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Browse Prior Art Database

Transparent Target Mode for a Small Computer System Interface

IP.com Disclosure Number: IPCOM000101377D
Original Publication Date: 1990-Aug-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 4 page(s) / 171K

Publishing Venue

IBM

Related People

Keener, DS: AUTHOR [+3]

Abstract

A technique is described whereby a transparent target-mode architecture provides a means of reducing the overhead of a host central processing unit (CPU) in small computer system interface (SCSI) (*) applications. The method implemented in this disclosure enables SCSI adapters used in host systems to emulate any type of SCSI target de vice and to execute all of the operations associated with that device, thereby minimizing the systems operational overhead.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 46% of the total text.

Transparent Target Mode for a Small Computer System Interface

       A technique is described whereby a transparent
target-mode architecture provides a means of reducing the overhead of
a host central processing unit (CPU) in small computer system
interface (SCSI) (*) applications.  The method implemented in this
disclosure enables SCSI adapters used in host systems to emulate any
type of SCSI target de vice and to execute all of the operations
associated with that device, thereby minimizing the systems
operational overhead.

      Typically, a SCSI device requires several of the possible eight
phases to complete a command.  These phases are: selection,
re-selection, data out, data in, message out, message in, command out
and status in.  A SCSI adapter used in conjunction with a host CPU
provides the means of coordinating the phases for the completion of
an operation. When used to emulate a target SCSI device, such as disk
storage units or printers, the SCSI adapter performs operations
requested by a SCSI initiator device.  These include:  a) the
selection from a given initiator; b) generating and responding to
messages from the initiator, including disconnection and reconnection
and synchron ous data transfer message exchange; c) responding to
commands from the given initiator; d) receiving or transmitting data;
e) giving final status; and f) giving a final message.

      The concept described herein provides an operational protocol
for the transfer of work from a host CPU to the SCSI adapter when the
CPU has been selected as a SCSI target.  A general-purpose target
mode is provided with two interrupts to the host for each target
command.  The first interrupt occurs after the command phase so as to
let the system know that a command is available for interpretation.
The second interrupt occurs after the data phase so as to let the
system know that the data is transferred and that the final status
and message can be sent to the initiator. Abort and reset messages
are handled automatically by the SCSI adapter and an error is
returned to the host if a command was in progress when an abort or
reset message was received.  Synchronous data transfer messages are
handled without host intervention.  Disconnection and reconnection
are controlled by option bits in an Enable/End target subsystem
control block (SCB).  The SCSI adapter handles the protocol
implementation so that if the host system does not have an Enable/End
SCB active in the SCSI adapter, a selection by an initiator will
result in a Not Ready indication being sent back to the initiator.

      Fig. 1 illustrates the Enable/End SCB which is used to enable
the SCSI adapter for target mode.  The SCB contains a mode control
field to tell the SCSI what type of target mode to enable and mode-2
is defined to be the transparent mode. To allow SCSI
disconnection/reconnection to free the SCSI bus while the system is
processing the operation, the no disk (ND) disconnect...