Browse Prior Art Database

Transfer and Set Mode in Channel

IP.com Disclosure Number: IPCOM000105589D
Original Publication Date: 1993-Aug-01
Included in the Prior Art Database: 2005-Mar-20
Document File: 2 page(s) / 89K

Publishing Venue

IBM

Related People

Betz, JF: AUTHOR [+2]

Abstract

Disclosed is a method that allows operating systems to provide virtual storage constraint relief (VSCR) by allowing I/O related control blocks that contain channel program prefixes to always reside in 31 bit addressable storage.

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

Transfer and Set Mode in Channel

      Disclosed is a method that allows operating systems to provide
virtual storage constraint relief (VSCR) by allowing I/O related
control blocks that contain channel program prefixes to always reside
in 31 bit addressable storage.

      All I/O Queue elements (IOQ) can be moved above the 16 megabyte
line if the prefix channel program (which resides in each IOQ), can
run as a format 1 channel program (having 31 bit addressing
capability) but switch modes to format 0 upon execution of a new type
of TIC (transfer in channel) CCW called the Transfer and Set Mode in
Channel CCW (TSMIC).  This would enable the prefix program to TIC to
a users channel program without needing to run in the same format
mode as the user's channel program.  In all cases, the subchannel
would indicate the mode of the channel program at that current
instant, so that a Store Subchannel (STSCH) instruction would detect
the current format, and the Interrupt Response Block (IRB) would
reflect the final format mode upon execution of the Test Subchannel
(TSCH) instruction.

      The channel subsystem (CSS) can implement a new version of the
Transfer In Channel (TIC) instruction, called the TSMIC instruction.
This new instruction will perform the same function as the TIC in
that it will branch to the instruction address contained as the
address operand of the instruction.  In addition to branching, the
instruction will cause the format flag in the subchannel to toggle to
the opposite format.  This capability causes the prefix program
residing above the 16 MB line to be able to toggle the format mode to
format 0 and branch to the format 0 channel program, thus allowing
the prefix program residing in the IOQ to be moved above the line.

      When calling the device support code to build the prefix
channel program, the I/O Supervisor (IOS) passes a flag indicating
which format mode is to be used by the prefix channel program.  IOS
would always pass the flag indicating a format 1 prefix program is to
be built, but save the real format and reset the format flag
indication upon return from the device support code.

      After the device dependent Start I/O Exit (SIO exit) builds the
channel program prefix, IOS scans the prefix channel program for only
those programs that have a prefix built and for which a mode switch
is required.  IOS then converts all TIC CCWs into TSMIC CCWs whenever
the target of the TIC is equal to the real address of the user's
channel program.  The maximum size of the channel prefix is 64 bytes
in MVS, or 8 CCWs (typically less because room is needed for the
channel program data).

      In this way, only those TICs that go to the user channel
program will cause a mode switch to occur.  This is needed because of
cases where a prefix channel program could conceivably TIC within
itself causing a premature format mode switch to format 0.  An
example of such a prefix program mig...