Patent Publication Number: US-2010122004-A1

Title: Message switching system

Description:
CROSS-REFERENCE 
     This application is a continuation of U.S. application Ser. No. 11/484,837, filed Jul. 11, 2006, and also claims priority to French application no. 06/02810, filed Mar. 31, 2006, each of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The present invention relates to a message switching system comprising at least two inputs and at least one output. 
     In a communication network on silicon chip (network on chip) linking numerous agents of the network, resources are pooled in order to obtain the best trade-off between network cost and performance. 
     In practice, directly linking two agents of the network when there are data interchanges between these agents would be excessively expensive. The data interchanges are grouped on common links in order to make best use of the performance of each link of the network. Such resource sharing requires the presence of arbitration elements or decision-making elements on the switches present at nodes of the network. 
     A switch is used to converge as input the data transferred over a number of links, and transfer it over a plurality of output links. 
     The data circulating on such links is in the form of data packets comprising a header and useful data. The header contains header data for routing or directing the packet through the network and can, if necessary, contain priority information concerning the data packets. 
     A switch includes a routing device which determines, for a packet arriving at an input of the switch, the output of the switch to which the packet is intended, according to header data in the packet header. 
     Furthermore, each output of the switch includes an arbitration device which decides, for the corresponding output, which packet will be assigned to it, in particular in cases of conflict between a number of data packets. 
     In the description that follows, the term “output assignment request” will be used to mean a request sent by an input, or, more specifically, by the logic associated with an input of the switch, according to data packets present at the inputs, and addressed to an arbitration device associated with an output. 
     An arbitration device decides, at a given instant, to which input, out of a plurality of inputs at which data packets are present and intended for the corresponding output, it must assign the output. When the output is available, it then notifies the chosen input, and the waiting data packet can then be transmitted by the associated output. 
     When the output bandwidth is not yet saturated, any types of arbitration criteria can be chosen, and particularly the most economical, such as the fixed priority or the random assignment of the output. 
     The effectiveness of an arbitration device is therefore judged in particular in cases of conflicting assignment of an output when the bandwidth is saturated on that output. 
     A good trade-off for dimensioning a network consists in having most of the time an unsaturated bandwidth, and to cope with occasional data interchange peaks resulting in a saturation of certain bandwidths, in having a system enabling such emergency conditions to be managed. 
     Various arbitration devices tending to overcome this type of problem exist. 
     For example, “round-robin” type arbitration devices use a fixed, rotating relative order for the inputs of a switch. After each assignment of the output, the minimum relative order is assigned to the next input in the circular list of inputs, or even better, to the input that has just had the output assigned. 
     However, such an arbitration device becomes costly and complicated when it involves handling a number of dynamic priority levels, because it is necessary to have a round-robin type arbitration for each priority level. Furthermore, the priorities are normally linked to a concept of service quality. There are normally few levels or degrees of priority, because the latter are associated with exceptional situations. The priority signals can be independent of the request signals and can therefore be positioned at any time. Furthermore, there can be retraction both on the request signals and on the priority signals. 
     It is necessary to have a rapid arbitration, because the request or priority signals arrive late in the clock cycle. 
     For example, arbitration devices serve the output assignment requests sent by inputs in their order of arrival (FCFS, for “first-come-first-served”, or LRU for “least recently used”). Such an arbitration device uses queues or counters. 
     However, if it is necessary to handle a number of requests arriving simultaneously, or if a high priority request arriving late needs to be assigned rapidly, it is then very difficult to rapidly update the queues used. 
     Such arbitration devices present problems of cost or effectiveness. 
     Thus, one object of the invention is to obtain a message switching system implementing an effective arbitration in the case of output bandwidth usage peaks, at low cost. 
     SUMMARY 
     Thus, according to one aspect of the invention, there is proposed a message switching system comprising at least two inputs and at least one output, first arbitration means dedicated to said output, and management means designed to determine a relative order OR(i,j) of one input relative to the other, for any pair of separate inputs belonging to the system and having sent requests for the assignment of said output. The management means are also designed to assign said output. The said management means comprise storage means designed to store said relative orders OR(i,j), initialization means designed to initialize said relative orders OR(i,j) such that only one of said inputs takes priority on initialization, and updating means designed to update all of said relative orders when a new request arrives at said first arbitration means, or when said output is assigned to one of said inputs. 
     Such a system enables bandwidth usage peaks to be managed on the link connected to an output of a switch, effectively and inexpensively. 
     In practice, storing the relative orders is a way of significantly improving the speed of arbitration, and enables a system to be produced simply and inexpensively that combines the FCFS (first-come-first-served) and LRU (least recently used) mechanisms. 
     Thus, a perfect equity between the inputs is assured, and since the relative orders are known for all the pairs of inputs, they are known for all the pairs of a subset of inputs, which makes it possible to use an input filtering without damaging the output assignment equity. 
     Ensuring a perfect arbitration equity is a way of avoiding congestion in the network at switch level. 
     According to an embodiment, said management means are designed to assign the relative order OR(i,j) of one input relative to the other, for any pair of separate inputs belonging to the system, according to the order of arrival of the corresponding requests. The input for which the corresponding request has arrived first takes priority. Said management means are designed, when a number of requests arrive simultaneously, to assign the relative order of one input relative to the other, for any pair of separate inputs belonging to the system, such that one input takes priority over another input when said output has recently been assigned to said input before the last assignment of said output to the other input. 
     In an embodiment, the system also comprises filtering means designed to filter the or each output assignment request sent by inputs to said output by selecting the or each highest priority and identical priority requests out of the or each assignment request, when said requests can be of different priorities. 
     According to an embodiment, said management means are designed, when a single request is selected by said filtering means, to assign said output to said single request selected by said filtering means, and when a number of requests are selected by said filtering means and one of said selected requests has arrived before the others, to assign said output to said request arriving before the others out of the selected requests. Furthermore, said management means are designed, when a number of requests are selected by said filtering means and have arrived simultaneously before the other selected requests, to assign said output to the request having the relative order that is highest out of said requests arriving simultaneously before the others. 
     In an embodiment, said updating means are also designed to process a request, sent early by one of said inputs and disappearing before said output is assigned to it, as if said output had been assigned to it. 
     For example, the relative order OR(i,j) of a pair of separate inputs i,j belonging to the system is one when the input j takes priority over the input i, and zero when the input i takes priority over the input j. 
     For example, the relative order OR(i,j) of a pair of separate inputs i,j belonging to the system is zero when the input j takes priority over the input i, and one when the input i takes priority over the input j. 
     For example, said storage means are designed to store said relative orders OR(i,j), for any pair of inputs belonging to the system such that i&lt;j. 
     For example, said storage means are designed to store said relative orders OR(i,j), for any pair of inputs belonging to the system such that i&gt;j. 
     According to an embodiment, the system also comprises second arbitration means dedicated to a second output. Said first and second arbitration means are designed to determine the input, corresponding to a request, to which said second output is assigned, such that only one other input has a higher priority request. 
     The second arbitration means provide a different choice from that of the first arbitration means. Everything proceeds as if arbitration were being applied for a first time, and then a second time taking into account the first arbitration. 
     Such a double arbitration is very rapid, because it is not done by stacking two arbiters. 
     In an embodiment, the system also comprises third and fourth arbitration means, mounted in parallel forming a second double arbiter ( 63   c ), means of assigning said two outputs, cooperating with said first and second arbitration means mounted in parallel and forming a first double arbiter, and with said second double arbiter ( 63   c ). Said first double arbiter ( 62   c ) receives the priority requests, and said second double arbiter ( 63   c ) receives the non-priority requests, said requests having only two possible priority levels, priority or non-priority. 
     Such a double arbitration with priority filtering occupies a smaller area, because many elements of the system are shared by the arbitration means, which means that the critical computations can be performed in parallel at low cost. 
     According to an embodiment, when at least two priority requests are received by said first double arbiter, said assignment means assign said two outputs to the two priority requests having the highest relative orders. 
     In an embodiment, when a single priority request is received by said first double arbiter, said assignment means assign said two outputs to said priority request and to the non-priority request having the highest relative order received by said second double arbiter. 
     According to an embodiment, when no priority request is received by said first double arbiter, said assignment means assign said two outputs to the two non-priority requests having the highest relative orders received by said second double arbiter. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       Other objects, features and advantages of the invention will become apparent from reading the description that follows, of a few by no means limiting examples, and given with reference to the appended drawings, in which: 
         FIG. 1  illustrates the relative orders OR(i,j) assigned to the pairs of inputs of the system according to an aspect of the invention; 
         FIG. 2  represents an exemplary embodiment of determining the signal for selecting the input i according to an aspect of the invention; 
         FIG. 3  represents an embodiment of the means of updating relative orders OR(i,j), according to an aspect of the invention; 
         FIG. 4  represents an embodiment of the filtering means, according to an aspect of the invention; 
         FIG. 5  illustrates a message switching system according to an aspect of the invention; 
         FIG. 6  illustrates a message switching system according to an aspect of the invention; 
         FIG. 7  illustrates a message switching system according to an aspect of the invention; and 
         FIG. 8  represents an exemplary embodiment of simultaneous determination of the signals for selecting the inputs i.sub.1 and i.sub.2 according to an aspect of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates how the relative orders OR(i,j) are processed. 
     For each pair of inputs of an arbitration module dedicated to an output of a message switching system, the relative orders OR(i,j) of one input relative to the other, at a given instant, are stored. For an n-input arbitration, it is therefore necessary to store n(n−1)/2 values. 
     For example, if the inputs of an arbitration module, which are the inputs of the system, are numbered from 0 to n−1, then for i and j integers between 0 and n−1 and such that i is different to j, the relative order OR(i,j) of the input j over the input i is defined as 1 when the input j takes priority over the input i and 0 otherwise. 
     Naturally, it would also be possible to define the relative order OR(i,j) of the input j over the input i, as being 0 when the input j takes priority over the input i, and 1 otherwise. 
     It is then sufficient to store OR(i,j) only for i&lt;j and take for OR(j,i) the inverse value of OR(i,j). 
     The term “inverse values” is used to mean the values 0 and 1. 
     Naturally, it would also be possible to store OR(i,j) only for i&gt;j, and to take for OR(j,i) the inverse value of OR(i,j). 
     There follows a description of the case in which, for i and j integers between 0 and n−1 and such that i is different to j, the relative order OR(i,j) of the input j over the input i is defined as being 1 when the input j takes priority over the input i and 0 otherwise, and in which OR(i,j) is stored only for i&lt;j, and for OR(j,i), the inverse value of OR(i,j) is taken. 
     On initialization, OR(i,j)=0 can arbitrarily be taken, regardless of i and j, and in this case at the outset, the input numbered 0 is higher priority than the input numbered 1, which is in turn higher priority than the input numbered 2 and so on. 
     The order of initialization can be anything but the OR(i,j) must, however, be consistent. 
     In practice, if OR(i,j)=OR(j,k)=OR(k,i), at the outset, there could be a situation in which no input is selected or even a more conflicting situation in which more than one input would be selected. 
     It is thus possible to calculate very rapidly the input assignment conditions from the moment when the requests are known. 
       FIG. 2  illustrates how the selection signal Grant(i) of the input i is calculated from the relative orders OR(i,j) of the other inputs relative to the input i. 
     For an input to be selected, there must be a request on that input and there must be no other request on a higher priority input. 
     The critical decision path beginning with requests is 1+log(n) gates. Only the OR(i,j) with i strictly less than and different to j are stored, with i therefore varying from 0 to n−2 and j varying from 1 to n−1, so there are therefore n(n−1)/2 coefficients to be stored. 
     The circuit is produced using a logical AND  1  and a logical AND  2  with signal inverter  3  at the output and a logical AND  4  with signal inverter at the output  5  and signal inverter  6  on the input corresponding to OR(i,j). The logical AND  1  receives at the input  7  the signal Req(i) representing the presence (1) or the absence (0) of request on the input i and its priority. The logical AND  1  also receives as input the inverted outputs of the logical ANDs  2  and  4 . Thus, for each signal Grant(i), one logical AND  1  and a plurality of logic gates  2 ,  3 ,  4 ,  5  and  6  are necessary. 
     Arbitration cannot be any faster, because log(n) gates is the optimum number of logic gates. 
     As illustrated in  FIG. 3 , the relative orders OR(i,j) are modified, either on arrival of each new request on the inputs i or j, or when an assignment of the output is performed in favor of one of these two inputs i or j. When a request arrives and obtains the output in the same cycle, it is not considered as a new request, in other words, the corresponding signal NewReq does not switch to 1. 
     The priorities OR(i,j) are updated when a new request arrives on the input i or j or when the assignment of the output is done on the input i or j. This makes it possible to take account of both the age of the requests, because their order of sending is retained through the corresponding OR(i,j), and the order in which the inputs have been served (output assigned) although there are no longer requests on these inputs. 
     For example, if a new request is defined as being a request not present in the preceding clock cycle, but not acknowledged in the current cycle, the value 0 is assigned to OR(i,j) when the output is assigned to the input j or when there is a new request on the input i and no request on the input j, and the value 1 is assigned to OR(i,j) when the output is assigned to the input i or when there is a new request on the input j and no request on the input i. 
     Thus, the advantages of the FCFS and LRU methods are combined, so ensuring a perfect equity in the assignment of the output. Furthermore, if this new implementation is compared with the existing solutions, excellent performance is obtained for a low cost in terms of number of logic gates. 
     The output is assigned only when the output link is free, the signal Upd is used to validate this assignment based on the choice made, identified by the single active Grant(i). The signals UpdGnt(i) and NewReq(i) are then calculated for each input. The signal NewReq(i) represents the new-request state of the request of the input i, and the signal UpdGnt(i) is representative of the fact that the input i has been chosen and that the output is ready to process the request associated with that input. 
     By not modifying the relative orders OR(i,j) of the old requests, the order of arrival of the requests is retained. Returning OR(i,j) to 1 when UpdGnt(i) is 1 or OR(i,j) to 0 when UpdGnt(j) is 1, is tantamount to assigning the lowest relative order to the input that has just been assigned the output, so, when, subsequently, two new assignment requests arrive simultaneously on their respective input, the order between them is fixed according to the times elapsed since they were last respectively assigned the output, the input to which the output has been assigned most recently being the lower priority. 
     The arrival of a new request on one of the inputs can compromise its relative order relative to the other inputs. If, on the other input, there is also a request, then the relative order has not to be modified even if it is also a new request, but, in the case where there is no request on the other input, this new request becomes a priority. OR(i,j) is set to 0 if there is NewReq(i) and not Req(j), and, symmetrically, OR(i,j) is set to 1 if there is NewReq(j) and not Req(i). 
     The assignment of the output to a request that has just arrived causes the corresponding input to be set to the minimum relative order whatever the state of the other inputs, so there is no benefit to be gained in handling this type of situation in the same way as new requests. The fact that a number of new requests arrive simultaneously therefore poses no problem. 
     Apart from initialization where an arbitrary order is fixed between the inputs, subsequently the relative orders are fixed so as to service the requests as equitably as possible. 
     This is performed, for example, as illustrated in  FIG. 3 . 
     When requests are submitted early, and can therefore, possibly disappear before obtaining the output, these requests are handled as if the output had been assigned to them. 
     Thus, a rapid, inexpensive, equitable arbitration is obtained, being able to manage several dynamic priority levels, and which retains both the order of the requests and the order of the assignments of the output, so ensuring a better regulation of the traffic for the network. 
     Thus, a logical AND  10  receives as input the values Upd and Grant(i) and delivers as output the value UpdGnt(i). The value UpdGnt(i) is duplicated on an inverter  11  at the input of a logical AND  12 . The logical AND  12  also receives as input the value Req(i). The output signal of the logical AND  12  is, on the one hand, transmitted to a flip-flop  13 , and, on the other hand, to a logical AND  14 . The output of the flip-flop  13  is transmitted to the logical AND  14  after inversion by an inverter  15 . UpdGnt(i) is 1 when the output is free to satisfy a waiting request. 
     Similarly, a logical AND  16  receives as input the values Upd and Grant(j) and delivers as output the value UpdGnt(j). The value UpdGnt(j) is duplicated on an inverter  17  at the input of a logical AND  18 . The logical AND  18  also receives as input the value Req(j). The output signal of the logical AND  18  is, on the one hand, transmitted to a flip-flop  19  and, on the other hand, to a logical AND  20 . The output of the flip-flop  19  is transmitted to the logical AND  20  after inversion by an inverter  21 . 
     The output of the logical AND  14  is transmitted as input to a logical AND  22 , also receiving as input Req(j) inverted by an inverter  23 . The output of the logical AND  22  is transmitted to a logical OR  24 , also receiving, as input, UpdGnt(j) output from the logical AND  16 , and a reset signal Reset. 
     The output of the logical OR  24  is transmitted after inversion by an inverter  25  to a logical AND  26 . The logical AND  26  also receives as input the output of the logical OR  27 . 
     The logical OR  27  receives as input the output of a logical OR  28  receiving as input UpdGnt(i) delivered by the logical AND  10  and the output of a logical AND  29 . 
     The logical AND  29  receives as input, on the one hand, NewReq(j) delivered by the logical AND  20  and, on the other hand, Req(i) inverted by an inverter  30 . 
     The output of the logical AND  26  is transmitted to a flip-flop  31 , which delivers as output OR(i,j). The signal OR(i,j) is transmitted by a feedback loop as input to the logical OR  37 . 
       FIG. 4  illustrates an embodiment of a filtering module  40  that can be used before an arbitration module dedicated to a single output, according to one aspect of the invention. 
     P 1  and P 2  are two signals for encoding a priority level, for a small number of priority or service quality levels, which is the usual case. In other words, when there is no urgency or priority, P 2 P 1 =00 (level 0), when there is a low level urgency, P 2 P 1 =01 (level 1) and when there is a high level urgency, P 2 P 1 =11 (level 2), which makes it possible to rapidly determine the maximum priority level P 1 P 2 max. 
     The requests are then filtered according to the maximum priority detected. Only those that have this maximum priority level are retained. 
     Then, out of the filtered requests, the oldest or, by default, the one whose input was last serviced a long time ago, is chosen. 
     Thus, an arbitration that takes account of dynamic priorities, or the age of the requests Req(i) or of the age of the assignments UpdGnt(i) is produced. The result is a very rapid and very equitable solution. 
       FIG. 4  illustrates an exemplary embodiment of a filtering module  40  comprising a module  41  for detecting the maximum priority level, comparators  42  for establishing, for each request Req(i), whether the priority of the request Req(i) is equal to said maximum priority detected by the detection module  41 . 
     The output of each comparator  42  is transmitted to the input of a logical AND  43  which also receives as input Req(i). 
     At the output, the maximum level priority requests are detected. 
     To support early requests and therefore the possibility of retraction, it is enough to consider that the requests that have disappeared are handled like the one that has been selected and validated (the one with UpdGnt(i)=1). 
     The following notation conventions apply:
         ˜A is the logical inverse of A &amp; and | are logical AND and logical OR operations Reg(A) is the logical value of A in the preceding cycle.       

     With the same definition for NewReq(i)=Req(i)&amp; ˜UpdGnt(i)&amp; ˜Reg(Req(i)&amp; ˜UpdGnt(i)), AutoGnt(i) is defined as being 1 after a retraction: 
     AutoGnt(i)=˜Req(i)&amp;Reg(Req(i) ˜UpdGnt(i)) 
     There is then introduced the concept of generalized full Grant: FullGnt(i)=UpdGnt(i)|AutoGnt(i) which is one when a request has been handled or has disappeared. The FullGnt(i) replaces the old UdpGnt(i) in the update equations of OR(i,j). In the case where FullGnt(i) and FullGnt(j) are both 1, OR(i,j) does not change. The new equations are therefore: 
       OR(i,j) set to 1 if ˜FullGnt(j)&amp;(FullGnt(i)|(NewReq(j)&amp; ˜Req(i))) 
       OR(i,j) set to 0 if [˜FullGnt(i)&amp;(FullGnt(j)|(NewReq(j)&amp; ˜Req(j)))]|Reset 
     It will be noted that these equations remain valid in the case of a system with multiple choices on a number of outputs as illustrated in  FIGS. 6 and 7 , because in this situation it is the cases with a number of simultaneous FullGnt that are being processed. 
     As illustrated in  FIG. 5 , a message switching system  51  comprises four inputs  52 ,  53 ,  54 ,  55 , and two independent outputs  56 ,  57 . 
     Each input has an associated routing module. The first, second, third and fourth inputs  52 ,  53 ,  54 ,  55  respectively comprise a first, second, third and fourth routing modules  58 ,  59 ,  60 ,  61 . 
     The outputs  56 ,  57  respectively comprise a dedicated arbitration module  62 ,  63 . 
     The arbitration modules  62 ,  63  respectively cooperate with a management module  64 ,  65 . 
     The management modules  64 ,  65  respectively comprise an initialization module  66 ,  67 , an updating module  68 ,  69  and a storage module  70 ,  71  for the relative orders OR(i,j). 
     Any request that has a dynamic priority greater than that of another request takes priority over the latter, and if two requests have the same dynamic priority level, the priority is the one given by the relative orders OR(i,j) of the corresponding inputs. 
     The term “dynamic priority” is used to mean a priority signal (or service quality signal) associated with a request at the input of a message switching system. 
     Such a device makes it possible to manage emergency situations while retaining as much as possible a certain equity between the inputs. 
     If two requests have the same dynamic priority level, then the priority is given to the oldest request, and if a number of requests of the same dynamic priority level arrive simultaneously, the priority is given to the one whose input was last assigned the output the longest time ago. 
     In  FIG. 6 , the elements having the same references as in  FIG. 5  are the same. 
     The management module  64  is common to the two arbitration modules  62   b  and  63   b . In  FIG. 6 , the double arbiter comprising the two modules  62   b  and  63   b  must choose two different requests simultaneously for the two outputs  56  and  57  that are equivalent. The two modules share the management module  64 . 
       FIG. 7  represents a variant of the system of  FIG. 6 , in which two double arbiters according to  FIG. 6  are used in parallel. Also, the management module  64  is shared. The elements having the same references are the same. 
     The two arbitration modules  62   c ,  63   c , are both dedicated to the two outputs  56 ,  57 . Each arbitration module  62   c  and  63   c  therefore comprises a double arbiter comprising the modules  62   b  and  63   b  as illustrated in  FIG. 6 . 
     Furthermore, an assignment module  72  cooperates with the two arbitration modules  62   c ,  63   c , and a counting module  77  controls the assignment module  72 . 
     In a system, as illustrated in  FIGS. 5 and 6 , the aim is to determine, out of the inputs presenting a request, the one for which there is no other input with a higher relative priority, and for a system as illustrated in  FIGS. 6 and 7 , it is necessary to add the search for the input for which there is only one other input with a request having a higher relative order. 
     The fact that for a given input i (with request) there is no non-zero OR(i,j) with j different from i is detected. 
     Furthermore, the fact that for a given input (with request) there is one and only one non-zero OR(i,j) with j different from i is determined. 
     A double arbiter is constructed by adding for each input a second computation tree based on AND/OR cells while keeping a single table of the relative orders stored in the storage module  70 , and the same initialization and updating modules. 
     This makes it possible to select two requests out of a plurality of requests without paying the cost of two independent arbitration modules, both in terms of execution time and in terms of number of gates used. 
     This is produced, for example, by means of the assembly of  FIG. 8 . 
     The signal Req(i) is transmitted as input to two logical ANDs  75  and  76 . 
     A counting module  77  delivers as output a signal representing the detection of a count equal to zero as input to the logical AND  75  which delivers as output Grant 1 ( i ). 
     Furthermore, the counting module  77  delivers as output a signal representing the detection of a count at one as an input to the logical AND  76  which delivers as output Grant 2 ( i ). 
     For i&lt;j, a plurality of logical ANDs  78  receive as input Req(j) and OR(i,j), and transmit their output signal, inverted by an inverter  81 , to the counting module  77 . 
     For i&gt;j, logical ANDs  79  receive as input Req(j) and OR(i,j) inverted by inverters  80 , and deliver their output signal, inverted by an inverter  81 , to the counting module  77 . 
       FIG. 8  illustrates how the selection signals Grant 1 ( i ) and Grant 2 ( i ) of the input i are calculated from the relative orders OR(i,j) of the other inputs relative to the input i, for a double arbiter. 
     Since the complexity is greater, there is a limit of two priority levels: priority and non-priority. 
     Since the computation trees for the signals Grant 1 ( i ) and Grant 2 ( i ) are inexpensive compared to the shared device (see  FIG. 3 ), duplicating the double arbiter is reduced to duplicating only its computation trees in order to have the computations performed in parallel. There is therefore a reduction in delays (parallelism) and a reduced cost (sharing of costly parts). The parallel connection of two double arbiters makes it possible to process in parallel and separately the priority requests (priority count). 
     The corresponding device is then similar to that of  FIG. 3 , except that the following applies: 
       UpdGrant( i )=(Grant( i )&amp;Upd1)|(Grant2( i )&amp;Upd2) 
     The priority requests are transmitted to a first arbitration module  62   c  and the non-priority requests are sent to a second arbitration module  63   c . The storage  70 , updating  68  and initialization  66  modules are common to both arbitration modules  62   c ,  63   c . At the same time, with the counting module  77 , it is determined whether there are one, zero or several priority requests. 
     In the case where there is no priority request, only the two results of the second arbitration module  63   c  are of interest. In the case where there is only one priority request, the corresponding input is necessarily selected on the Grant 1  of the first arbitration module  62   c  and the second input corresponds to the selection on the Grant 1  of the second arbitration module  63   c . In the other cases, there are at least two priority requests and only the results of the first arbitration module  62   c  are of interest. 
     Such a system has a theoretical arbitration time corresponding to approximately 1.5 log(n)+5 gates, and is therefore very rapid. With the OR(i,j) described previously, the age of the requests and the time elapsed since the last Grant can be taken into account, so the arbitration remains totally equitable. 
     Given the low number of logic gates involved, the computation trees of the double arbiter are duplicated and the table of relative orders of the storage module  70 , and the table updating  68  and initialization  66  modules are shared, the first arbitration module  62   c  being dedicated to the priority requests and the second arbitration module  63   c  being dedicated to the non-priority requests. Thus, a double arbiter is obtained with priority filtering using a double arbiter, that is, four arbitration trees pooling the same relative orders OR(i,j). 
     A count of the number of priority requests provides a way of selecting, if there is more than one priority request, the two inputs with priority requests that have the highest relative orders, if there is only one priority request, the input with priority request and the one with a non-priority request for which the relative order is the higher, and if there is no priority request, the two inputs with non-priority requests for which the relative orders are the highest. 
     The relative orders are, furthermore, fixed according to the seniority of the requests. When a request disappears or obtains the output, the respective inputs have a priority returned to the minimum priority, the relative orders between these inputs remaining unchanged. Any new request on the input j, without output assignment for the input i or the input j and without the disappearance of the request on the input i, sets the relative order OR(i,j) to 0. 
     In such a system, only the computation trees of the Grant  1 ( i ) and Grant 2 ( i ) are doubled, so the increase in the number of gates remains limited to approximately 20%. Compared to the conventional solution that consists in placing two arbitration modules in series, the gains in speed and surface area are significant.