Browse Prior Art Database

Method of Implementing "Fairness" Arbitration in Accessing a Computer System Bus

IP.com Disclosure Number: IPCOM000100564D
Original Publication Date: 1990-May-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 8 page(s) / 317K

Publishing Venue

IBM

Related People

Brown, WW: AUTHOR [+3]

Abstract

A technique is described whereby an inequality in bus access of multi- arbitrating device arbitration is eliminated by means of a "fairness" Multi-Master Inactive State Override function. The concept applies to computer systems, such as the MICRO CHANNEL*, which uses a low-end parallel bus. The design is such that unnecessary wait conditions do not take place on devices in simple systems, such as with one processor, or only two arbitrating devices.

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Method of Implementing "Fairness" Arbitration in Accessing a Computer System Bus

       A technique is described whereby an inequality in bus
access of multi- arbitrating device arbitration is eliminated by
means of a "fairness" Multi-Master Inactive State Override function.
The concept applies to computer systems, such as the MICRO CHANNEL*,
which uses a low-end parallel bus.  The design is such that
unnecessary wait conditions do not take place on devices in simple
systems, such as with one processor, or only two arbitrating devices.

      Typically, in a computer system's Multi-Master environment,
several arbitrating devices must share the resources provided by the
system. When access to those system resources is through the system
bus, arbitrating devices must cooperate with each other in sharing
the system bus, known as "fairness".  The procedure for gaining
control of the system bus is generally known as "arbitration".

      When a device requires use of the system bus, it will drive a
-PREEMPT signal line active.  If another device is presently using
the system bus, the current transactions must be completed before
release to the system.  The system logic then drives a +ARB/-GNT
signal line high, which indicates to all the devices waiting to use
the system bus that an arbitration cycle is to begin.  The devices
then drive their +ARB(0-3) signal lines with their respective
arbitration level.  Once a "winner" has been determined, the system
logic then drives the +ARB/-GNT signal line low, indicating that a
bus owner has been determined.  The device, whose arbitration level
matches that which is present on the arbitration bus, now "owns" the
system bus.

      Since the arbitration level is a priority level, devices which
have the highest priority levels will tend to dominate the use of the
system bus, unless fairness is practiced.  To practice fairness, a
device will enter an Inactive State once it has been PREEMPTed by
another device requiring use of the system bus.  A device in the
Inactive State will remain so until all other devices requiring use
of the system bus have had their turn.  When there are no active
devices waiting to use the system bus, indicated by an inactive
-PREEMPT, the devices in the Inactive State may Exit the Inactive
State and again drive the -PREEMPT signal line to indicate a
requirement for the system bus.

      When only two devices are contending for bus ownership, the
process of practicing fairness will cause them to simply alternate
ownership of the bus.  The major concern, in this case, is the
reduction in performance due to the wasting of every other bus cycle
for an arbitration cycle.

      When three or more devices are contending for bus ownership,
the problem of equal access to system resources through system bus
ownership in a Multi-Master environment begins to appear.  The
following examples illustrate various conditions:
At time zero, three Devices; 1,2 and...