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Status-Pending Indicator for Multiple, Virtual, Concurrently Active Channel I/O Devices

IP.com Disclosure Number: IPCOM000101963D
Original Publication Date: 1990-Sep-01
Included in the Prior Art Database: 2005-Mar-17
Document File: 3 page(s) / 123K

Publishing Venue

IBM

Related People

Svec, RS: AUTHOR

Abstract

Disclosed is a circuit for indicating that device status is available for presentation to a channel for any of multiple, concurrently active I/O devices within a control unit. This circuit makes a "mark on the wall" for each device where there is status to present when the device is made virtual, or status is generated for that device while it remains virtual, and also notes whether the channel has refused that status at least once.

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

Status-Pending Indicator for Multiple, Virtual, Concurrently Active Channel I/O Devices

       Disclosed is a circuit for indicating that device status
is available for presentation to a channel for any of multiple,
concurrently active I/O devices within a control unit.  This circuit
makes a "mark on the wall" for each device where there is status to
present when the device is made virtual, or status is generated for
that device while it remains virtual, and also notes whether the
channel has refused that status at least once.

      In multiple-device Channel to Channel architecture, it is
possible for a Channel to Channel Adapter (CTCA) control unit to have
all of its devices concurrently active on commands issued by the
channel [1,2].  Because the channel will only be actively attached to
one of these devices in any given instant, the channel adapter will
put the remaining devices on "hold" in some sort of memory, or
virtual device storage.

      When a device is virtual, it can either have unfinished
business with the channel, or, because of the way the Channel to
Channel Architecture works, can spontaneously have new business with
the channel, without ever having been connected to the channel in the
first place.  This business is called "pending status".

      The channel adapter must try to offer any pending status to the
channel for any device that has such status. The channel, however,
has the option of refusing the pending status on a device by device
basis.  Only after the channel has once refused such status for any
given device, can the channel adapter simply wait until the channel
signals willingness to accept such held status.  Status that the
channel has refused is called "stacked status".

      This circuit provides an indication to the control unit if a
virtual device has "pending" or "stacked" status to offer to the
channel.  Since the device is virtual, or "stored away", there is no
longer a connection to the logic that would normally tell the channel
of the need to transact business.  The circuit consists of the
following elements:
 1.   An address decoder which automatically decodes the device
address of any device being stored into the virtual device storage.
The output of this decoder is one signal line for each possible CTCA
device.
      2.   Signal generating logic associated with the virtual device
storage, whose outputs indicate whether there is pending status being
put away into virtual storage or not, and whether the channel is
requesting that the status be stacked.
      3.   A Pending Status Register (PSR).  This register has a
two-bit position for each possible CTCA device in the channel
adapter.  Outputs from the PSR are a signal line for each possible
device, indicating if there is, or is not, status that should be
offered to the channel.

      4.   A PSR count register, an...