Patent Publication Number: US-7216182-B2

Title: Shared storage arbitration

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an arbitration unit, to a channel, and to a method for arbitrating accesses to a shared storage. 
     An electric or electronic system may comprise functional units that output data that has to be written to some kind of storage. There might exist different latency requirements with regard to the output data of the various functional units. Besides that, said functional units might generate different bandwidths of output data. Alternatively or additionally, said system might comprise functional units that have to be provided with input data that has to be read from some kind of storage. For some of these functional units, a low latency read access might be required. For other functional units, a high latency might be acceptable. Some functional units might require a high bandwidth of read data, while for others, a low bandwidth might be sufficient. 
     In order to meet the requirements imposed by the various functional units, one might provide the electric or electronic system with at least two different storages. This is not a very efficient solution, though. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an improved arbitration for controlling accesses to a shared storage. The object is solved by the independent claims. Preferred embodiments are shown by the dependent claims. 
     The arbitration unit according to an embodiment of the present invention is adapted for controlling accesses to a shared storage. The arbitration unit comprises a set of interfaces that connect a plurality of units with said arbitration unit. Outgoing data streams are transmitted from the arbitration unit via respective ones of said interfaces to at least one of said units, and incoming data streams are transmitted from at least one of said units via respective ones of said interfaces to the arbitration unit. The arbitration unit further comprises a control logic that is connected to each of said interfaces. Said control logic is adapted for segmenting write data of incoming data streams in order to set up write accesses to said shared storage, for scheduling a sequence of at least one of write and read accesses to said shared storage, and for distributing read data obtained during said read accesses to outgoing data streams. 
     The arbitration unit might e.g. receive a stream of write data from one of said units. A block of write data that has been received via one of said interfaces is broken up into a plurality of smaller data packets, whereby the size of the data packets is such that they can be written to the shared storage during one single write access. Besides that, the arbitration unit might have to handle read requests that are issued by one or more of said units. In accordance with said read requests, the arbitration unit might perform read accesses to the shared storage. The arbitration unit schedules a sequence of at least one of write and read accesses to the shared storage, whereby “scheduling” shall mean to define the temporal order of said write and read accesses. The data that is obtained from the storage during the read accesses might be reassembled into data blocks. Said data blocks might be transmitted, via one of the interfaces, to a respective unit, in order to fulfil the read request of said unit. 
     The arbitration unit according to an embodiment of the present invention can be adapted in many ways to the properties of said units. The requirements imposed by said units can e.g. be considered by scheduling the memory accesses accordingly. The arbitration unit may choose the temporal order of the memory accesses in dependence on the required properties of the incoming or outgoing data streams, e.g. independence on the bandwidth or the latency required for a certain data stream. For example, a low latency read access might have to be scheduled immediately. For performing a high throughput write access, it might be necessary to perform a series of consecutive write accesses. 
     In order to allow for low latency memory accesses, each one of said read or write accesses must not take too long. The size of the data packets transferred during a single memory access is chosen in accordance with a trade off between latency and throughput. 
     According to a preferred embodiment of the invention, one or more of the interfaces comprise buffers that are adapted for buffering at least one of the incoming and outgoing data streams. Preferably, said buffers are realized as FIFO (First In First Out) buffers. For example, in a buffer that corresponds to a certain interface, write data that is received via said interface can be buffered. Even if the corresponding write access cannot be scheduled immediately, the write data can be accepted immediately. When a read request is processed, the read data that has been fetched from the memory might as well be buffered in a buffer, in order to provide a continuous data stream to the respective unit that has requested said data. 
     In another embodiment of the present invention, each of the interfaces is connected with a respective one of the functional units. Thus, each of the interfaces is assigned to a certain one of the functional units, and the respective properties of said functional unit can be taken into account. Incoming data streams that are received from a certain functional unit via said interface can be processed in accordance with the requirements imposed by said functional unit. Also outgoing data streams that are routed via said interface to a certain functional unit can be processed in accordance with the properties of said functional unit. 
     According to another preferred embodiment of the invention, one or more of the units transmit read requests to the arbitration unit, whereby at least some of said read requests indicate a start address and a size of a data block that is to be read from the shared storage. One or more of the functional units might as well transfer write requests to the arbitration unit before a corresponding stream of write data is sent from the respective functional unit to the arbitration unit, whereby at least one of said write requests might indicate the size of the block of write data and a start address. The arbitration unit converts each of the read or write requests into a number of corresponding read or write accesses. As soon as the arbitration unit has received a respective read request or write request, it can start scheduling the corresponding read accesses or write accesses. Said requests, which are received some time in advance, are helpful for generating an optimized sequence of read and write accesses. 
     According to another preferred embodiment of the invention, said read and write accesses to the shared memory are scheduled in accordance with priorities that are assigned to at least some of the arbitration unit&#39;s interfaces, or to at least some of the incoming and outgoing data streams. The temporal order of the read and write accesses is determined by the control logic of the arbitration unit. If a high priority is assigned to an incoming data stream, the write accesses corresponding to said incoming data stream will be processed more quickly than other read and write accesses. In case of a very high priority, the write accesses corresponding to said incoming data stream might even be scheduled immediately. If a high-priority read request is received, the corresponding read accesses will be carried out favorably, while low-priority write and read accesses will be postponed. As soon as the high-priority accesses have been processed, the low-priority accesses will be taken care of. 
     Preferably, whenever an incoming or outgoing data stream has to be processed with low latency, a correspondingly high priority is assigned to said data stream. By assigning a high priority, it can be achieved that the corresponding read or write accesses are scheduled more quickly than other read and write accesses. As a result, a low-latency storage access can be provided. Thus, the arbitration unit&#39;s control logic allows to fulfill different kinds of latency requirements imposed by the functional units. 
     According to another preferred embodiment, the priorities are modified in a way that the amount of switching between read accesses and write accesses is kept small. Each time said switching is performed, an extra delay will occur. In order to avoid said extra delays, it is preferable to first process a group of write accesses, followed by a group of read accesses, etc. When a read access is being processed, it will be most favorable if the following memory access is a read access as well. This can e.g. be achieved by incrementing the priorities of the read requests, while the priorities of the write requests are kept constant. In contrary, when a write access is being performed, it is most favorable to process a write request next. In this case, one might e.g. increase the priorities assigned to the write requests, while the priorities of the read requests are kept constant. 
     According to another embodiment, the priorities assigned to the read requests and to the write requests are modified in a way that a continuous transmission of data is promoted. If several memory accesses that relate to adjacent address ranges have to be processed, said memory accesses should preferably be carried out in succession in order to avoid any time delays due to re-addressing. Therefore, when a memory access that relates to a certain address range is processed, the priorities assigned to memory accesses that relate to adjacent address ranges are increased, and as a consequence, said memory accesses will most probably be scheduled next. As a consequence, an uninterrupted transmission of data is enhanced, and the amount of extra time delays due to re-addressing is kept small. 
     According to another preferred embodiment, the priority that is assigned to a certain interface is modified in dependence on the fill level of the buffer that corresponds to said interface. The higher the fill level of said buffer gets, the higher the priority of the corresponding data stream will become. By increasing the priority of the buffered data stream in dependence on the fill level, it can be achieved that memory accesses corresponding to said data stream are scheduled in a preferred manner. As a consequence, the fill level of the respective buffer decreases, and overflow can be avoided. 
     In a preferred embodiment of the invention, the arbitration unit is employed in a channel of an automated test equipment (ATE). The channel is responsible for at least one of: providing stimulus data to at least one device under test (DUT), and receiving response data from said at least one DUT. The channel&#39;s arbitration unit is responsible for coordinating the memory accesses to a shared storage. Said shared storage might e.g. be adapted for storing at least one of instructions and sequencer data. Said instructions and said sequencer data might be used for generating a stream of test data. The shared memory might as well be utilized for storing result data that is obtained by evaluating response data obtained from the at least one DUT. 
     In prior art solutions, a channel of an automated test equipment encompassed at least two different memories, a low-latency SRAM adapted for providing a stream of instructions to a sequencer unit, and a high-latency DRAM adapted for storing sequencer data as well as result data. The use of two different memories with completely different properties allowed to fulfill the latency and bandwidth requirement of the channel&#39;s functional units. 
     According to embodiment of the present invention, said at least two distinct memories are replaced by one shared memory that is controlled by an arbitration unit. The shared memory might e.g. be used for storing at least one of instructions, vector data, and result data. Thus, the set-up of the channel is simplified. A channel according to an embodiment of the present invention is cheaper and smaller than a channel according to the prior art. 
     Preferably, the units of said channel comprise at least one of: a sequencer, a result processing unit, an interface module, and a microprocessor core. The sequencer unit reads instructions and sequencer data from the shared memory and generates a stream of output data that might e.g. be provided to at least one of the drive path and the receive path of the channel. The result processing unit is responsible for evaluating the response data obtained from said at least one DUT. The result processing unit might generate a stream of result data that is written to the shared memory. The interface module is adapted for establishing a data link between the channel and a central facility. Via said interface module, data might be exchanged between the channel&#39;s shared memory and said central facility. Besides that, the channel might comprise a microprocessor core. 
     According to a preferred embodiment, outgoing data streams are transmitted from the arbitration unit to the sequencer, whereby said outgoing data streams might comprise at least one of instructions and sequencer data. Preferably, a high priority is assigned to an outgoing data stream that comprises instructions for said sequencer. Thus, it is made sure that the instruction stream is not disrupted. 
     According to another preferred embodiment, the arbitration unit is adapted for receiving an incoming data stream from the result processing unit, whereby said incoming data stream might comprise result data. Said result data might e.g. be generated by comparing response data obtained from said at least one DUT with expected data. 
     According to another embodiment of the invention, the arbitration unit exchanges data streams of high priority with said interface module. Especially if there do not exist any handshake signals, data that is exchanged between the interface module and the arbitration unit will have to be taken care of immediately. 
     Preferably, the shared storage is implemented as a dynamic RAM. A dynamic RAM (DRAM) is cheaper and smaller than a SRAM. Preferably, a RDRAM comprising a synchronous clocked interface is employed. Due to said clocked interface, a data exchange of high bandwidth can be realized. 
     Further preferably, the arbitration unit comprises a memory maintenance unit, whereby said memory maintenance unit forwards maintenance requests to the shared storage. In case a DRAM is used, said maintenance requests might for example comprise requests for performing a memory refresh. 
     The invention can be partly or entirely embodied or supported by one or more suitable software programs, which can be stored on or otherwise provided by any kind of data carrier, and which might be executed in or by any suitable data processing unit. Software programs or routines are preferably applied for scheduling the sequence of write and read accesses to the shared storage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and many of the attendant advantages of the present invention will be readily appreciated and become better understood by reference to the following detailed description when considering in connection with the accompanied drawings. Features that are substantially or functionally equal or similar will be referred to with the same reference sign(s). 
         FIG. 1  shows a channel of an automated test equipment (ATE); 
         FIG. 2  shows a memory unit comprising a shared storage and an arbitration unit; and 
         FIG. 3  shows the structure of a slice train comprising slices of read data and write data. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     In  FIG. 1 , a channel  1  of an automated test equipment (ATE) is shown, whereby the channel  1  is responsible for at least one of: providing stimulus data to a DUT, and analyzing response data obtained from said DUT. The channel  1  doesn&#39;t have to be a channel of an automated test equipment; it might as well be a channel within any kind of multi-channel architecture. The channel  1  comprises a sequencer unit  2  that receives sequencer instructions  3  and sequencer data  4  from a shared memory  5  that is preferably implemented as a RDRAM. According to embodiments of the present invention, the shared memory  5  is accessed via an arbitration unit  6 . If the DUT comprises at least one DRAM memory, the sequencer unit  2  might also read refresh data  7  from the shared memory  5 . Said refresh data  7  is responsible for periodically initiating a refresh of the at least one DRAM memory within the DUT. 
     In accordance with the sequencer instructions  3 , the sequencer unit  2  generates an output data stream  8  that might comprise both drive data and expected data. The output data stream  8  is provided both to the drive path  9  and to the compare unit  10  of the receive path  11 . The drive path  9  comprises a waveform table, which is a look-up table adapted for converting the vectors of the output data stream  8  into a corresponding sequence of waveforms. Each waveform comprises a set of edges, together with timing information for said edges. At the output of the drive path  9 , a stream of stimulus data  12  is obtained, and said stream of stimulus data  12  is provided to the pin  13  of the DUT. 
     Alternatively or additionally, a stream of response data  14  might be obtained from the pin  13  of the DUT. In the compare unit  10 , the response data  14  is compared with expected data that is transmitted as a part of the output data stream  8 . For this reason, the output data stream  8  is also provided to the compare unit  10  of the receive path  11 . The compare unit  10  generates a stream of result data  15  comprising the results of the comparison. Said stream of result data  15  is provided to the result processing unit  16 . The result processing unit  16  comprises a counter  17  that keeps track of the error count. Furthermore, the result processing unit  16  generates an error map  18  by recording the results as a function of the respective cycle. The error map data  19  is written, via the arbitration unit  6 , to the shared memory  5 . Besides that, the result processing unit  16  generates an overview map  20 , with one bit of said overview map representing 4 kByte of result data  15 . 
     When a DUT is tested, different kinds of errors might occur simultaneously. If only a subset of said errors is to be tracked and analyzed, it will be required to mask out all the other errors. For this purpose, cycle mask data  21  that is read from the shared memory  5  is provided to the result processing unit  16 . Said cycle mask data  21  defines those parts of the result data stream  15  that have to be masked out. 
     As described so far, the shared memory  5  contains sequencer instructions, sequencer data and cycle mask data. Besides that, the shared memory  5  contains result data that is received from the result processing unit  16 . For exchanging the shared memory&#39;s content with a workstation, the channel  1  comprises a workstation interface  22  adapted for establishing a data link  23  with the workstation. Via the workstation interface  22 , data  24  can be exchanged between the workstation and the shared memory  5 . Said data  24  is routed via the arbitration unit  6 . Furthermore, the channel  1  comprises an embedded microprocessor core  25 . Between said microprocessor core  25  and the shared memory  5 , data  26  might be exchanged as well, whereby said data is also routed via the arbitration unit  6 . 
     According to embodiments of the present invention, the arbitration unit  6  segments write data of incoming data streams into corresponding data packets in order to set up write accesses to the shared memory  5 . Said incoming data streams might comprise streams of result data  19 , of data  24  exchanged with the workstation interface  22 , and of data  26  exchanged with the embedded microprocessor core  25 . The arbitration unit  6  schedules a sequence of write and read accesses to the shared memory  5 . Read data obtained during the read accesses is distributed to the outgoing data streams. Said outgoing data streams might comprise the sequencer instructions  3 , the sequencer data  4 , the refresh data  7 , and the cycle mask data  21 . The outgoing data streams might also comprise data  24  for the workstation interface  22 , and data  26  for the embedded microprocessor core  25 . 
     In  FIG. 2 , the channel&#39;s memory unit is depicted. The memory unit comprises a RDRAM memory  28 , a RAM controller  29 , and an arbitration unit  30 . The arbitration unit  30  comprises a set of interfaces  31 – 37  for the various incoming and outgoing data streams. Via a first interface  31 , data is exchanged with the workstation interface  22  shown in  FIG. 1 . The workstation may write to and read from the shared memory, and therefore, the corresponding request req may be a write request or a read request. A write request is adapted for indicating the start address of the write access and the size of the data block that is to be written to the shared memory. Correspondingly, in a read request, the start address and the size of a data block that is to be read from the shared memory are specified. 
     In case of a write access, write data Dw is buffered in a FIFO (First In First Out) buffer  38 . The FIFO buffer  38  issues so-called slice requests req to the arbitration unit  30 . There, priorities are assigned to the slice requests of the various interfaces, and the slice requests are handled in accordance with their respective priorities. 
     In the example shown in  FIG. 2 , priorities ranging from “1” to “9” are assigned to the various slice requests. The priority assigned to a data stream that is transmitted via one of the interfaces is modified in dependence on the corresponding FIFO buffer&#39;s fill level. For example, in case the FIFO buffer  38  is empty, a priority of “7” is assigned to a slice request req that corresponds to the first interface  31 . When the fill level of the FIFO buffer  38  increases, it becomes more urgent to handle the corresponding data stream. Accordingly, the priority assigned to the slice request req of the first interface  31  is increased from “7” to “8” or “9” in dependence on the fill level of the FIFO buffer  38 . 
     The workstation interface  22  might as well transmit a read request to the FIFO buffer  38 . One or more slice requests req corresponding to said read request are forwarded to the arbitration unit  30  and there, priorities ranging from “7” to “9” are assigned to said slice requests. 
     The arbitration unit  30  receives slice requests req from all the interfaces that are connected to said arbitration unit  30 . The arbitration unit  30  schedules a sequence of write and read data slices, whereby each of said data slices is of fixed size. For example, each of said data slices might comprise 256 bits of data. The write data Dw of a data block received via the interface  31  is broken up into a number of data slices, with each data slice comprising 256 bit of write data. Said data slices are scheduled in accordance with their respective priority. Then, said data slices are transmitted, as a part of a slice train  39 , from the arbitration unit  30  to the RAM controller  29 . The slice train  39  also comprises data slices that have been read from the shared memory, and each of said data slices also comprises 256 bit of read data. The RAM controller  29  exchanges read and write-data  40  with the RDRAM memory  28 . 
     As an example, let us assume that the workstation interface  22  has issued a read request to the first interface  31 . Slice requests that correspond to said read request are transmitted to the arbitration unit  30 . The data slices are scheduled by the arbitration unit  30 , and the corresponding read accesses are performed by the RAM controller  29 . Within the slice train  39 , the obtained data slices of read data are transmitted from the RAM controller  29  to the arbitration unit  30 . There, data slices corresponding to various outgoing data streams are received and distributed to the FIFO buffers of the corresponding interfaces. The data slices that relate to the read request issued by the workstation interface  22  are written in sequential order to the FIFO buffer  38 . The FIFO buffer  38  comprises a buffer for buffering incoming data as well as a buffer for buffering outgoing data. The workstation interface  22  can fetch the requested data block of read data Dr from the FIFO buffer  38 . 
     Via the second interface  32 , sequencer instructions are provided to the sequencer unit  2  shown in  FIG. 1 . The requested sequencer instructions are read from the RDRAM memory  28  and are transmitted within the slice train  39  to the arbitration unit  30 . The arbitration unit  30  writes said sequencer instructions to the FIFO buffer  41 . 
     The third interface  33  is responsible for providing sequencer data to the sequencer unit  2 . Sequencer data that has been read from the shared memory is buffered in a corresponding FIFO buffer  42 . 
     The fourth interface  34  is adapted for providing refresh data to the sequencer unit  2 . Refresh data that has been read from the shared memory is buffered in the FIFO buffer  43 . 
     The error map  18 , which is part of the result processing unit  16  shown in  FIG. 1 , provides a stream of result data to the fifth interface  35  of the arbitration unit  30 . The result data is buffered in a FIFO buffer  44 . 
     The stream of cycle mask data is an outgoing data stream that is provided to the result processing unit  16  of  FIG. 1 . The cycle mask data that has been read from the RDRAM memory  28  is buffered in the FIFO buffer  45 . Via the sixth interface  36 , the cycle mask data is forwarded to the result processing unit  16 . 
     The seventh interface  37  is adapted for exchanging data with the embedded microprocessor core  25  of  FIG. 1 . The corresponding FIFO buffer  46  comprises a buffer for buffering incoming data as well as a buffer for buffering outgoing data. 
     In  FIG. 2 , the respective priorities that are assigned to the incoming and outgoing data streams of the various interfaces are depicted. The three priorities that are shown for each one of the interfaces  31 – 37  correspond to different fill levels of the respective FIFO buffers  38 ,  41 – 46 . The highest priority is assigned to data that is exchanged, via the interface  31 , with the workstation. Data that is provided by the workstation has to be processed immediately, because there do not exist any handshaking signals between the arbitration unit, the workstation interface and the workstation. 
     From  FIG. 2 , it can be seen that a priority ranging from “6” to “8” is assigned to the stream of sequencer instructions that is read from the shared memory. Thus, it can be achieved that the stream of sequencer instructions is not disrupted. Priorities ranging from “5” to “7” are assigned to the stream of result data received via the interface  35 . Due to this rather high priority, the stream of result data received from the error map  18  can be continuously written to the RDRAM memory  28 . To the data streams that are exchanged with the channel&#39;s microprocessor core  25 , a low priority ranging from “1” to “3” is assigned. As a consequence, the latency of the microprocessor core&#39;s memory accesses will be rather high. 
     Maintenance requests  47  that relate to the maintenance of the RDRAM memory  28  are generated by the RAM controller  29 . Said maintenance requests  47  might for example comprise requests for performing a refresh of the RDRAM memory  28 . The maintenance requests  47  are forwarded to the maintenance unit  48 . In order to schedule said maintenance requests, slice requests are transmitted from the maintenance unit  48  to the arbitration unit  30 . Initially, the priority “1” is assigned to a maintenance request. After some time, it is checked whether the maintenance request has been processed or not. If the maintenance request has not been processed yet, the priority of said maintenance request is increased to “5”. After some more time has elapsed, it is checked once more whether the maintenance request has been taken care of. If the maintenance request still hasn&#39;t been processed, the priority is even set to “14” in order to enforce that the respective maintenance request is scheduled. Thus, it can been made sure that a periodic refresh of the RDRAM memory  28  is performed. 
       FIG. 3  shows how a plurality of read and write requests are converted into a slice train  49 , whereby the time that has elapsed is indicated from the left to the right. At the point of time  50 , a first read request  51  is received via an interface A, whereby said read request specifies both the start address (“0”) and the size (“96 byte”) of the data block that has to be read from the memory. The first read request  51  might for example be a read request for reading sequencer data from the memory, and therefore, the priority “3” might be assigned to said first read request  51 . 
     An interface slice request  52  from interface A is set in order to indicate to the arbitration unit&#39;s control logic that a request with priority “3” from interface A has to be processed. In  FIG. 3 , the interface slice requests from interfaces A, B, and C are shown underneath the slice train  49 . At the point of time  53 , the next data slice of the slice train  49  has to be selected. There exists only one interface slice request  52  at said point of time  53 , and therefore, in accordance with the interface slice request  52 , 32 byte are read from the addresses  0  to  32  of the RDRAM memory  28 . The obtained data is transmitted, as a data slice  54 , from the memory to the arbitration unit. 
     The first read request  51  relates to a data block of 96 bytes. After the transmission of the data slice  54 , there remain 64 byte that still have to be processed. For this reason, an interface slice request  55  from interface A remains active. The priority of the interface slice request  55  is increased from 3 to 3+1+1=5. 
     The first reason why the priority of the interface slice request  55  is increased is that whenever a data slice comprising read data is transmitted, all the priorities of interface slice requests that correspond to read accesses are increased, while the priorities of interface slice requests that correspond to write accesses are kept constant. Whenever a data slice comprising write data is transmitted, the priorities of interface slice requests that correspond to write accesses are increased, while the priorities of interface slice requests that correspond to read accesses remain constant. By doing this, it can be achieved that the amount of switching between write accesses and read accesses is reduced. Any switching between write accesses and read accesses gives rise to an extra time delay that will further on be referred to as a “read-to-write bubble” or a “write-to-read bubble”. By reducing the amount of switching between write accesses and read accesses, said extra time delays are reduced, and the RDRAM memory can be accessed more efficiently. 
     The second reason why the priority of the interface slice request  55  is increased is that after a certain data slice has been transmitted, the priority of data slices that relate to an adjacent address range is increased in order to promote continuous read and write operations. After the data slice  54  has been read from the shared memory, it is favorable to read an adjacent data slice from said shared memory, because in this case, no re-addressing is necessary. In order to promote continuous read and write operations, the priorities of subsequent data slices might e.g. be incremented by one. By doing this, extra time delays due to re-addressing, which will further on be referred to as “re-addressing bubbles”, are reduced. 
     In accordance with these two reasons, the priority of the interface slice request  55  is incremented from “3” to “4” in order to avoid any switching between read and write operations, and said priority is further increased from “4” to “5” in order to promote the transmission of an adjacent block of data. 
     At the point of time  56 , only the interface slice request from interface A is active. Accordingly, 32 byte of read data that are read from the addresses  32  to  64  are transmitted, as a data slice  57 , from the memory to the arbitration unit. 
     At the point of time  58 , a write request  59  is received by the arbitration unit  30  via an interface B. The write request  59  indicates that 64 byte of write data starting at the address  256  have to be written to the memory. Said write data might e.g. be a block of result data, and correspondingly, a priority of “5” might be assigned to said write data. At the point of time  58 , the interface slice request  60  from interface B is set. 
     A second read request  61  is received at the point of time  62 , and said second read request  61  indicates that 32 byte of read data have to be read from the memory starting at the address  512 . The read request  61 , which is received via the interface C, might for example relate to fetching instructions from the memory, and therefore, a priority of “6” might initially be assigned to said read request. At the point of time  63  when the second read request  61  is received, a read access is being processed. In order to avoid any switching from read to write, the priority of the second read request  61  is incremented from “6” to “7”. At the point of time  62  when the second read request  61  is received, the interface slice request  63  from interface C is set. 
     The transmission of the data slice  57  is finished at the point of time  64 . At said point of time  64 , the interface slice request  65  from interface A is still active, because the first read request  51  hasn&#39;t been completed yet. A third data slice corresponding to the first read request  51  still has to be transmitted. At the point of time  64 , the priority “7” of the interface slice request  63  is higher than the respective priorities of the interface slice requests  60  and  65 . Accordingly, a data slice  67  that corresponds to the second read request  61  is scheduled next. 
     After a time delay  66  that is caused by re-addressing, the data slice  67  is transmitted, said data slice  67  comprising 32 bytes of read data from the addresses  512  to  544 . By transmitting the data slice  67 , the read request  61  is completed, and accordingly, the interface slice request  63  from interface C is reset to zero. 
     At the point of time  68 , two interface slice requests from the interfaces A and B are active. The priority of the interface slice request  69  has been changed from “5” to “4”, because now, re-addressing would be necessary for processing said slice request. At the point of time  68 , the interface slice request  70  has the highest priority, and accordingly, the write request  59  is processed next. After a time delay  71  caused by a “read-to-write bubble”, a data slice  72  comprising 32 byte of write data is transmitted. Said write data, which is received via the interface B, is written to the memory addresses  256  to  288 . In order to promote a continuous transmission of write data from the arbitration unit to the shared memory, the priority of the interface slice request  73  is changed from “5” to “7”. 
     At the point of time  74  during the transmission of the data slice  72 , the fill level of the FIFO buffer corresponding to interface A might exceed a certain predefined fill level, and correspondingly, the priority of the interface slice request  75  is increased from “3” to “4”. 
     At the point of time  76 , it is decided to transmit another data slice  77  of write data corresponding to the write request  59 . Said write data, which is received via the interface B, is written to the memory addresses  288  to  320  of the shared memory. By transmitting the data slice  77 , the write request  59  is completed, and accordingly, the interface slice request  73  is reset to zero. 
     At the point of time  78 , only the interface slice request  79  from interface A is active. because the first read request  51  hasn&#39;t been completed yet. After a time delay  80  caused by a “write-to-read bubble”, a third data slice  81  corresponding to the first read request  51  is transmitted from the shared memory to the arbitration unit. Said data slice  81  comprises 32 byte of read data that have been read from the memory addresses  64  to  96 .