Jun-01
Included in the Prior Art Database: 2005-Mar-19
Document File: 2 page(s) / 61K

Publishing Venue

IBM

Related People

Brice Jr, FW: AUTHOR [+3]

Abstract

The CANCEL SUBCHANNEL (XSCH) instruction is enhanced to clear internal indications of "busy" within the ESA/3901 channel subsystem. This facilitates the use of the instruction by programming to perform channel-path recovery.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Cancel Subchannel Enhancement

      The CANCEL SUBCHANNEL (XSCH) instruction is enhanced to clear
internal indications of "busy" within the ESA/3901 channel subsystem.
This facilitates the use of the instruction by programming to perform
channel-path recovery.

      As originally described in [*], the CANCEL SUBCHANNEL (XSCH)
instruction was created to allow programming to cancel an I/O request
that is currently queued in a subchannel.  This earlier article
described how the XSCH instruction is useful in I/O recovery
situations, particularly channel-path recovery.  The ability to issue
XSCH in place of CLEAR SUBCHANNEL (CSCH) when the I/O request has not
yet started avoids the use of needless device-level error-recovery
procedures.  Instead, the original I/O request is simply re-issued
following a successful higher-level recovery.

      A problem with the originally-disclosed XSCH instruction was
that channel-path recovery typically involves the use by programming
of Sense Channel-Path Identifier (SNID) and Set Channel-Path
Identifier (SPID) CCW commands.  These commands are essentially
control-unit commands and are processed without regard to any
device-busy conditions at the devices to which the commands are
addressed.  However, these commands can be prevented from ever
reaching the control unit by internal indications of "busy" that are
maintained within the channel subsystem.  Thus, false "busy"
conditions that the recovery actions hope to resolve can impair the
reliability of the XSCH instruction as a recovery...