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Structure for Controlling Small Computer System Interface Logical Devices in an Independent And Overlapped Manner

IP.com Disclosure Number: IPCOM000100880D
Original Publication Date: 1990-Jun-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 3 page(s) / 133K

Publishing Venue

IBM

Related People

McNeill, AB: AUTHOR [+2]

Abstract

The Small Computer System Interface (SCSI) architecture has the ability to connect a broad range of devices. These devices include direct access devices, sequential access devices, printers, scanners, optical devices and communication devices. There are sequences of phases to complete each command; for example, to send a reset to a device there is a selection phase to connect to the appropriate device followed by a message phase to send a reset message. Each device type requires different commands to access it and a variety of sequences to complete the command. Because devices have dead time (e.g.

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Structure for Controlling Small Computer System Interface Logical Devices in an Independent And Overlapped Manner

       The Small Computer System Interface (SCSI) architecture
has the ability to connect a broad range of devices.  These devices
include direct access devices, sequential access devices, printers,
scanners, optical devices and communication devices.  There are
sequences of phases to complete each command; for example, to send a
reset to a device there is a selection phase to connect to the
appropriate device followed by a message phase to send a reset
message.  Each device type requires different commands to access it
and a variety of sequences to complete the command.  Because devices
have dead time (e.g., a printer while it is printing; a fixed disk
while it is seeking; a tape drive while it is rewinding), each
device, if possible, releases the SCSI bus (disconnects) during dead
time to allow other commands to start up and then gets back on the
bus (reconnects) when the dead time is over.  A structure is needed
which overlaps any SCSI device (all device types allowing for
disconnect/reconnect).

      The SCSI logical device control structure is based on logical
device numbers (LDNs) which are assigned to SCSI devices (physical
unit/logical unit pairs).  A subsystem control block (SCB) is sent
from the host system to an adapter for a given LDN.  In the case of
the adapter as an initiator, this command is translated into data
structures for each phase of the given command sequence and an
expected phase sequence byte is assigned to the given LDN.  Each LDN
has its own data structure area in memory for each phase. These
structures include a message area for the message-out phase, a
command area for the command-out phase and a data transfer control
block (DTCB) area for the data phase, as shown in Fig. 1.  By setting
these phases up ahead of time, once connected to the SCSI bus, the
transfer time is significantly reduced.  Once this translation is
complete, the system command is ready for SCSI bus connection.  The
LDN is then placed on an arbitration queue which is a queue of LDNs
waiting for a free SCSI bus to use to select or reselect a peripheral
device.  In the case of the adapter as a target, the LDN is enabled
and in general the DTCBs are set up for the maximum system buffer
size.

      SCSI bus utilization is controlled by an arbitration queue, a
device reselect table, a SCSI-expected phase byte and a SCSI phase
sequence status byte illustrated in the table of Fig. 2.  The SCSI
phase sequence status byte is a log of the actual phase sequence.
This phase status byte consists of a series of 8 bits which are
turned...