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Device Arbitration and Selection Method for Personal Computers

IP.com Disclosure Number: IPCOM000113628D
Original Publication Date: 1994-Sep-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 6 page(s) / 287K

Publishing Venue

IBM

Related People

Cook, SE: AUTHOR [+4]

Abstract

Described is an architectural implementation for Personal Computers (PCs) to provide fairness in arbitration and selection of devices. The method applies to devices which use the same Small Computer System Interface (SCSI) bus by allowing fairness in the usage of buffer pools among two or more initiators.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 23% of the total text.

Device Arbitration and Selection Method for Personal Computers

      Described is an architectural implementation for Personal
Computers (PCs) to provide fairness in arbitration and selection of
devices.  The method applies to devices which use the same Small
Computer System Interface (SCSI) bus by allowing fairness in the
usage of buffer pools among two or more initiators.

      Typically, PCs incorporating a SCSI-2 adapter can have multiple
target devices accessed by multiple initiators.  For the IBM PS/2
fast/wide SCSI-2 adapter, buffers are allocated through the use of an
established Buffer Pool Request (BPR) control element by the system.

      Fig. 1 shows the format used to establish a BPR control
element.  The BPR control element is used to add buffers to the
buffer pool for use by the adapter when acting as a target SCSI
device.  The adapter will fill the buffers with data received from
the SCSI bus during processing of SCSI send commands from particular
SCSI initiators which have been assigned to corresponding automatic
SCSI target mode entity identifications (IDs).

      Up to thirty-two entities may be assigned at one time to use
the internal, or external, SCSI bus, but not to both buses
simultaneously.  One or more SCSI Information Event Elements (IEEs)
are sent to the corresponding information entity IDs specified in the
assign entity ID management request for each send command that is
processed by the adapter.  Fig. 2 shows the format used to establish
a SCSI IEE.  The SCSI information element contains a list of buffers
from the buffer pool which are filled with data in response to the
send commands.

      Up to ten buffers may be added to the pool with each
established BPR.  Each buffer added must be at least sixty-four bytes
long.  Up to thirty-two establish BPRs may be active at one time,
therefore, up to 320 buffers may be made available to the adapter at
any one point in time.  A standard reply element will be returned if
not suppressed when the establish BPR has been accepted by the
adapter.  A standard error element will be returned if the thirty-two
established BPRs are already active, or if invalid parameters, or
buffer descriptors are found in the request.  Buffers are used in the
order established.  A release BPR may be sent by the system to cancel
all outstanding established BPR.

      Each buffer is described in the request element
entity-to-entity field by way of a sixteen byte entry.  The first
four bytes of each entry describes the type of buffer and gives a
parameter descriptor number (N...N) as a buffer identifier.  Bit 15
of this field, C, (Fig. 2) is used to indicate if additional buffer
descriptors follow the current descriptor.  The remaining bytes
specify the buffer size and system memory address.

      It is up to the system software to determine when to send
additional establish BPRs.  A Drypool Event Element (DEE) will be
sent to the system whenever t...