Abstract:
A system, comprising, a first apparatus; and a second apparatus which transmits first data of a first amount to said first apparatus, wherein said first apparatus comprises, a memory element which stores said first data, a first controller which transmits a first signal for adjusting said first amount when said first amount need to be adjusted or a second signal corresponding to a second amount of a second data extracted from said memory element when said first amount does not need to be adjusted, to said second apparatus through a signal line, wherein said second apparatus comprises, a second controller which updates a third signal corresponding to a third amount of third data that said first apparatus permits of receiving, based on said first signal, a third controller which determines to transmit said first data when said third amount is equal to or larger than said first amount.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a technique of data transmission from a transmitting apparatus to a receiving apparatus, and more particularly, to a technique of data transmission control based on a permissible amount of transmissible data. 
         [0002]    When data is transmitted from the transmitting apparatus to the receiving apparatus, it is necessary to perform flow control for controlling data transmission from the transmitting apparatus so that a buffer (receiving data buffer) provided for the receiving apparatus does not become full. In the past, as shown in Laid-open No. 61-63140, a flow control was performed in which a special character indicating whether data transmission is allowed or not is transmitted from the receiving apparatus to the transmitting apparatus to control data transmission. However, the following problems reside in the flow control performed by transmitting the special character. First, the flow control cannot be performed when a data transmission speed is high. Second, reliable control cannot be performed. Third, it is difficult to handle transaction processing. 
         [0003]    Accordingly, as one of flow control systems used when data is transmitted from the transmitting apparatus to the receiving apparatus, there has been used credit-based flow control in which the receiving apparatus transmits a credit to the transmitting apparatus, and the transmitting apparatus transmits data whose amount corresponds to the received credit. In the credit-based flow control, the receiving apparatus generates a credit based on the amount of data processed in the receiving data buffer or an amount of available space of the receiving data buffer, and transmits the credit to the transmitting apparatus as a reception credit. At this time, the transmitting apparatus should receive data whose amount corresponds to the reception credit. The transmitting apparatus receives the reception credit from the receiving apparatus and adds the reception credit to a transmittable credit stored therein. When the transmittable credit is equal to or more than a credit corresponding to the amount of data to be transmitted (transmission credit), the transmitting apparatus transmits data to the receiving apparatus. The credit expresses an amount of data. In the technical field of the flow control for data transmission, the amount of data is not directly expressed in units of bytes or kilobytes, and the amount of data or the amount of data transmitted is often expressed in units of credits on the assumption that a predetermined amount of data is defined as one credit in advance. An example of the credit-based flow control is disclosed in Laid-open No. 2004-235728. 
         [0004]      FIG. 7  shows an example of a configuration of a conventional flow control system in which data transmission is controlled by the credit-based flow control. The conventional flow control system performs flow control when data is transmitted from a transmitting apparatus to a receiving apparatus. The conventional flow control system is configured by a transmitting apparatus  101  for transmitting data and a receiving apparatus  102  for receiving data transmitted from the transmitting apparatus  101 . The transmitting apparatus  101  shown in  FIG. 7  serves as an interface to the receiving apparatus. Similarly, the receiving apparatus  102  shown in  FIG. 7  serves as an interface to the transmitting apparatus. Between the transmitting apparatus  101  and the receiving apparatus  102 , there are a data line through which data is transmitted and received as a data signal  109  and a signal line through which a credit is notified as a credit notification signal  110  from the receiving apparatus  102  to the transmitting apparatus  101 . 
         [0005]    The receiving apparatus  102  includes a first-in, first-out (FIFO) receiving data buffer  107  for temporarily storing data received from the transmitting apparatus  101 , and a credit notification control circuit  108  for calculating a credit (reception credit) to be notified to the transmitting apparatus  101 . Data stored in the receiving data buffer  107  can be taken out from another circuit (not shown) included in the receiving apparatus. In the receiving apparatus  102 , when the received data which is stored in the receiving data buffer  107  is transmitted to another circuit of the receiving apparatus in order to process the received data, a reception data credit signal  111  which is used to notify of a credit corresponding to the amount of taken out received data is transmitted from the receiving data buffer  107  to the credit notification control circuit  108 . The credit notification control circuit  108  calculates a reception credit to be notified to the transmitting apparatus  101 , based on the reception data credit signal  111 , and notifies the transmitting apparatus  101  of the calculated reception credit. 
         [0006]    The transmitting apparatus  101  includes a transmission data buffer  103  for storing transmission data, a transmission credit calculating circuit  104  for calculating and holding a credit of data stored in the transmission data buffer  103  as a transmission credit, a transmittable-credit calculating and holding circuit  105  for calculating and holding a transmittable credit in the transmitting apparatus  101 , and a flow control circuit  106  for performing flow control. Transmission data is sent from a circuit (not shown) provided in the transmitting apparatus and serving as a data transmission source to the transmission data buffer  103 . Transmission data is temporarily stored in the transmission data buffer  103 . The transmission data buffer  103  transmits the stored transmission data to the receiving apparatus  102  under the control of the flow control circuit  106 . 
         [0007]    The transmission credit calculating circuit  104  notifies, when data is transmitted from the transmission data buffer  103  to the receiving apparatus  102 , the transmittable-credit calculating and holding circuit  105  of a credit corresponding to the amount of transmission data transmitted from the transmission data buffer  103 . The transmittable-credit calculating and holding circuit  105  subtracts the credit notified by the transmission credit calculating circuit  104  from the transmittable credit stored in the transmittable-credit calculating and holding circuit  105  when the transmission data buffer  103  transmits data to the receiving apparatus  102 . And, when the transmittable-credit calculating and holding circuit  105  receives the reception credit from the receiving apparatus  102 , the transmittable-credit calculating and holding circuit  105  adds the reception credit to the transmittable credit in order to update the transmittable credit. Note that, at the time of initialization of the flow control system, a credit corresponding to the capacity of the receiving data buffer  107  obtained when the receiving data buffer  107  is vacant is generally set up as a transmittable credit. 
         [0008]    The flow control circuit  106  compares the transmittable credit stored in the transmittable-credit calculating with holding circuit  105  with the transmission credit stored in the transmission credit calculating circuit  104 , and performs the flow control according to a result of the comparison. Specifically, when the transmittable credit is equal to or larger than the transmission credit, the flow control circuit  106  orders the transmission data buffer  103  to transmit data to be transmitted to the receiving apparatus  102 . On the other hand, when the transmittable credit is smaller than the transmission credit, the flow control circuit  106  temporarily stops transmitting data until the transmittable credit becomes equal to or larger than the transmission credit. 
         [0009]    Next, an operation of the conventional flow control system shown in  FIG. 7  is described with reference to a flowchart shown in  FIGS. 8A to 8C .  FIG. 8A  shows a data transmission process performed in the transmitting apparatus  101  and a transmittable credit update process performed in the transmittable-credit calculating and holding circuit  105 .  FIG. 8B  shows data receiving process performed in the receiving apparatus  102  and a credit update process performed in the credit notification control circuit  118 .  FIG. 8C  shows a transmittable credit update process performed by receiving a reception credit in the transmitting apparatus  101 . As described above, before a data transmission is started, the transmittable credit stored in the transmittable-credit calculating and holding circuit  105  is equal to the credit of the receiving data buffer  107 . In other words, the amount of available space of the receiving data buffer  107  is set up as the transmittable credit. The transmittable credit is referred to as a credit “B”, for example. 
         [0010]    First, an operation of the transmitting apparatus  101  when the transmitting apparatus  101  receives data whose amount is expressed by the credit “A” from a transmission source circuit of the transmitting apparatus, or another system is described. 
         [0011]    Referring to  FIG. 8A , transmission data having the credit “A” is stored in the transmission data buffer  103 (step  201 ). The transmission credit stored in the transmission credit calculating circuit  104  is updated to “A” (step  202 ). The transmittable credit “B” stored in the transmittable-credit calculating and holding circuit  105  is compared with the transmission credit “A” (step  203 ). When “A” is equal to or smaller than “B” as a result of the comparison, the transmittable credit is equal to or larger than the transmission credit. In other words, the amount of available space of the receiving data buffer  107  is equal to or larger than the amount of data stored in the transmission data buffer  103 . Accordingly, the flow control circuit  106  transmits the data having the credit “A” from the transmission data buffer  103  to the receiving apparatus  102  (Step  204 ). After data is transmitted, the transmission data buffer  103  becomes vacant. By transmission data which has the credit “A”, the transmittable credit stored in the transmittable-credit calculating and holding circuit  105  is updated to ” (B minus A)” (step  205 ). On the other hand, when “A” is larger than “B” in Step  203 , the flow control circuit  106  does not transmit data and keeps waiting until “A” becomes equal to or smaller than the transmittable credit. Note that, in the case of the credit-based flow control, it is assumed that the transmission data having the credit “A” is transmitted to the receiving apparatus  102  as a group of data. Thus, an operation that the transmission data is split into pieces of partial data according to a space of the receiving data buffer  107  and the partial data is transmitted to the receiving apparatus  102  is not performed. 
         [0012]    After the transmitting apparatus  101  transmits the transmission data having the credit “A” as described above, the receiving apparatus  102  receives the transmission data having the credit “A” (step  206 ). The received data is stored in the receiving data buffer  107 . When a circuit of the receiving apparatus  102  takes out the transmission data having the credit “A” from the receiving data buffer  107 , the credit stored in the credit notification control circuit  108  is updated to “A” (step  207 ). The credit notification control circuit  108  notifies the transmitting apparatus  101  of a reception credit “A” by using a credit notification signal  110  (step  208 ). 
         [0013]    After the receiving apparatus  102  transmits the credit notification signal  110 , the transmittable-credit calculating and holding circuit  105  of the transmitting apparatus  101  receives the reception credit “A” from the receiving apparatus  102  (step  209 ,  FIG. 8C ). Then, the transmittable-credit calculating and holding circuit  105  adds “A” to the transmittable credit (the transmittable credit is “(B minus A) ” at the time just before the addition) stored therein to update the transmittable credit to “B” (which is equal to the sum of “(B minus A)” and “A”). The flowchart returns to Step  201  and repeats the above-described processing to transmit next transmission data. 
         [0014]    In the flow control described above, the transmittable credit which represents an available space of the receiving data buffer  107 is compared with the credit of transmission data, and then the transmission data is transmitted to the receiving apparatus  102 . Thus, a buffer overflow of the receiving data buffer  107  is prevented. Note that, in this flow control system, receiving data is taken out from the receiving data buffer  107 . Except when a process for the reception credit corresponding to the receiving data taken out from the receiving data buffer  107  is not finished in the transmittable-credit calculating and holding circuit  105 , the transmittable credit generally matches a credit corresponding to the amount of available space of the receiving data buffer  107 . 
         [0015]    There are cases in which the receiving apparatus  102  needs to stop data transmission while the above-mentioned flow control is being performed, regardless of the available space of the receiving data buffer  107 . Those cases include a case of changing the system configuration of the receiving apparatus  102  (for example, a crossbar route is changed to change the data routing destination). However, in the above-mentioned conventional credit-based flow control system, since whether to perform data transmission is judged in the transmitting apparatus  101 , it is impossible for the receiving apparatus  102  to instruct the transmitting apparatus  101  to stop data transmission from the transmitting apparatus  101  when the receiving data buffer  107  has available space. Further, there is a case in which, to evaluate a test data that is transmitted from transmitting apparatus  101  and has a quite small credit, it is desired to receive only data having a quite smaller credit than that corresponding to the amount of available space (i.e., transmittable credit) of the receiving data buffer  107 . However, since the receiving apparatus  102  cannot issue an instruction, data having larger amount than the test data is transmitted instead of the test data. 
         [0016]    To stop data transmission from the transmitting apparatus without issuing a stop instruction from the receiving apparatus, there is a method in which when the receiving apparatus receives data, the receiving apparatus does not transmit a reception credit to the transmitting apparatus when the receiving apparatus receives data irrespective of whether data is taken out from the receiving data buffer. According to this method, the transmittable credit of the transmitting apparatus keeps decreasing every time the transmission data is transmitted to the receiving apparatus and finally becomes zero or a value close to zero. As a result, every piece of transmission data has a credit larger than the transmittable credit, so the transmitting apparatus does not transmit the transmission data to the receiving apparatus. A method of this type is disclosed in Laid-open No. 2004-72547, for example. However, with this method, it is impossible to properly grasp an elapsed time from when transmission of a reception credit from the receiving apparatus is stopped to when data transmission from the transmitting apparatus is stopped. Further, when transaction data is transmitted, a reply that includes a reception credit is returned from the receiving apparatus to the transmitting apparatus after the transmission data is transmitted from the transmitting apparatus to the receiving apparatus. If the reception credit is not transmitted to the transmitting apparatus, the reply is not returned. Accordingly, in a case of transaction processing having a time-out concept, the transmitting apparatus may detect a reply time-out and regard the reply time-out as a system failure. For example, the credit-based flow control is performed on PCI-Express used for an internal bus in a computer system or a bus which connects a computer system and an extension board. However, since a time-out concept resides therein, the receiving apparatus has to return a reply to a transaction issued by the transmitting apparatus within a fixed period of time. Thus, in the case of PCI-Express, the method of preventing data transmission from the transmitting apparatus without transmitting a reception credit to the transmitting apparatus cannot be used. 
         [0017]    In Laid-open No. 09-200290, there is disclosed credit-based flow control in which data transmission from the transmitting apparatus is stopped by not transmitting information used to update a credit stored in the transmitting apparatus from the receiving apparatus. 
         [0018]    Note that if a new signal line for transmitting a transmission stop instruction from the receiving apparatus to the transmitting apparatus is provided, the receiving apparatus can instruct the transmitting apparatus to stop data transmission, thereby solving the above-mentioned problem. However, providing the new signal line for transmitting a transmission stop instruction can cause a large increase in cost, and further, it maybe difficult to provide the new signal line. In addition, each of the transmitting apparatus and the receiving apparatus needs to be configured considering the new signal line. So, to stop data transmission by transmitting the transmission stop instruction through the new signal line is hardly applied to an existent flow control system or a system which uses an apparatus of different vendors. 
       SUMMARY OF THE INVENTION 
       [0019]    As described above, in the conventional credit-based flow control system, it is impossible to stop or suppress data transmission by an instruction from the receiving apparatus. This problem is caused by a configuration in which a credit corresponding to the amount of available space or an increase in the amount of available space of the receiving data buffer is constantly notified from the receiving apparatus to the transmitting apparatus. However, even if transmission of a credit from the receiving apparatus to the transmitting apparatus is prevented, the receiving apparatus cannot predict the time when data transmission from the transmitting apparatus ends, and reply time-out occurs in the transaction which requires return of a reply. When the new signal line is provided to instruct to stop data transmission from the receiving apparatus to the transmitting apparatus, the cost increases, and modification of the hardware is required. 
         [0020]    A system, comprising: a first apparatus; and a second apparatus which transmits first data having a first amount to said first apparatus, wherein said first apparatus comprises: a memory element which stored said first data; a first controller which transmits a first signal for adjusting said first amount when said first amount need to be adjusted or a second signal indicating a second amount of a second data extracted from said memory element when said first amount does not need to be adjusted, to said second apparatus through a signal line; and wherein said second apparatus comprises: a second controller which updates a third signal indicating a third amount of third data that said first apparatus permits of receiving, based on said first signal; and a third controller which determines to transmit said first data when said third amount is equal to or larger than said first amount. 
         [0021]    An apparatus which receives first data having a first amount from another apparatus, comprising: a memory element which stored said first data; and a controller which transmits a first signal for adjusting said first amount when said amount need to be adjusted or a second signal indicating a second amount of a second data extracted from said memory element when said first amount does not need to be adjusted, to said another apparatus through a signal line. 
         [0022]    An apparatus which transmits first data having a first amount to another apparatus having a memory element for storing said first data, comprising: a first controller which receives a first signal for adjusting said first amount when said another apparatus need to adjust said first amount or a second signal indicating a second amount of second data extracted from said memory element when said first amount does not need to be adjusted, from said another apparatus through a signal line; wherein said first controller updates a third signal indicating a third amount of third data that said another apparatus permits of receiving, based on said first signal; and a second controller which determines to transmit said first data when said third amount is equal to or larger than said first amount. 
         [0023]    A system, comprising: a first apparatus; and 
         [0024]    a second apparatus which transmits first data to said first apparatus, wherein said first apparatus comprises: a memory element which stores said first data; a first storage element which stores a first signal indicating a first amount of second data extracted from said memory element; a second storage element which stores a second signal indicating a second amount that is smaller than that of said first signal; a select circuit which transmits said first signal to said second apparatus while said first apparatus accepts a data transmission from said second apparatus, and transmits said second signal to said second apparatus when said first apparatus suspends said data transmission from said second apparatus; and 
         [0025]    said second apparatus comprises: a third storage element which stores said first signal or said second signal as a third signal; a fourth storage element which stores a fourth signal indicating a third amount of third data that is ready to be transmitted to said first apparatus; and a controller which permits said data transmission when said third amount indicated by said fourth signal is equal to or smaller than that of said third signal. 
         [0026]    A method for controlling a system having a first apparatus and a second apparatus transmitting first data having a first amount to said first apparatus, comprising: storing said first data in a memory element; transmitting a first signal for adjusting said first amount when said first amount need to be adjusted or a second signal indicating a second amount of a second data extracted from said memory element when said first amount does not need to be adjusted, to said second apparatus through a signal line; and updating a third signal indicating a third amount of said third data that said first apparatus permits of receiving, based on said first signal; and 
         [0027]    determining to transmit said first data when said third amount is equal to or larger than said first amount. 
         [0028]    A method of controlling a apparatus, comprising: receiving first data having a first amount from another apparatus; storing said first data in a memory element; and transmitting a first signal for adjusting said first amount when said amount of said data need to be adjusted or a second signal indicating a second amount of a second data extracted from said memory element when said first amount does not need to be adjusted, to said another apparatus through a signal line. 
         [0029]    A method for controlling an apparatus which transmits first data having a first amount to another apparatus having a memory element for storing said first data, comprising: receiving a first signal for adjusting said first amount when said another apparatus need to adjust said first amount or a second signal indicating a second amount of second data extracted from said memory element when said first amount does not need to be adjusted, from said another apparatus through a signal line; updating a third signal indicating a third amount of said third data that said another apparatus permits of receiving, based on said first signal; and determining to transmit said first data when said third amount is equal to or larger than said first amount. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    Other feature and advantages of the invention will be made apparent by the following detailed description and the accompanying drawings, wherein: 
           [0031]      FIG. 1  is a block diagram illustrating a configuration of a flow control system according to a first embodiment of the present invention; 
           [0032]      FIG. 2  is a block diagram illustrating a configuration of a credit notification control circuit; 
           [0033]      FIG. 3  is a block diagram illustrating a configuration of a transmittable-credit calculating and holding circuit; 
           [0034]      FIG. 4A  is a flowchart illustrating a process for suspending data transmission between a transmitting apparatus  301  and a receiving apparatus  302  and canceling the suspension of the data transmission; 
           [0035]      FIG. 4B  is a flowchart illustrating an operation of the transmitting apparatus  301  for updating a transmittable credit; 
           [0036]      FIG. 5  is a block diagram illustrating a configuration of a flow control system according to a second embodiment of the present invention; 
           [0037]      FIG. 6A  is a flowchart illustrating an operation of a transmitting apparatus  701  for updating the credit; 
           [0038]      FIG. 6B  is a flowchart illustrating an operation of a receiving apparatus  702  for updating the credit; 
           [0039]      FIG. 6C  is a flowchart illustrating an operation of the transmitting apparatus  701  for updating the credit; 
           [0040]      FIG. 6D  is a flowchart illustrating a case in which the receiving apparatus  702  issues an instruction to suspend data transmission or to cancel the suspension; 
           [0041]      FIG. 7  is a block diagram illustrating a configuration of a conventional flow control system based on a credit; 
           [0042]      FIG. 8A  is a flowchart illustrating flow control operation based on a transmittable credit; 
           [0043]      FIG. 8B  is a flowchart illustrating an operation of a receiving section for notifying a credit; and 
           [0044]      FIG. 8C  is a flowchart illustrating an operation of a transmitting section for updating a transmittable credit. 
           [0045]    In the drawings, the same reference numerals represent the same structural elements. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0046]    A first embodiment of the present invention will be described in detail below. 
         [0047]      FIG. 1  illustrates a configuration of a flow control system based on a credit. In the flow control system, a flow control is performed by transmitting a reception credit from a receiving apparatus  302  to a transmitting apparatus  301 , similarly to the conventional flow control system shown in  FIG. 7 . The flow control system according to the first embodiment is different from that of  FIG. 7  because not only the reception credit capable of increasing a value of transmittable credit stored in the transmitting apparatus  301  but also the reception credit capable of decreasing the value of a transmittable credit can be transmitted from the receiving apparatus  302  to the transmitting apparatus  301 . Specifically, a credit having a negative value is transmitted from the receiving apparatus  302  to the transmitting apparatus  301  to update, for example, a value of the transmittable credit to  0 , to thereby suspend data transmission from the transmitting apparatus  301  irrespective of a credit of a transmission data stored in a transmission data buffer. In order to cancel the suspension of data transmission, it is only necessary to transmit a credit having an appropriate positive value to the transmitting apparatus  301  from the receiving apparatus  302 . Hereinafter, the flow control system shown in  FIG. 1  is described in detail. 
         [0048]    The flow control system shown in  FIG. 1  includes the transmitting apparatus  301  which transmits data, and the receiving apparatus  302  which receives the data transmitted from the transmitting apparatus  301 . Between the transmitting apparatus  301  and the receiving apparatus  302 , there is provided at least one data line through which data is transmitted and received as a data signal  309 , and a plurality of signal lines through which a credit is notified by using a credit notification signal  310  from the receiving apparatus  302  to the transmitting apparatus  301 , each serving as an interface. 
         [0049]    The receiving apparatus  302  includes a FIFO receiving data buffer  307  for temporarily storing receiving data, and a credit notification control circuit  308  for calculating a credit to be notified to the transmitting apparatus  301 . In a case in which the receiving data stored in the receiving data buffer  307  is transferred therefrom to an internal circuit (not shown) of the receiving apparatus  302  so as to be processed, a receiving data credit signal  3081  for notifying the credit of the transferred receiving data is transmitted from the receiving data buffer  307  to the credit notification control circuit  308 . Further, the receiving data buffer  307  transmits a receiving buffer credit signal  3080  for notifying the credit (i.e., receiving buffer credit) indicating the space currently available in the receiving data buffer  307  to the credit notification control circuit  308 . The credit notification control circuit  308  is configured so as to be supplied with a transmission suspension instruction signal  311  for instructing suspension of data transmission between the transmitting apparatus  301  and the receiving apparatus  302  from the internal circuit (not shown) of the receiving apparatus  302 . 
         [0050]    As shown in  FIG. 2 , the credit notification control circuit  308  includes a credit selector  3082  for selecting one of the receiving buffer credit signal  3080  and the receiving data credit signal  3081  in response to the transmission suspension instruction signal  311 , a credit storage register  3083  for storing the credit of one of the credit selected in the credit selector  3082 , and a sign storage register  3084  for storing “1” when the sign storage register  3084  receives the transmission suspension instruction signal  311 . A credit is represented by a binary signed integer. A negative credit has a most significant bit (MSB) of “1”, and a positive credit has a most significant bit of “0”. Accordingly, the credit notification control circuit  308  generates the credit by setting the content of the sign storage register  3084  for the most significant bit of the credit and setting the content stored in the credit storage register  3083  for lower-order bits. The credit notification control circuit  308  notifies the generated credit as a reception credit to the transmitting apparatus  301 . 
         [0051]    The transmitting apparatus  301  includes a transmission data buffer  303  for storing transmission data, a transmission credit calculating circuit  304  for calculating the transmission credit, a transmittable-credit calculating and holding circuit  305  for calculating and holding the transmittable credit, and a flow control circuit  306  for performing the flow control. Transmission data temporarily stored in the transmission data buffer  303  is transmitted from an internal circuit of the transmitting apparatus  301  or from another system. The transmission data buffer  303  transmits the transmission data stored therein to the receiving apparatus  302  under the direction of the flow control circuit  306 . The transmission credit calculating circuit  304  calculates the transmission credit and, in a case in which the transmission data is transmitted from the transmission data buffer  303 , outputs a transmission data credit signal  3053  for notifying the transmittable-credit calculating and holding circuit  305  of the transmission credit. The flow control circuit  306  compares the transmittable credit held in the transmittable-credit calculating and holding circuit  305  with the transmission credit held in the transmission credit computing circuit  304 . In a case, in which the transmittable credit is equal to or larger than the transmission credit, the transmission data is transmitted to the receiving apparatus  302 . In a case in which the transmittable credit is smaller than the transmission credit, the transmission data is not transmitted until the transmittable credit becomes equal to or larger than the transmission credit. 
         [0052]    As shown in  FIG. 3 , the transmittable-credit calculating and holding circuit  305  includes a credit split circuit  3050  for splitting the credit that has received as a notification from the credit notification signal  310  into a code signal  3051  (i.e., MSB of the credit) and a credit signal  3052  (lower-order bits excluding MSB of the credit), a transmittable credit storage register  3054  for storing the transmittable credit, a transmittable credit calculating circuit  3055  for calculating the transmittable credit, and a code storage register  3056  for storing “1” in a case in which it is determined by the calculation in the transmittable credit calculating circuit  3055  that the transmittable credit has a negative value. The transmittable credit held in the transmittable credit storage register  3054  is notified to the flow control circuit  306  by a transmittable credit notification signal  3057 . 
         [0053]    When the transmission data buffer  303  transmits the transmission data to the receiving apparatus  302 , the transmittable credit calculating circuit  3055  subtracts the credit notified by the transmission data credit signal  3053  from the transmittable credit stored in the transmittable credit storage register  3054 . Further, upon reception of the credit notification signal  310  having the code signal  3051  of “0” from the receiving apparatus  302 , the transmittable credit calculating circuit  3055  calculates the transmittable credit by adding the credit of the credit signal  3052  to the transmittable credit stored in the transmittable credit storage register  3054  and updates the transmittable credit. Further, upon reception of the credit notification signal  310  having the code signal  3051  of “1 from the receiving apparatus  302 , the transmittable credit calculating circuit  3055  calculates the transmittable credit by subtracting the credit of the credit signal  3052  from the transmittable credit stored in the transmittable credit storage register  3054  and updates the transmittable credit. 
         [0054]    In this embodiment, one of the plurality of signal lines for the credit notification signal  310  is used for transmitting the code signal  3051  of the credit notification signal  310  from the receiving apparatus  302  to the transmitting apparatus  301 . 
         [0055]    Next, an operation of the flow control system is described. In initializing the flow control system, the transmittable credit is generally assigned a credit corresponding to the space of the receiving data buffer  307  when the receiving data buffer  307  is vacant. In the flow control system, until the transmission data buffer  303  of the transmitting apparatus  301  receives the transmission data, a credit corresponding to the space of the receiving data buffer  307  (i.e., receiving buffer credit) is stored as a transmittable credit in the transmittable credit storage register  3054 . In this case, the credit is assumed to be “B”. 
         [0056]    First, a process in a case in which the transmission suspension instruction signal  311  is not inputted to the credit notification control circuit  308  yet is described below. In this case, the content of the code storage register  3084  is “0” and the credit selector  3082  selects the receiving data credit signal  3081 . Therefore, the receiving data credit signal  3081  is stored in the credit storage register  3083 . As a result, a credit having the most significant bit of “0” and corresponding to an amount of data extracted from the receiving data buffer  307  is transmitted from the receiving apparatus  302  to the transmitting apparatus  301  as the reception credit. The most significant bit of the credit (the reception credit) is “0”, so the credit has a positive value. Also, in this case, the code signal  3051  is “0”. So, a credit corresponding to the credit signal  3052  is added to the transmittable credit. That is, a similar operation as that illustrated in  FIGS. 8A to 8C  is performed. 
         [0057]    Next, referring to  FIGS. 4A and 4B , explained is an operation of the credit notification control circuit  308  in a case in which the receiving apparatus  302  has received an instruction of suspending transmission with the transmitting apparatus or an instruction of canceling the suspension of transmission with the transmitting apparatus based on the transmission suspension instruction signal  311  is described below.  FIG. 4A  illustrates an operation of the receiving apparatus  302  in a case in which the receiving apparatus  302  has received the instruction of suspending transmission with the transmitting section or the instruction of canceling the suspension of transmission with the transmitting section based on the communication suspension instruction signal  311 .  FIG. 4B  illustrates an operation of the transmittable-credit calculating and holding circuit  305  for updating the transmittable credit upon reception of a credit which is sent from the receiving apparatus  302 . 
         [0058]    As shown in  FIG. 4A , the receiving apparatus  302  receives the instruction of suspending transmission with the transmitting apparatus based on the transmission suspension instruction signal  311  (Step  211 ). Then, the content of the code storage register  3084  is updated to “1” (Step  212 ). Since the content of the code storage register  3084  is “1”, the receiving buffer credit signal  3080  is selected by the credit selector  3082  (Step  213 ). Accordingly, the receiving buffer credit (“B” in this case) is stored in the credit storage register  3083 . The content of the code storage register  3084  and the content of the credit storage register  3083  are combined, and a credit “1B” having the most significant bit of “1” and the lower-order bits of “B” is notified to the transmitting apparatus  301  (Step  214 ). The most significant bit of the credit “1B” is “1” because the value stored in the code storage register  3084  is “1”. Since the most significant bit of the credit “1B” is “1”, the credit “1B” has a negative value even though the credit “1B” has an absolute value of “B”. 
         [0059]    As described above, the receiving apparatus  302  transmits the credit notification signal  310  of the credit “1B” as the reception credit. Then, as shown in  FIG. 4B , the transmittable-credit calculating and holding circuit  305  receives the credit “1B” (Step  215 ). The received credit is split into the code signal  3051  and the credit signal  3052  by the credit split circuit  3050  (Step  216 ). In this case, the code signal  3051  is “1” and the credit signal  3052  is “B”. Upon reception of the credit having the code signal  3051  of “1”, the transmittable credit calculating circuit  3055  subtracts the credit (“B”) notified by the credit signal  3052  from the value stored in the transmittable credit storage register  3054 . Therefore, the transmittable credit is “0”, which is obtained by subtracting “B” from “B”. As a result, the transmittable credit calculating circuit  3055  stores the transmittable credit of “0” in the transmittable credit storage register  3054  (Step  217 ). The transmittable-credit calculating and holding circuit  305  notifies the transmittable credit of “0” to the flow control circuit  306  using the transmittable credit notification signal  3057  (Step  218 ). 
         [0060]    If the transmitting apparatus  301  receives the transmission data having the credit of “D” from the internal circuit or another system in a case in which “0” is set as the transmittable credit, the transmission data having credit of “D” is stored in the transmission data buffer  303  (Step  201 ), as shown in  FIG. 8A . Next, the transmission credit stored in the transmission credit calculating circuit  304  is updated to “D” (Step  202 ), as shown in  FIG. 8A . Then, the flow control circuit  306  compares the transmittable credit (“0” in this case) with the transmission credit (“D” in this case) in Step  203 . Herein, because “D” is larger than,“0”, the flow control is performed so as to wait in Step  203  without transmitting the transmission data until “D” becomes equal to or smaller than the transmittable credit. 
         [0061]    As described above, when the receiving apparatus  302  receives the instruction of suspending transmission with the transmitting apparatus  301  based on the transmission suspension instruction signal  311 , the data transmission between the receiving apparatus  302  and the transmitting apparatus  301  is suspended. 
         [0062]    Next, an operation for restarting the data transmission from the state where the data transmission between the receiving apparatus  302  and the transmitting apparatus  301  is suspended is described below. 
         [0063]    As shown in  FIG. 4A , the receiving apparatus  302  receives the transmission suspension instruction signal  311  indicating that the suspension of the data transmission needs to be canceled(Step  211 ). As a result, the code storage register  3084  in the credit notification control circuit  308  is updated to “0” (Step  212 ). The receiving buffer credit signal  3080  is selected by the credit selector  3082  (Step  213 ). Accordingly, the credit (“B” in this case) indicated by the receiving buffer credit signal  3080  is stored in the credit storage register  3083 . The credit “0B” is notified to the transmitting apparatus  301  (Step  214 ). The credit “0B” has the most significant bit of “0” and the lower-order bits of “B”. The most significant bit of “0” corresponds to the stored value of “0” in the code storage register  3084 . 
         [0064]    The receiving apparatus  302  transmits the credit notification signal  310  of the credit “0B” as the reception credit. Then, as shown in  FIG. 4B , the transmittable-credit calculating and holding circuit  305  receives the credit “0B” from the receiving apparatus  302  (Step  215 ). The received credit “0B” is split into the code signal  3051  (“0” in this case) and the credit signal  3052  (“B” in this case) by the credit split circuit  3050  (Step  216 ). Since the code signal  3051  is “0”, the transmittable credit calculating circuit  3055  adds the credit notified by the credit signal  3052  to the credit stored in the transmittable credit storage register  3054 . Therefore, the transmittable credit becomes “B”, which is obtained by adding “0” to “B”. Subsequently, the transmittable credit calculating circuit  3055  stores the transmittable credit “B” in the transmittable credit storage register  3054  (Step  217 ). The transmittable credit “B” is notified by the transmittable credit notification signal  3057  to the flow control circuit  306  (Step  218 ). After the transmittable credit “B” is notified to the flow control circuit  306  as described above, the data transmission from the transmission data buffer  303  to the receiving apparatus  302  is initiated according to the procedure shown in  FIG. 8A . 
         [0065]    The available space of the receiving data buffer  307  changes because data is extracted therefrom when the data transmission with the transmitting apparatus  301  is suspended. In this case, the credit storage register  3083  stores the receiving buffer credit used when the suspension of transmission was indicated based on the transmission suspension instruction signal  311  (Step  213 ). Further, in order to deal with the change of the available space of the receiving data buffer  307  due to the extraction of data while suspending the data transmission, the receiving data credit  3081  corresponding to the extracted data is transmitted to the transmitting apparatus  301  as a reception credit. 
         [0066]    The operations when the instruction of suspending transmission with the transmitting apparatus  301  and the instruction of canceling the suspension of transmission with the transmitting apparatus  301  are received using the transmission suspension instruction signal  311  have been explained as above. In each case, both the transmittable credit and the receiving buffer credit are “B”. As long as the transmittable credit is set so as to correspond to the space of the receiving data buffer  307  when the system is initialized, the value of the transmittable credit is same as that of the receiving buffer credit, except for a case in which the cancellation of transmission is instructed or the like. However, in some cases, the transmittable credit and the receiving buffer credit included in the receiving buffer credit signal  3080  may be different from each other. 
         [0067]    For example, the receiving data credit included in the receiving data credit signal  3081  should be transmitted to the transmitting apparatus  301  when data is extracted from the receiving data buffer  307 , but when the receiving data credit has not been transmitted completely the transmittable credit (“C” in this case) stored in the code storage register  3054  is smaller than the receiving buffer credit “B” of the receiving data buffer  307 . The transmittable credit “C” is smaller than the receiving buffer credit “B”. In this case, the transmitting apparatus  301  receives the credit “1B” from the receiving apparatus  302  (Step  215 ), and the transmittable credit calculating circuit  3055  performs calculation to obtain the transmittable credit “(C-B)” (Step  216 ). It means that the transmittable credit has a negative value. As a result, “1” is stored in the code storage register  3056  and the transmittable credit “(C-B)” is stored in the transmittable credit storage register  3054  (Step  217 ). The transmittable-credit calculating and holding circuit  305 notifies the transmittable credit “(C-B)” to the flow control circuit  306  by using the transmittable credit notification signal  3057  (Step  218 ). Also in this case, the transmission credit of transmission data is always larger than the transmittable credit. Therefore, the transmission of the transmission data is prevented. 
         [0068]    Herein, a method for canceling the suspension of the data transmission in this case is explained. In a case in which the transmittable credit “(C-B)” having a negative value is stored in the transmittable credit storage register  3054  and “1” is stored in the code storage register  3056 , the transmitting apparatus  301  receives the credit “0B” from the receiving apparatus  302  (Step  215 ). The transmittable credit calculating circuit  3055  performs calculation, in which “B” is added to “(C-B)”, to obtain the transmittable, credit “C” (Step  216 ). It means that the transmittable credit has a positive value. Accordingly, “0” is stored in the code storage register  3056  and the transmittable credit “C” is stored in the transmittable credit storage register  3054  (Step  217 ). The transmittable-credit calculating and holding circuit  305  notifies the transmittable credit “C”, which is larger than 0, to the flow control circuit  306  by using the transmittable credit notification signal  3057  (Step  218 ). As a result, the transmitting apparatus  301  establishes a state where the transmitting apparatus  301  can transmit data to the receiving apparatus  302 . However, the transmission of the receiving data credit included in the receiving data buffer credit signal  3081  in order to deal with the extraction of data from the receiving data buffer  307  has not been finished at this time. Therefore, the transmittable credit “C” is smaller than the receiving buffer credit “B” corresponding the space of the receiving data buffer  307 . After that, by transmitting the receiving data credit included in the receiving data credit signal  3081  as the reception credit, the process for the receiving data credit is performed in the transmitting apparatus  301  according to the above-mentioned procedure shown in  FIGS. 8A to 8C . As a result, the transmittable credit corresponds with the credit corresponding to the space of the receiving data buffer  307 . In consideration of reply time-out in transaction process, it is preferable that the receiving data credit included in the receiving data credit signal  3081  be initially transmitted to the transmitting apparatus  301  previous to the suspension of the data transmission between the transmitting apparatus  301  and the receiving apparatus  302 . 
         [0069]    Also in a case in which the transmission data is transmitted from the transmission data buffer  303  to the receiving apparatus  302  but the transmission data has not been received by the receiving data buffer  307 , the transmittable credit may be smaller than the receiving buffer credit included in the receiving buffer credit signal  3080 . In this case, it is possible to suspend the data transmission from the transmitting apparatus  301  or to cancel the suspension by performing the similar process described above. 
         [0070]    In this embodiment, it is possible not only to suspend the data transmission from the transmitting apparatus  301  but also to limit the amount of the transmission data transmitted from the transmitting apparatus  301  based on the instruction from the receiving apparatus  302 . To limit the amount of the transmission data, the credit corresponding to the amount of data which the receiving apparatus  302  wants to receive is set to “E”, and the credit having a negative value and an absolute value of “(B-E)” is transmitted to the transmitting apparatus  301  in order to give an instruction of limiting the data transmission. In order to cancel the limitation, the credit having a positive value and an absolute value of “(B-E)” is transmitted to the transmitting apparatus  301 . 
         [0071]    In the above description, the credit transmitted from the receiving apparatus  302  to the transmitting apparatus  301  is represented by a binary signed integer. Alternatively, it is also possible to use a binary complement integer, which is generally used for operation performed by a computer. Whether the binary signed integer or the binary complement integer should be used may be determined based on the addition logic especially used for adding the reception credit to the transmittable credit in the transmittable-credit calculating and holding circuit  305 . In a case in which the binary complement integer is used, a complement calculation circuit for obtaining a complement of the receiving data credit signal  3081  is provided in the credit notification control circuit  308  whereby the complement of the receiving data credit signal  3081  may be inputted in the credit storage register  3083 . 
         [0072]    Assume a case in which it is already possible for the transmitting apparatus  301 , upon receiving of a credit having a negative value, to reduce the value of the transmittable credit based on the value of the credit. In this case, when the flow control according to this embodiment is applied to a conventional system, it is preferable that only the receiving apparatus  302  may have the configuration described above. Considering the implementation of a general integer arithmetic operation, a transmitting apparatus  301  capable of reducing the value of the transmittable credit upon receiving the credit having a negative value can be regarded as wide spread. Therefore, it may be necessary to provide only the receiving apparatus  302  with the above-mentioned configuration in order to establish the flow control system according to this embodiment. 
         [0073]    Subsequently, a flow control system according to a second embodiment of the present invention will be explained. In the second embodiment, the flow control system according to the present invention is applied to an interface of a PCI-Express protocol.  FIG. 5  shows the flow control system according to the second embodiment. 
         [0074]    The flow control system shown in  FIG. 5  includes the transmitting apparatus  701  for transmitting a transmission data and a receiving apparatus  702  for receiving the transmission data transmitted from the transmitting apparatus  701 . Between the transmitting apparatus  701  and the receiving apparatus  702 , there are a plurality of data lines through which data itself is transmitted and received as a data signal  711 , and a plurality of signal lines through which credit notification signal  710  is transmitted from the receiving apparatus  702  to the transmitting apparatus  701 , each serving as an interface. 
         [0075]    The receiving apparatus  702  includes a receiving data buffer  707 , an accumulator  722 , a CREDIT Received  709 , an accumulator  723 , a CREDIT Allocated  708 , a credit selector  712 . The receiving data buffer  707  of the FIFO type temporarily stores the data transmitted from the transmitting apparatus  701 . The accumulator  722  accumulates the credit of the data transmitted from the transmitting apparatus  701  every time the accumulator  722  receives the data transmitted from the transmitting apparatus  701 . The CREDIT Received  709  is a register for storing the accumulation result obtained by the accumulator  722 . The accumulator  723  accumulates the credit of data extracted from the receiving data buffer  707 . The accumulator  723  accumulates the credit every time the data is extracted from the receiving data buffer  707  and transmitted from the receiving data buffer  707  to the internal circuit of the receiving apparatus  702 . The CREDIT Allocated  708  is a register for storing the accumulation result obtained by the accumulator  723 . The credit selector  712  selects one of the credit stored in the CREDIT Allocated  708  and the credit stored in the CREDIT Received  709 . A transmission suspension instruction signal  713  for instructing suspension of data transmission between the transmitting apparatus  701  and the receiving apparatus  702  is supplied from the internal circuit of the receiving apparatus  702  to the credit selector  712 . The credit selector  712  selects one of the credit from the credit stored in the CREDIT Allocated  708  and the credit stored in the CREDIT Received  709  based on the transmission suspension instruction signal  713 . And, the credit selector  712  transmits the selected credit by using a credit notification signal  710  indicating the reception credit. The credit stored in the CREDIT Allocated  708  is a credit which is to be notified to the transmitting apparatus  701  based on PCI-Express standard when the data transmission between the transmitting apparatus  701  and the receiving apparatus  702  is not suspended. 
         [0076]    The transmitting apparatus  701  includes a transmission data buffer  703 , an accumulator  721 , a CREDIT Consumed  704 , a CREDIT Limit  705 , a flow control circuit  706 . The transmission data buffer  703  temporarily stores the transmission data transmitted from the internal circuit of the transmitting apparatus  701 , an external system. The accumulator  721  accumulates the credit of the transmission data every time the transmission data is stored in the transmission data buffer  703 . The CREDIT Consumed  704  is a register for storing the accumulation result obtained by the accumulator  721 . The CREDIT Limit  705  is a register for storing the reception credit received from the receiving apparatus  702 . The flow control circuit  706  performs the flow control. The credit is notified from the receiving apparatus  702  point by point. When the credit notified from the receiving apparatus  702  changes, the credit stored in the CREDIT Limit  705  also changes immediately. 
         [0077]    In PCI-Express protocol, each credit is accumulated by each of the accumulators  721 ,  722 , and  723  using a circulation-type operation in which an overflow is ignored. The flow control circuit  706  compares the credit stored in the CREDIT Limit  705  with the credit stored in the CREDIT Consumed  704 . In a case in which the credit stored in the CREDIT Limit  705  is equal to or larger than the credit stored in the CREDIT Consumed  704 , the transmission data stored in the transmission data buffer  703  is transmitted. In a case in which the credit stored in the CREDIT Limit  705  is smaller than the credit stored in the CREDIT Consumed  704 , the data transmission is temporarily suspended. The suspension of the data transmission is continued until the credit stored in the CREDIT Limit  705  becomes equal to or larger than the credit stored in the CREDIT Consumed  704 . 
         [0078]    The flow control system of this embodiment, in which the credit is calculated by accumulating, is greatly different from the flow control system shown in  FIG. 1 , in which the credit is added or subtracted based on the amount of data transmitted or received, in terms of the calculation process of the credit. However, each credit of this embodiment corresponds to each credit in the flow control system shown in  FIG. 1 . The credit stored in the CREDIT Consumed  704  corresponds to the transmission credit. The credit stored in the CREDIT Limit  705  corresponds to the transmittable credit. The credit stored in the CREDIT Allocated  708  corresponds to the receiving data credit. The credit stored in the CREDIT Received  709  corresponds to the receiving buffer credit. 
         [0079]    Subsequently, an operation of the flow control system shown in  FIG. 5  is explained referring to  FIGS. 6A to 6D .  FIG. 6A  illustrates an operation of the data transmission and updating the credit stored in the CREDIT Consumed  704  performed in the transmitting apparatus  701 .  FIG. 6B  illustrates an operation of receiving the transmission data, updating the credit stored in the CREDIT Received  709 , and updating the credit stored in the CREDIT Allocated  708 , which are performed in the receiving apparatus  702 .  FIG. 6C  illustrates an operation of updating the credit stored in the CREDIT Limit  705  based on the credit notification signal  710  performed in the transmitting apparatus  701 .  FIG. 6D  illustrates an operation performed by the receiving apparatus  702  in a case in which the receiving apparatus  702  receives the transmission suspension instruction signal  713  indicating the instruction of suspending transmission with the transmitting apparatus  701  or the instruction of canceling the suspension of transmission with the transmitting apparatus  702 . 
         [0080]    In a case in which the data is-not being transmitted and there is no credit unprocessed in the system, in PCI-Express, the credit stored in the CREDIT Allocated  708  corresponds to the credit stored in the CREDIT Limit  705  in general. In the following, the credit stored in the CREDIT Allocated  708  and the credit stored in the CREDIT Limit  705  are assumed to be “A”. Accordingly, the credit which is stored in the accumulator  723  is also “A”. Further, it is assumed that no transmission data has been transmitted to the transmission data buffer  703  and the credit stored in the CREDIT Consumed  704  and the credit stored in the CREDIT Received  709  are both “0”. Accordingly, the accumulator  721  and the accumulator  722  also stores the credit whose value is “0”. 
         [0081]    First, a process performed in a case in which the transmitting apparatus  701  has received the transmission data of the credit “C” from the internal circuit of the transmitting apparatus, will be explained. 
         [0082]    As shown in  FIG. 6A , the transmission data of the credit “C” is stored in the transmission data buffer  703  (Step  221 ). The credit “C” is accumulated by the accumulator  721  and the credit stored in the CREDIT Consumed  704  is updated (Step  222 ). In this case, since the accumulator  721  has the initial value of “0”, the credit stored in the CREDIT Consumed  704  is updated to “C”. The flow control circuit  706  compares the credit “A” stored in the CREDIT Limit  705  with the credit “C” stored in the CREDIT Consumed  704  (Step  223 ). In a case in which “C” is equal to or smaller than “A” as a result of the comparison, the data of the credit “A” is transmitted to the receiving apparatus  702  based on the flow control performed by the flow control circuit  706  (Step  224 ). Meanwhile, in a case in which “C” is larger than “A”, the flow control is performed so as to wait in Step  223  without transmitting the transmission data until “C” becomes equal to or smaller than the credit stored in the CREDIT Limit  705 . 
         [0083]    The transmitting apparatus  701  transmits data of the credit “C” as described above. Then, as shown in  FIG. 6B , the receiving apparatus  702  receives data of the credit “C” (Step  225 ). The credit “C” is accumulated by the accumulator  722  to thereby update the credit stored in the CREDIT Received  709  (Step  722 ). In this case, since the accumulator  722  has the initial value of “0”, the credit stored in the CREDIT Received  709  is updated to “C”. Then, data of the credit “C” is transferred from the receiving data buffer  707  to the internal circuit of the receiving apparatus  702 . The credit “C” is accumulated by the accumulator  723  and the credit stored in the CREDIT Allocated  708  is updated to “(A+C)” (Step  227 ). The credit selector  712  selects the credit stored in the CREDIT Allocated  708 , and the credit “(A+C)” stored in the CREDIT Allocated  708  as the reception credit is notified to the transmitting apparatus  701  by using the credit notification signal  710  (Step  228 ). 
         [0084]    When the receiving apparatus  702  transmits the credit notification signal  710 , the transmitting apparatus  701  receives the credit “(A+C)” from the receiving apparatus  702  (Step  229  of  FIG. 6C ) The credit of the CREDIT Limit  705  is updated to “(A+C)” (Step  230 ). Then, the flowchart returns to Step  221  to repeat the above-mentioned process in order to transmit the subsequent transmission data. 
         [0085]    Herein, it is assumed that the receiving apparatus  702  has received the instruction of suspending transmission with the transmitting apparatus  701  (Step  231  of  FIG. 6D ). The instruction is notified by the transmission suspension instruction signal  713 . Then, the credit selector  712  selects the credit stored in the CREDIT Received  709  when the transmission suspension instruction signal  713  instructs the credit selector  712  to suspend transmission with the transmitting apparatus  701 . The credit “C” stored in the CREDIT Received  709  as the reception credit is notified to the transmitting apparatus  701  by using the credit notification signal  710 . The transmitting apparatus  701  receives the credit “C” from the receiving apparatus  702  (Step  229   FIG. 6C ). Then, the credit stored in the CREDIT Limit  705  is updated to “C” (Step  230 ). It is assumed that the transmitting apparatus  701  has received the transmission data of the credit “D” in a case in which the credit stored in the CREDIT Limit  705  is “C”. In this case,, the data of the credit “D” is stored in the transmission data buffer  703  (Step  221  of  FIG. 6A ). Further, the credit stored in the CREDIT Consumed  704  is updated to “(C+D)” based on the accumulation performed by the accumulator  721  (Step  222 ). The flow control circuit  706  compares the credit “C” stored in the CREDIT Limit  705  with the credit “(C+D)” stored in the CREDIT Consumed  704  (Step  223 ). Since “(C+D)” is larger than “C”, the flow control is performed by the flow control circuit  706  so as to wait in Step  223  without transmitting the transmission data until “(C+D)” becomes equal to or smaller than the credit stored in the CREDIT Limit  705 . 
         [0086]    As described above, based on the transmission suspension instruction signal  713  giving the instruction of suspending transmission with the transmitting apparatus  701  , the data transmission between the transmitting apparatus  701  and the receiving apparatus  702  is suspended. The process of suspending the data transmission is explained as follows. 
         [0087]    In the above-mentioned process, the credit stored in the CREDIT Received  709  is the sum of the credit of the data received by the receiving data buffer  707 , that is, the sum of the credit of the transmission data which is supposed to be transmitted from the transmission data buffer  703 . Therefore, the credit stored in the CREDIT Received  709  is equal to or smaller than the sum of the credit (that is, the credit stored in the CREDIT Consumed  704 ) of the transmission data stored in the transmission data buffer  703 . Accordingly, by compulsorily making the credit stored in the CREDIT Limit  705  coincide with the credit stored in the CREDIT Received  709 , the data transmission is prevented. 
         [0088]    Subsequently, explanation will be made on a process for canceling the suspension of transmission. 
         [0089]    The receiving apparatus  702  receives the instruction of canceling the suspension of transmission with the transmitting apparatus  701  (Step  231  of  FIG. 6D ). The instruction is notified by the transmission suspension instruction signal  713 . As a result, the credit selector  712  selects the credit “(A+C)” stored in the CREDIT Allocated  708  (Step  232 ). The credit notification signal  710  notifies the selected credit “(A+C)” to the transmitting apparatus  701 . As described above, the credit notification signal  710  is transmitted. Then, the transmitting apparatus  701  receives the credit “(A+C)” from the receiving apparatus  702  (Step  229  of  FIG. 6C ). As a result, the credit stored in the CREDIT Limit  705  is updated to “(A+C)” (Step  230 ). The credit “(A+C)” is larger than the credit “C” of the transmission data. Therefore, it is possible to advance from Step  223  to Step  224  in  FIG. 6A  whereby the data transmission is restarted from the transmission data buffer  703  to the receiving apparatus  702 . In other words, by using the credit stored in the CREDIT Allocated  708 , which is a proper credit to be notified to the transmitting apparatus  701 , the operation of the flow control system performed in the normal conditions is restarted whereby the data transmission is restarted. 
         [0090]    If a credit larger than the credit stored in the CREDIT Received  709  by a predetermined value is notified to the transmitting apparatus  701  instead of notifying the credit stored in the CREDIT Received  709 , the flow control system according to the second embodiment may be configured such that only the transmission data having a credit, which is equal to or smaller than the credit larger than the credit stored in the CREDIT Received  709  by the predetermined value, is transmitted from the transmitting apparatus  701  irrespective of the currently available space of the receiving data buffer  707 . 
         [0091]    In a case in which the flow control system according to this embodiment is applied to the conventional system using PCI-Express, the conventional transmitting apparatus can be used as it is. Based on PCI-Express standard, the receiving apparatus  702  generally includes not only the CREDIT Allocated  708  being a register but also the CREDIT Received  709  being a register. Therefore, only by adding to the conventional receiving apparatus the credit selector  712  capable of being switched in response to the transmission suspension instruction signal  713 , it is possible to apply the conventional receiving apparatus to the flow control of the present invention. 
         [0092]    According to the present invention, in a case of suspending or controlling the data transmission from the transmitting apparatus  301 ,  701  in the receiving apparatus  302 ,  702 , the receiving apparatus  302 ,  702  notifies the credit capable of reducing the value of the transmittable credit (in a case of PCI-Express, CREDIT Limit) to the transmitting apparatus  301 ,  701 . As a result, according to the flow control process under the normal conditions based on the credit of the transmitting apparatus  301 ,  701 , the data transmission from the transmitting apparatus  301 ,  701  to the receiving apparatus  302 ,  702  is prevented or limited. Therefore, the present invention produces an effect of suspending the data transmission from the transmitting apparatus  301 ,  701  at any time according to the instruction from the receiving apparatus  302 ,  702 , irrespective of the available space of the receiving data buffer  307 ,  707 , while eliminating a need to additionally provide a signal line for notifying the suspension of transmission. 
         [0093]    Further, the flow control system of the present invention is different from the one in which the data transmission is suspended by not sending a reply which is supposed to be sent. Therefore, according to the present invention, a problem of reply time-out does not arise, and as a result, the reliable data transmission and flow control can be performed. 
         [0094]    Further, in consideration of applying the flow control system of the present invention to the conventional system including the transmitting apparatus and the receiving apparatus of the conventional system, the flow control system of the present invention can be realized only by modifying the receiving apparatus. Therefore, according to the present invention, it is possible to save work and cost for implementation.