WO1987004828A1

WO1987004828A1 - Apparatus and method responding to an aborted signal exchange between subsystems in a data processing system

Info

Publication number: WO1987004828A1
Application number: PCT/US1987/000182
Authority: WO
Inventors: Robert E. Stewart; Paul J. Natusch; Eugene L. Yu; James B. Keller; John F. Henry, Jr. (Deceased)
Original assignee: Digital Equipment Corporation
Priority date: 1986-01-29
Filing date: 1987-01-29
Publication date: 1987-08-13
Also published as: KR910007030B1; CN1012224B; CA1275328C; KR880700901A; AU7026387A; EP0292499A1; US4858173A; IN170450B; JPH01501425A; IL81424A; ES2004366A6; IL81424A0; CN87102175A

Abstract

In a data processing system in which access to a second subsystem by a first subsystem through a system bus is determined by an arbitration unit, the situation where a requesting subsystem that receives access to the system bus, but is unable to use that access for interaction with the target subsystem, is resolved by providing a busy signal to the arbitration unit and to the subsystems. The busy signal causes the subsystems to reinstitute a request for access to the system bus when the subsystem has an aborted transaction. The busy signal enforces a delay in the next arbitration for the system bus until any subsystem, with an aborted transaction as a result of the busy signal, can reassert the request for access signal. Apparatus can be included with the arbitration unit that permits rearbitrating access to the bus using the priority conditions in effect at the time of the original arbitration.

Description

APPARATUS AND METHOD RESPONDING TO AN ABORTED SIGNAL EXCHANGE BETWEEN SUBSYSTEMS IN A DATA PROCESSING SYSTEM

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to data processing systems and more particularly to the exchange of logic signals between data processing subsystems on a system bus in the data processing system. The present invention addresses the problem occurring when the exchange of logir signals is initiated, but the receiving data processing subsystem is unable to utilize the logic signals.

2. Description of the Related Art

Referring to Figure 1, a typical data processing system is shown. The data processing system includes at least one central processing unit 10 ( or 11), at least one input/output unit 13 (or 14), a memory unit 15 and a system bus 19 coupling the plurality of units or subsystems of the data processing system. The central processing unit processes groups of logic signals according to software and/or firmware instructions. The logic signal groups to be processed as well as the current program in execution are typically stored in the memory unit 15. A console unit 12 can be coupled to the central processing unit(s) and includes the apparatus and stored instructions to initialize the system. The console unit 12 can act as a terminal during the operation of the data processing system. The input/output units provide the interface between the data processing system and terminal units, mass storage units, communication units, and any other units to be coupled to the data processing syste .

However, a problem can arise when access of the system bus is awarded to a subsystem, the appropriate activity for the implemen ation of the signal exchange is initiated, but the signal exchange does not result In successful processing by the receiving unit. Such a situation can occur when the main memory unit is busy and the signals applied thereto can not be processed by the the memory unit. The memory unit can return a signal that indicates that the logic signals were or were not processed by the memory subsystem through a Confirmation Acknowledge signal or a Confirmation Busy signal. However, by the time that the first subsystem is able to retry the access to the memory subsystem, the arbitration unit can have given access to other subsystems, and the other subsystems can have initiated memory activity which causes the first subsystem to receive a memory busy signal again upon rearbitration by the first subsystem. Thus a situation can occur wherein a subsystem of lower priority can prevent a subsystem having a higher priority from gaining access to a data processing system resource, such as a main memory subsystem.

A possible solution to the problem of the inability of a subsystem to acquire access to a memory subsystem is to delay the arbitration of the access to the system bus until after the exchange of the signals has taken place. In this solution, the next sequential arbitration is delayed a predetermined period to insure that the processing of the transferred signals has taken place or that the the requesting subsystem has been able to renew the request for access to the bus. This solution results in unacceptable delays in the exchange of signal groups between subsystems. A need has therefore been felt for apparatus and method for interaction of the data processing subsystems with the system bus that permits access to the system bus based on the priority of the subsystem requesting access while preventing inadvertent monopoly of the system bus by an inappropriate data processing subsystem.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved data processing system. It is a further object of the present invention to provide a data processing system that can provide for aborted exchanges of signals between data processing subsystems without permitting an Inappropriate subsystem to access the system bus.

It is a particular object of the present

Invention to provide apparatus for identifying and communicating to a signal-issuing data processing subsystem that the target subsystem is currently unable to process signals from the system bus.

It is a another particular object of the present invention, to provide a -memory subsystem with apparatus for generating a Memory Busy signal when the memory subsystem is unable to process the signals applied to the system bus.

It is still another object of the present invention to provide apparatus associated with the memory subsystem that, upon determination that the memory subsystem can no longer process incoming data signal groups, applies a Memory Busy signal to the remaining subsystems and to unit controlling access to the system bus. It is yet another particular object of the present invention to restore the priority state determining access to a data processing system bus to the priority state before an aborted transfer of logic signals prior to rearbitration for access to the system bus.

The aforementioned and other objects are accomplished, according to the present invention, by providing a target data processing subsystem with apparatus for generating a busy signal when the target subsystem is unable, at least temporarily, to process additional signal groups. The busy signal is applied to the other subsystems as a status signal and to the bus arbitration unit, the unit controlling access to the system bus, to prevent further access to the system bus, at least to the busy subsystem, until the busy signal is removed. The busy signal is applied to the arbitration unit for as long as the condition causing the busy signal is present, but for at least a period of time sufficient for a subsystem with an aborted signal group transfer to rerequest access to the system bus. According to another embodiment of the invention, after access to the system bus has been determined, the arbitration unit provides an up-date of the priority parameters resulting from the assignment of system bus access. The busy signal is applied to the arbitration unit and to the other data processing subsystems. With respect to the other data processing subsystems, the application of the memory busy signal concurrently with the application of command signals to the system bus aborts the transaction specified by the command. With respect to the arbitration unit, the busy signal results in suspension of the granting of any further access to the system bus except by the memory unit and causes the arbitration state to return to the state prior to any aborted command. When the busy signal is removed, the arbitration unit then redetermines which of the requesting subsystems should receive access, but performs the redetermination using the priority parameters in effect at the time of the original access determination. The invention has particular application to the memor subsystem wherein the subsystem can still be processing logic signals resulting from a earlier logic signal transfer. In this example, the memory subsystem issues a Memory Busy signal and the access to the system bus is redetermxned by the arbitration unit. These and other features of the present invention will be understood upon reading of the following description along with the drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of the components of a data processing system capable of utilizing the present Invention.

FIG. 2 is a block diagram of a data processing system having a unit controlling the determination of the next data processing subsystem, referred to as an arbitration unit, to have access to the system bus.

FIG. 3 is a block diagram of the components of an arbitration unit. FIG. 4 is a timing diagram illustrating how a subsystem requesting access to the system bus can be unable to gain access to the system bus for more than one access attempt. FIG. 5 is a timing diagram showing the signals on the system bus to eliminate the problem of the potential inability of data processing subsystem to access the system bus.

FIG. 6 is a block diagram of the arbitration unit includine the apparatus implementing one aspect of the present invention.

^'DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 1. Detailed Description of the Figures

Figure 1 has previously been described in relation to the related art.

Referring next to Figure 2, the data processing system and the data processing subs tems, as show and described ith reference to Figure 1 , is illustrated. In addition, an arbitration unit 20 is indicated that can control the application of the groups of logic signals to the system bus. The arbitration unit 20 can operate in the following manner. When any of the subsystems has the requirement that the subsystem must place a group of logic signals on the system bus, a Request logic signal is applied to a mechanism coupling the data processing subsystem and the arbitration unit 20. In the simplest situation, the data processing subsystem and the arbitration unit 20 are coupled by a conductor, a logic signal applied thereto indicating not only the presence of a Request for access to the system bus, but also the identity of the subsystem requesting the access. The coupling mechanism can be part of the system bus or can be distinct from the system bus. Based on an algorithm preventing monopoly of the system bus by a., particular subsystem, assigning priority parameters to e various data processing unit subsystems, and taking account of any other factor deemed important in the access to the system bus, the arbitration unit 20 determines the next data processing subsystem to have access to the system bus and an Enable logic signal Is applied to the selected subsystem by a coupling means, typically a conductor. The presence of this Enable logic signal communicates to the selected data processing subsystem that the logic signal group can be applied to the system bus. The selected subsystem can apply a Hold logic signal to the arbitration unit 20. The Hold logic signal is used in those situations where the selected data processing subsystem must apply a plurality of logic signal groups to the system bus in adjacent cycles, for example a write operation as opposed to a read operation. The Hold signal prevents the Enable signal from being removed from the selected data processing subsystem until the necessary system .clock cycles have passed for the plurality of related logic signal groups to have been applied to the system bus. In addition to the signals identified above, the subsystem to which the logic signals are transferred has apparatus to identify the receipt of the transferred signal groups and provides a confirmation signal, typically a Confirmation Acknowledge signal or a Confirmation Busy signal to the sending subsystem upon determination of a status of the signal group transfer .

Referring next to Figure 3, the components of the arbitration unit 20 are sho n. At least one request signal is applied to the arbitration unit 20 indicating that access to the system bus 19 is required by the subsystem. These request signals are applied to decision logic unit 31. Also applied to decision logic unit 31 are signals from priority state unit 32. The signals from the priority state unit 32 control logic elements in the decision logic unit 31 such that, in the situation where a plurality of Request signals are applied to the decision logic unit 31, the selected Enable signal will be based on preassigned priorities. However, once the subsystem is selected, then the priority assigned to that subsystem will typically change. To provide for this change in priority, the Enable signals and the priority state signals are applied to up-date logic unit 32. Up-date logic unit 32 applies appropriate signals to the storage elements of priority state unit 32, causing signals representing the new priority state to be applied to decision logic unit 31. Also shown in Figure 3 is the application of the subsystem busy signal, i.e. the Memory Busy signal, to the decision network 31.. According to one aspect of the invention, the subsystem busy signal can prevent subsystem "lock-out", described below, without the use of the priority up-date unit 33.

Referring to Figure 4, a timing diagram of the operation of the interaction between subsystems, competing for access to the system bus, is shown.

The arbitration unit 20 is enabled to select the next subsystem to have access to the system bus during clock cycle T„. The arbitration of the Request signals occurs during that system clock cycle. During the same system clock cycle the result of that arbitration in the form of an Enable signal is transmitted to the selected subsystem. At the end of the arbitration cycle, the priority parameters can be up-dated to reflect the successful access to the system bus by the selected subsystem. During the next timing cycle, T. , the command/address information is applied to the system bus. (When the access to the system bus involves a Write command, then during cycle_a T, , subsystem 1 asserts a Hold signal and the Command/Address signal, and the arbitration unit maintains the Enable signal. During the next cycle, T- , the data signal group to be stored is applied to the system bus.) During the second timing cycle after the command/address cycle, i.e. T-, the subsystem to which the command was addressed responds with a Ack owled ement Confirmation signal indicating successful transfer of the signal groups. If that subs stem sends the Busy Confirmation signal instead, the transfer of signal groups has been aborted. Then the subsystem can not attempt to gain access to the system bus again until during cycle T, to execute the original operation. However, a second subsystem (c.f. Figure 4) can, in the intervening interval, have gained access during period T- to the system bus. The result of this access can have been successful resulting in a Confirmation Acknowledge signal in period T_. As a result of that activity, the first subsystem can receive a Confirmation Busy signal in period T-.. Viewed in another manner , the confirmation signal is delayed to the extent that the subsystem #1 requesting access to the system bus can not have received knowledge of the aborted signal transfer until after a subsequent arbitration. When subsystem #1 attempts to gain access to the system bus, the memory subsystem can again be busy because of the memory activity started by subsystem #2.

Referring next to Figure 5, a timing diagram of the technique for avoiding the inability of a data processing subsystem to gain access to the memory subsystem by means of the system bus is illustrated. Typically, the problem in executing the command of the requesting subsystem is that the target subsystem is the main memory 15 and the main memory can b-_ busy during a attempted access. According to Figure 4, the signal, indicating that the processing of signals transferred to the main memory subsystem 15 was not successful, Is not communicated arbitration unit 20 and to the requesting subsystem sufficiently early for the subsystem to respond in an appropriate manner. According to the present Invention, when a memory activity results in a condition indicating that the' processing of further signal is temporarily not possible, then a Memory Busy signal is transferred to all subsystems and to the arbitration unit 20. According to the preferred embodiment of the present invention, a interaction with the memory subsystem will not be aborted unless the the Memory Busy signal occurs during the Command/Address cycle or during the first data signal group cycle. When the Memory Busy signal occurs in either of these interaction cycles, then the issuing subsystem determines that th.e transaction was- aborted, and as a result, the issuing subsystem (with the aborted transaction) rerequest access to the system bus. However, the Memory Busy signal applied to decision logic unit 31 prevents arbitration for access to the system bus until the Memory Busy signal is removed. Such time period is long enough for the issuing subsystem (with the aborted- transaction) to reapply the Request signal (typically in two clock cycles).

Referring next to Figure 6, the arbitration unit, according to another aspect of the present invention, is illustrated. The decision logic unit 31 receives Request signals from the subsystems when the subsystems require access to the system bus, in the specific example, for exchange of logic signals with the the memory subsystem 15. As a result of the Request signals, an Enable signal is generated by logic gates in the decision logic unit 31. The operation of the logic gates of unit 31 is determined by signals from the priority state logic 32, the signals being determined by storage elements in the priority state unit 32 that are a reflection of the current priorities assigned to the subsystems of the data processing system. As before the priority up-date logic unit 33 receives signals from the priority state unit 32 indicating the current state of the priority assignments and receives the Enable signals. Based on a predetermined algorithm, a new current priority assignment is produced and can be applied to the priority state unit 32 through controllable switch 65. Also receiving the current priority state is the priority state silo 64. This silo stores the priority parameters or state upon which the arbitration decision for the aborted transfer was made. This priority state can also be applied to the priority state logic unit through the controllable switch 65. The signals actually applied to the priority state logic unit through switch 65 are ^'determined by the Memory Busy signal, such that, upon application of the Memory Busy signal, switch 65 selects the priority state silo unit 64. (When the Memory Busy signal is not applied, he next state is determined by the priority up-date unit 33.) After a Memory Busy signal (which indicates that the logic signal transfer has been aborted) is generated by the memory subsystem, then the subsystem having received access to the system bus, being unsuccessful in the transfer of data on the system bus, can participate in the rearbitration of the bus subject to the same priorities that existed at the time of the original bus arbitration.

2. Operation of the Preferred Embodiment

In data processing systems of the related art, the lack of ability of the data processing subsystems to respond to an aborted transaction resulted from the use of a confirmation signal that was issued after the determination was made as to the next subsystem to gain access. The Memory Busy signal prevents inadvertent lock-out of a subsystem by performing two functions simultaneously. First, the arbitration unit is prevented from providing access to the system bus for the duration of the Memory Busy cycle, and second, the Memory Busy signal is asserted for sufficient time to permit the subsystems having transactions aborted by the Memory Busy signal to reinstitute the request for access to the system bus. ith respect to the subsystems, the above-described relationship of the assertion of the Memory Busy signal and the signals being applied to the system bus determine when a transaction has been aborted. Therefore, the subsystem can reapply the Request signal to the arbitration unit 20. The enforced duration of the Memory Busy cycle insures that the subsystem will have an opportunity to be included in the next arbitration procedure.

As shown in Figures 3 and 4, arbitration units 20 have apparatus to up-date the priority states of the subsystems. The algorithm embedded in the arbitration unit 20 of Figure 3 operates under the assumption that the data processing subsystem having _I6_

current access to the system bus had been successful in executing the_ command signal transfer and in performing the function defined by the command signals resulting from the previous arbitration process. The algorithm does not address the possibility that the access to the system bus might not result in effectuating the function required by the subsystem which was awarded access to the system bus. Indeed, the timing of the arbitration procedure is such that the arbitration unit issues the Enable signal (the signal indicating to a subsystem that the arbitration process has granted the subsystem access to the bus) before the arbitration unit detects the Confirmation Busy signal. The conditions establishing the status of the Confirmation signal can not be determined at an earlier time.

The second embodiment of the present invention, shown in Figure 6, takes into account the late receipt of Memory Busy signals by providing a priority state silo unit 64. For reestablishing the last previous priority state of the system in response to assertion of the Memory Busy signal.

Referring again to Figure 6, priority state silo unit 64 stores the priority state of the system at the point when the unsuccessful transaction was arbitrated. In response to assertion of the Memory

Busy signal, the contents of the silo unit 64 are reintroduced into priority state unit 32, thus restoring priority state unit 32 to its condition immediately prior to the unsuccessful transaction. Thus, in the event that the unit whose transaction likely was aborted requests access to the system bus, the priority state will be essentially the same as at the time of the arbitration hich resulted in an unsuccessful transaction. Thus, when the Memory Busy signal is de-asserted, the ■ rerequesting unit has a higher priori in receiving access to the bus than the other units that were requesting bus access at the time of the rerequesting unit's earlier arbitration.

It will be clear that the rearbitration of access to the system bus using the previous priority state does not ensure that the previously successful subsystem will receive access to the system bus on rearbitration . A new Request signal from a subsystem having a higher priority can have been applied to the arbitration unit 20 between the first and the second bus arbitration cycles.

It will also be clear that the technique of rearbitration of requests for access to the system bus has been described with particulari y in relation to the memory unit. The technique of rearbitration in the event of an aborted access to any subsystem of the data processing system can utilize this technique when aborted accesses have performance penalties significant enough to permit the use of the additional apparatus.

The foregoing description is included to illustrate the operation of the preferred embodiment and is not meant to limit the scope of the invention. The scope of the invention is to be limited only by the following claims. From the foregoing description, many variations will be apparent to those skilled in the art that would yet be encompassed by the spirit and scope of the invention.

Claims

What is Claimed i^ :

1. An arbitration unit for determining access to a data processing system bus by a plurality of subsystems applying first signals to said arbitration unit, comprising: state means for providing state signals indicative of a priority of each of said subsystems; and decision means responsive to said state signals and to said first signals and to a third signal for selecting a one of a plurality of second signals.

2. The arbitration unit of Claim 1 wherein a one of said subsystems is a main memory subsystem, said third signal being a memory busy signal.

3. The arbitration unit of Claim 2 wherein said first signals are request signals for access to said system bus signals, and wherein each of said second signals enables access to said system bus by an associated subsystem

4. The arbitration unit of Claim 1, further comprising up-date means responsive to said selected one second signal for changing said state signals from said state means in a pred etermined manner .

5 . The a rb i t rat i o n un it o f Claim 1 , f u rt he r comprising: storage means coupled to said state means for storing previous state signals; and gate means for applying said stored state signals to said state means in response to a third signal.

6. The method of providing for interaction between subsystems in a data processing system comprising the steps of: applying state signals to an decision portion of an arbitration unit, said state signals determined by priorities assigned to each of said subsystems; when a first subsystem requires access to a second subsystem, applying a first signal to said arbitration unit by said first subsystem; selecting a subsystem to receive access to said second subsystem based on said priorities and said applied first signals; changing said state signals based on new priorities determined by said selecting a subsystem; and when said access by said selected subsystem is not successful, selecting a subsystem bases on said priorities hen a condition causing said unsuccessful access .

7. Apparatus for ensuring access to a system bus in a data processing system comprising: a plurality of subsystems associated with said data processing system, each of said subsy tems including; first signal means for generating a first signal when access to said svstem bus is required, and second signal means for providing a second signal hen said subsystem is busy, herein first means includes apparatus responsive to said second signals from other of said subsystems and to selected internal conditions for generating said first signal; and arbitration means responsive to said first signals for generating a third signal providing access to said system bus by a selected subsystem, said arbitration means further including means responsive to said second signal for preventing generation of said third signal until a subsystem having said selected condition can generate said first signal.

8. The method for insuring access to a target subsystem in a data processing system by other subsystems comprising the step of disabling an arbitration unit granting access to said target subsystem to a one of said other subsystems applying first signals to said arbitration unit until said target subsystem is free and until a subsystem having an aborted transaction with said target subsystem can apply said first signal to arbitration unit.

AMENDED CLAIMS

[received by the International Bureau on 03 August 1987 (03.08.87); original claims 1-6 amended; 7 and 8 cancelled and new claims 7-9 added (4 pag_ts)]

1. An arbitration unit for incorporation in a data processing system having a plurality of subsystems interconnected by a bus means, said arbitration unit being connected to each of said plurality of subsystems for determining access to said bus means by any subsystems which attempt to communicate with a first subsystem, said first subsystem being capable of sending a busy signal to said arbitration unit, and said subsystems other than said first subsystem each being capable of sending a request signal to said arbitration unit for requesting access to said first subsystem, wherein said arbitration unit comprises state means for storing signals representing a priority state of said subsystems other than said first subsystem, and decision means connected to receive said request signals, said busy signal and said priority state signals, said decision means outputting enable signals for granting access to said bus means by one of said subsystems other than said first subsystem in response to receipt of said request signals and said priority state signals, and said decision means being disabled during receipt of said busy signal.

2. The arbitration unit as defined in claim 1, wherein said first subsystem comprises a main memory unit which outputs said busy signal in response to said main memory unit being unable to process signals from a subsystem requesting access.

3. The arbitration unit as defined in claim 1, wherein said subsystems other than said first subsystem have a first set of predetermined priorities when said state means stores signals representing a first priority state. 4- The arbitration unit defined in claim 3- further comprising update means connected to receive said first priority state signals from said state means and said enable signals from said decision means, wherein said update means is capable of outputting signals representing a second priority state in response to receipt of said first priority state signals and said enable signals, said state means being connected to receive and store said second priority state signals in place of said first priority state signals, said subsystems other than said first subsystem having-a second set of predetermined priorities different than said first set when said state means stores said second priority state signals.

5. The arbitration unit as defined in claim 4, further comprising storage means connected to receive and store said first priority state signals sent from said state means to said decision means, and gating means connected to receive said first priority state signals from said storage means, said second priority state signals froo said update means, and said busy signal from said first subsystem, wherein said gating means sends said first priority state signals to said state means in response to receipt of said busy signal and sends said second priority state signals to said state means in response to the absence of said busy signal.

6. A method for enabling interaction between a plurality of subsystems of a data processing system, said subsystems being connected by a bus means, comprising the following steps:

(a) storing signals representing a first priority state of subsystems other than a first subsystem, wherein said subsystems other than said first subsystem have a first set of predetermined priorities;

(b) respectively outputting a request signal from any of said subsystems other than said first subsystem fox -respectively requesting access to said first subsystem,

(c) outputting a busy signal from said first subsystem when access thereto by a subsystem requesting access is not possible due to the busy state of said first subsystem;

(d) enabling access to said bus means by a requesting subsystem in accordance with said first priority state in response to the outputting of any request signals during the absence of said busy signal or denying access by any requesting subsystem in response to the outputting of any request signals during the presence of said busy signal; and

(e) storing signals representing a second priority state of said subsystems other than said first subsystem, wherein said subsystems other than said first subsystem have a second set of predetermined priorities, in response to enabling access by a subsystem in accordance with said first priority state, and determining the next access in accordance with said second priority state, or storing signals representing said first priority state of said subsystems other than said first system in response to denying access by any requesting subsystems to said first subsystem and determining the next access in accordance with said first priority state.

7. A method for preventing the lockout of a first system requesting access to a target system due to the busy state of said target system after access to a bus means has been granted, comprising the following steps:

(a) disabling the means for granting access to said bus means in response to a busy signal from said target system indicating said busy state;

(b) maintaining said stored priority state so that, during arbitration of the next access to said target system_*.said first subsystem has the same priority relative to other subsystems that it had when access to said bus means was granted;

(c) repeating a request by said first subsystem for access to said target system immediately after termination of said busy signal; and

(d) arbitrating contention between said first and any other subsystems requesting access to said target system in accordance with said stored priority state.

&. A data processing system having a plurality of subsystems connected by a bus means, and an arbitration unit for resolving contention between subsystems requesting access to a first subsystem, said arbitration unit being connected to each of said plurality of subsystems, wherein each of said subsystems other than said first subsystem comprises means for sending request signals respectively to said arbitration unit for requesting access to said first subsystem, said first subsystem comprises means for sending a busy signal to said arbitration unit when access to said first subsystem is not possible, and said arbitration unit comprises means for outputting enable signals for enabling access to said bus means by one of said subsystems other than said first subsystem in accordance with a predetermined priority state, said means for outputting enable signals being disabled by said busy signal, said busy signal enduring until at least the soonest time when said one subsystem can send another request signal for access to said first subsystem following an aborted communication with said first subsystem.

9. The data processing system as defined in claim 8, wherein said arbitration unit further comprises means for providing an updated priority state in dependence on said predetermined priority state and said enable signals.