Browse Prior Art Database

Suppress Illegal Length Indication On the Small Computer System Interface While Still Detecting Length Errors

IP.com Disclosure Number: IPCOM000100242D
Original Publication Date: 1990-Mar-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 3 page(s) / 114K

Publishing Venue

IBM

Related People

Chartrand, DM: AUTHOR [+3]

Abstract

Hardware and software can be implemented on the receiv- ing (requestor) side of a direct memory access channel to detect length errors on read operations when the amount of data to be transferred is not equal to the amount of data requested. This hardware and software can detect cases where the amount of data transferred is less than requested, and also when there is more data to be transferred than requested. It can detect the case of more data to be transferred without overflowing the buffer assigned to the data. The server of the data need not notify the requester of the length error when this detection mechanism is used. This is useful for processing records of unknown length on an I/O device (such as tape devices).

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

Suppress Illegal Length Indication On the Small Computer System Interface While Still Detecting Length Errors

       Hardware and software can be implemented on the receiv-
ing (requestor) side of a direct memory access channel to detect
length errors on read operations when the amount of data to be
transferred is not equal to the amount of data requested. This
hardware and software can detect cases where the amount of data
transferred is less than requested, and also when there is more data
to be transferred than requested.  It can detect the case of more
data to be transferred without overflowing the buffer assigned to the
data.  The server of the data need not notify the requester of the
length error when this detection mechanism is used.  This is useful
for processing records of unknown length on an I/O device (such as
tape devices).

      For the purposes of this description, a small computer system
interface (SCSI) implementation of the invention will be discussed.
However, this method is actually broader than the small computer
system interface implementation.

      The small computer system interface - 2 architecture defines a
suppress illegal length indication bit which can be used on read
commands to tape devices.  Using this bit causes the device to return
good completion status to the system even if the block of data on the
tape was actually smaller or larger than requested by the system.
Other architectures using this method may define a different way of
suppressing the illegal length indication.  Using this bit results in
a performance improvement because the host is not required to clear
an error status on the device by issuing a request sense command to
find the cause of the error.  But, it is almost always necessary for
the system to know about the length error even if the suppress
illegal length indication bit is set.  The following invention allows
the system to detect these errors even when they are not indicated by
the device.  This allows the system to use the suppress illegal
length indication bit to increase the small computer system interface
performance while still being able to detect the length errors.

      Throughout this description, the term underlength will be used
to denote the error condition where the block of data on the tape is
smaller than the size requested by the system.  The term overlength
will be used to denote the error condition where the block of data on
the tape is larger than the size requested by the system.  The
components involved in this error detection sche...