Patent Publication Number: US-11392517-B2

Title: Access control method, bus system, and semiconductor device

Description:
TECHNICAL FIELD 
     The disclosure relates to an access control method, a bus system that performs an access control with use of the access control method, and a semiconductor device including the bus system. 
     BACKGROUND ART 
     A processor may be often coupled to a memory through, for example, a bus and a memory controller. Moreover, the processor may write data to the memory, or read data from the memory, in performing operation processing. Examples of a control method in a case in which the processor writes the data to the memory may include posted transfer (also referred to as bufferable) and non-posted transfer (for example, PTL 1). 
     In the posted transfer, for example, when the processor (a master) makes a write request to a memory controller (a slave), the memory controller makes a response to the processor, prior to an end of data writing to the memory. In the posted transfer, the response may be made at earlier timing, which brings enhancement in transfer performance. This leads to expectation of enhancement in a processing speed in a whole system. However, in a case in which the processor makes such a write request to a certain address, and thereafter, makes a read request, latest data has not been written to the memory yet. Accordingly, there is possibility that old data before re-writing may be read. Thus, in the posted transfer, there is possibility of a disadvantage related to so-called coherency (data consistency). 
     In the non-posted transfer, for example, when the processor (the master) makes the write request to the memory controller (the slave), the memory controller makes the response after the end of the data writing to the memory. In this case, the response may be made at later timing than that of the posted transfer. This results in lowered transfer performance, causing possibility of a lower processing speed in the whole system, but allows for suppression of the coherency-related disadvantage. 
     CITATION LIST 
     Patent Literature 
     
         
         [PTL 1] Specification of U.S. Pat. No. 7,194,566 
       
    
     SUMMARY OF THE INVENTION 
     As described, in the memory access, it is desired to enhance the transfer performance while suppressing the coherency-related disadvantage, with expectation of further enhancement in the transfer performance. 
     It is therefore desirable to provide an access control method, a bus system, and a semiconductor device that make it possible to enhance transfer performance while suppressing a coherency-related disadvantage. 
     An access control method according to an embodiment of the disclosure includes: allowing one master device of a plurality of master devices to generate a request for access to a device to be accessed; allowing a slave device to identify, on a basis of the request for the access, the one master device that has generated the request for the access; and allowing the slave device to make a response to the one master device, at response timing that corresponds to the one master device identified. 
     A bus system according to an embodiment of the disclosure includes a plurality of master devices and a slave device. The plurality of master devices are able to generate a request for access to a device to be accessed. The slave device makes, on a basis of the request for the access, a response to one master device of the plurality of master devices that has generated the request for the access, at response timing that corresponds to the one master device. 
     A semiconductor device according to an embodiment of the disclosure includes a plurality of master devices and a slave device. The plurality of master devices are able to generate a request for access to a device to be accessed. The slave device makes, on a basis of the request for the access, a response to one master device of the plurality of master devices that has generated the request for the access, at response timing that corresponds to the one master device. 
     In the access control method, the bus system, and the semiconductor device in the embodiments of the disclosure, the request for the access is generated by the one master device of the plurality of master devices. On the basis of the request for the access, processing is performed on the device to be accessed. At this occasion, the response from the slave device to the one master device is made at the response timing that corresponds to the one master device. 
     According to the access control method, the bus system, and the semiconductor device in the embodiments of the disclosure, the response to the one master device is made at the response timing that corresponds to the one master device. Hence, it is possible to enhance the transfer performance while suppressing the coherency-related disadvantage. It is to be noted that some effects described here are not necessarily limitative, and any of other effects described herein may be achieved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWING 
         FIG. 1  is a block diagram of one example of a configuration of a bus system according to an embodiment of the disclosure. 
         FIG. 2  is a block diagram of one example of a configuration of a memory controller illustrated in  FIG. 1 . 
         FIG. 3  is a block diagram of one example of a configuration of a response controller illustrated in  FIG. 2 . 
         FIG. 4  is a table that summarizes one example of operation of the response controller illustrated in  FIG. 3 . 
         FIG. 5  is a block diagram of one example of a configuration of another memory controller illustrated in  FIG. 1 . 
         FIG. 6  describes one example of operation of a bus system illustrated in  FIG. 1 . 
         FIG. 7  describes another example of the operation of the bus system illustrated in  FIG. 1 . 
         FIG. 8  describes another example of the operation of the bus system illustrated in  FIG. 1 . 
         FIG. 9  describes one example of operation of a bus system according to a comparative example 1. 
         FIG. 10  describes one example of operation of a bus system according to a comparative example 2. 
         FIG. 11  describes another example of the operation of the bus system according to the comparative example 2. 
         FIG. 12  describes one example of operation of a bus system according to a comparative example 3. 
         FIG. 13  describes another example of the operation of the bus system according to the comparative example 3. 
         FIG. 14  is a block diagram of one example of a configuration of a response controller according to a modification example. 
         FIG. 15  describes one example of operation of a bus system according to another modification example. 
         FIG. 16  describes another example of the operation of the bus system illustrated in  FIG. 15 . 
         FIG. 17  is a block diagram of one example of a configuration of a response controller related to the bus system illustrated in  FIG. 15 . 
         FIG. 18  is a block diagram of one example of a configuration of a bus system according to another modification example. 
         FIG. 19  is a block diagram of one example of an interconnect unit according to another modification example. 
         FIG. 20  is a block diagram of one example of a configuration of a bus system according to another modification example. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     In the following, some embodiments of the disclosure are described in detail with reference to the drawings. 
     [Example of Configuration] 
     [Example of Overall Configuration] 
       FIG. 1  illustrates an example of a configuration of a bus system according to an embodiment. A bus system  1  may be, in this example, a so-called SoC (System on Chip) system that includes one chip. It is to be noted that since an access control method and a semiconductor device according to embodiments of the disclosure are embodied by this embodiment, description thereof is made together. 
     The bus system  1  may include an information processing unit  9 , DMA (Direct Memory Access) controller  20 , an interconnect unit  30 , memory controllers  40  and  60 , and memories  50  and  70 . In this example, circuits except for the memories  50  and  70  may be integrated in one chip. It is to be noted that this is non-limiting. In one alternative, all circuits may be integrated in one chip. 
     The information processing unit  9  may perform operation processing. In this example, the information processing unit  9  may include a cash  10  and two cores  11  and  12 . The cash  10  may be a level 2 (L2) cash memory. The cores  11  and  12  may each be a so-called processor core. The information processing unit  9  may be coupled to the interconnect unit  30  through a bus B 1 . As the information processing unit  9 , a combination of a processor available from ARM Ltd. (e.g., Cortex-A9 MPCore) and an IP (Intellectual Property) such as a level 2 cash controller available from ARM Ltd. (e.g., L2C-310) may be used. As a bus interface with the interconnect unit  30 , for example, an AXI (Advanced eXtensive Interface) may be used. 
     An identifier MI may be assigned to each of the cash  10  and the cores  11  and  12 . The identifier MI may be different for each of the cash  10  and the cores  11  and  12 . In this example, the identifier MI may be an 8-bit parameter. Specifically, in this example, the identifier MI of the cash  10  may be “0x10” in hexadecimal number notation (“00010000” in binary number notation). The identifier MI of the core  11  may be “0x11” (“00010001” in the binary number notation). The identifier MI of the core  12  may be “0x12” (“00010010” in the binary number notation). Upper-order four bits (bits b 4  to b 7 ) of the identifiers MI of the cash  10  and the cores  11  and  12  may be same as one another. In other words, in this example, the upper-order four bits of the identifiers MI may be the same, in consideration that data transfer performance through the bus interface is same, in blocks (i.e., the cash  10  and the cores  11  and  12 ) of the information processing unit  9 . Moreover, lower-order four bits (bits b 0  to b 3 ) of the identifiers MI of the cash  10  and the cores  11  and  12  may be different from one another. 
     In making a write request to the memory  50  or the memory  70 , the information processing unit  9  may supply the interconnect unit  30  with a write address WrADD and write data WrDATA, and receive a response signal RE from the interconnect unit  30 . The write address WrADD may include, for example, the identifier MI and transfer mode information MODE, in addition to address information. The transfer mode information MODE may indicate whether a device that makes the write request (in this example, the information processing unit  9 ) requests for posted transfer or non-posted transfer. In one specific example, the information processing unit  9  may set the transfer mode information MODE as “1”, in making the write request by the posted transfer, and set the transfer mode information MODE as “0”, in making the write request by the non-posted transfer. Moreover, the write address WrADD may further include, for example, information on a bit width or a burst length of the data to be written, and security information. 
     It is to be noted in a case with use of the AXI as the bus interface, the transfer mode information MODE may correspond to a parameter AWCACHE [0] of the AXI. Moreover, the identifier MI may be assigned to a sideband signal of the AXI. At this occasion, an existing sideband signal such as a parameter AWUSER may be used, or alternatively, a sideband signal may be newly defined. 
     Furthermore, in making a read request to the memory  50  or the memory  70 , the information processing unit  9  may supply the interconnect unit  30  with a read address RdADD, and receive read data RdDATA from the interconnect unit  30 . 
     The DMA controller  20  may be a controller that controls DMA transfer to the memories  50  and  70 . The identifier MI may be assigned to the DMA controller  20 , as with the blocks of the information processing unit  9 . Specifically, in this example, the identifier MI of the DMA controller  20  may be “0x20” (“00100000” in the binary number notation). The DMA controller  20  may be coupled to the interconnect unit  30  through a bus B 2 , as with the information processing unit  9 . Moreover, in making the write request to the memory  50  or the memory  70 , the DMA controller  20  may supply the interconnect unit  30  with the write address WrADD and the write data WrDATA, and receive the response RE from the interconnect unit  30 , as with the information processing unit  9 . Furthermore, in making the read request to the memory  50  or the memory  70 , the DMA controller  20  may supply the interconnect unit  30  with the read address RdADD, and receive the read data RdDATA from the interconnect unit  30 , as with the information processing unit  9 . 
     In the figure, the information processing unit  9  and the DMA controller  20  may be coupled to the interconnect unit  30 , but this is non-limiting. In one alternative, another device may be coupled. In this case, in one desirable example, the identifier MI may be assigned to the device. The identifier MI of the device may be different from the identifiers MI of the information processing unit  9  (the cash  10  and the two cores  11  and  12 ) and the DMA controller  20 . For example, another DMA controller  120  may be coupled that has substantially same transfer performance as that of the DMA controller  20 . In this case, for example, the identifier MI of the DMA controller  120  may be “0x21” (“00100001” in the binary number notation). In other words, in this example, the upper-order four bits of the identifiers MI may be the same, in consideration that the data transfer performance of the DMA controllers  20  and  120  is substantially the same. 
     The interconnect unit  30  may arbitrate the access to the memories  50  and  70  by the information processing unit  9  and the DMA controller  20 . The interconnect unit  30  may be coupled to the information processing unit  9  through the bus B 1 . The interconnect unit  30  may be coupled to the DMA controller  20  through the bus B 2 . The interconnect unit  30  may be coupled to the memory controller  40  through a bus B 11 . The interconnect unit  30  may be coupled to the memory controller  60  through a bus B 12 . 
     When the information processing unit  9  or the DMA controller  20  makes the write request to the memory  50 , the interconnect unit  30  may transfer, to the memory controller  40 , the write address WrADD and the write data WrDATA supplied from the device that has made the write request. The interconnect unit  30  may transfer, to the device that has made the write request, the response signal RE supplied from the memory controller  40 . Moreover, when the information processing unit  9  or the DMA controller  20  makes the read request to the memory  50 , the interconnect unit  30  may transfer, to the memory controller  40 , the read address RdADD supplied from the device that has made the read request. The interconnect unit  30  may transfer, to the device that has made the read request, the read data RdDATA supplied from the memory controller  40 . 
     Similarly, when the information processing unit  9  or the DMA controller  20  makes the write request to the memory  70 , the interconnect unit  30  may transfer, to the memory controller  60 , the write address WrADD and the write data WrDATA supplied from the device that has made the write request. The interconnect unit  30  may transfer, to the device that has made the write request, the response signal RE supplied from the memory controller  60 . Moreover, when the information processing unit  9  or the DMA controller  20  makes the read request to the memory  70 , the interconnect unit  30  may transfer, to the memory controller  60 , the read address RdADD supplied from the device that has made the read request. The interconnect unit  30  may transfer, to the device that has made the read request, the read data RdDATA supplied from the memory controller  60 . 
     The memory controller  40  may control operation of the memory  50 . The memory controller  40  may be coupled to the interconnect unit  30  through the bus B 11 . The memory controller  40  may generate a control command COM, on the basis of the write address WrADD and the read address RdADD supplied from the interconnect unit  30 . The memory controller  40  may supply the control command COM to the memory  50 . Moreover, the memory controller  40  may generate an address ADD, on the basis of the write address WrADD and the read address RdADD supplied from the interconnect unit  30 . The memory controller  40  may supply the address ADD to the memory  50 . Furthermore, the memory controller  40  may supply the memory  50  with the write data WrDATA supplied from the interconnect unit  30 , as data DATA. The memory controller  40  may supply the interconnect unit  30  with the data DATA supplied from the memory  50 , as the read data RdDATA. In performing data writing to the memory  50 , the memory controller  40  may allow a write buffer  44  incorporated (described later) to temporarily store the write address WrADD and the write data WrDATA. Thereafter, the memory controller  40  may perform the data writing to the memory  50 , on the basis of information stored in the write buffer  44 . 
     Further, the memory controller  40  may also have a function of generating the response signal RE. At this occasion, the memory controller  40  may decide timing of generation of the response signal RE, on the basis of the identifier MI and the transfer mode information MODE. In one specific example, as described later, when the information processing unit  9  or the DMA controller  20  makes the write request by the non-posted transfer (MODE=0) to the memory  50 , the memory controller  40  may generate the response signal RE at timing of an end of the data writing to the memory  50 . The memory controller  40  may supply the response signal RE to the device that has made the write request, through the interconnect unit  30 . In other words, in this case, the non-posted transfer may be performed. Moreover, when the information processing unit  9  or the DMA controller  20  makes the write request by the posted transfer (MODE=1) to the memory  50 , the memory controller  40  may select a transfer scheme, on the basis of the identifier MI. Specifically, when the information processing unit  9  (the cash  10  and the cores  11  and  12 ) makes the write request, the memory controller  40  may generate the response signal RE at the timing of the end of the data writing to the memory  50 . The memory controller  40  may supply the response signal RE to the device that has made the write request, through the interconnect unit  30 . In other words, in this case, the non-posted transfer may be performed. Moreover, when the DMA controller  20  makes the write request, the memory controller  40  may generate the response signal RE at timing at which the data writing has been performed to the write buffer  44  incorporated (described later). The memory controller  40  may supply the response signal RE to the device that has made the write request, through the interconnect unit  30 . In other words, in this case, the posted transfer may be performed. 
     As described, in the memory controller  40 , when the information processing unit  9  makes the write request, the response signal RE may be generated at the timing of the end of the data writing to the memory  50 . This makes it possible to suppress a coherency-related disadvantage. Specifically, if the information processing unit  9  makes the write request to a certain address of the memory  50 , and immediately thereafter, makes the read request, there is possibility that data before re-writing may be read. In contrast, in the memory controller  40 , when the information processing unit  9  makes the write request, the response signal RE may be generated at the timing of the end of the data writing to the memory  50 . This allows for reading of data after re-writing, even if the information processing unit  9  makes the read request immediately after the write request to the same address as that of the write request. Hence, it is possible to suppress the coherency-related disadvantage. 
     The memory controller  60  may control operation of the memory  70 , as with the memory controller  40 . The memory controller  60  may be coupled to the interconnect unit  30  through the bus B 12 . Moreover, in performing the data writing to the memory  70 , as with the memory controller  40 , the memory controller  60  may allow a write buffer  64  incorporated (described later) to temporarily store the write address WrADD and the write data WrDATA. Thereafter, the memory controller  60  may perform the data writing to the memory  70 , on the basis of information stored in the write buffer  64 . Unlike the memory controller  40 , the memory controller  60  may have a snoop mechanism. In one specific example, upon receipt of the read request for data stored in the write buffer  64 , the memory controller  60  may read the data from the write buffer  64 . The memory controller  60  may output the data thus read, as the read data RdDATA. Thus, reading the data from the write buffer  64 , instead of reading data from the memory  70 , makes it possible to suppress the coherency-related disadvantage. 
     Further, the memory controller  60  may also have the function of generating the response signal RE, as with the memory controller  40 . At this occasion, the memory controller  60  may decide the timing of the generation of the response signal RE, on the basis of the transfer mode information MODE. In one specific example, as described later, when the information processing unit  9  or the DMA controller  20  makes the write request by the non-posted transfer (MODE=0) to the memory  70 , the memory controller  60  may generate the response signal RE at the timing of the end of the data writing to the memory  70 . The memory controller  60  may supply the response signal RE to the device that has made the write request, through the interconnect unit  30 . In other words, in this case, the non-posted transfer may be performed. Moreover, when the information processing unit  9  or the DMA controller  20  makes the write request by the posted transfer (MODE=1) to the memory  70 , the memory controller  60  may generate the response signal RE at the timing at which the data writing has been performed to the write buffer  64  incorporated (described later). The memory controller  60  may supply the response signal RE to the device that has made the write request, through the interconnect unit  30 . In other words, in this case, the posted transfer may be performed. 
     The memories  50  and  70  may store data, and serve as a workspace memory of the information processing unit  9 . The memories  50  and  70  may each include, for example, DRAM (Dynamic Random Access Memory). The memory  50  may receive the control command COM and the address ADD from the memory controller  40 , and supply or receive the data DATA to or from the memory controller  40 . Similarly, the memory  70  may receive the control command COM and the address ADD from the memory controller  60 , and supply or receive the data DATA to or from the memory controller  60 . 
     [Memory Controller  40 ] 
       FIG. 2  illustrates one example of a configuration of the memory controller  40 . The memory controller  40  may include a write interface  41 , a read interface  48 , and a controller  49 . 
     The write interface  41  may be an interface related to write access. In one specific example, the write interface  41  may temporarily store the write address WrADD and the write data WrDATA supplied from the interconnect unit  30 , and transfer the write address WrADD and the write data WrDATA to the controller  49 . Moreover, the write interface  41  may have the function of generating the response signal RE on the basis of the write address WrADD. The write interface  41  may include the write buffer  44 , a response controller  80  and a response signal generator  43 . 
     The write buffer  44  may be a buffer memory that temporarily stores the write address WrADD and the write data WrDATA. Moreover, the write buffer  44  may supply the controller  49  with the write address WrADD and the write data WrDATA stored, on the basis of an instruction from the controller  49 . 
     The response controller  80  may generate a parameter PW, on the basis of the write address WrADD. 
       FIG. 3  illustrates one example of a configuration of the response controller  80 . The response controller  80  may include an identifier acquiring unit  81 , a transfer mode information acquiring unit  85 , a register  82 , a logical AND operating unit  83 , a comparison unit  84 , and a logical AND circuit  86 . 
     The identifier acquiring unit  81  may acquire the identifier MI from the write address WrADD. The transfer mode information acquiring unit  85  may acquire the transfer mode information MODE from the write address WrADD. 
     The register  82  may store parameters MIMask and MIMatch. The parameters MImask and MIMatch may each be an 8-bit parameter. In the memory controller  40 , the parameter MImask may be set as “0x10” in the hexadecimal number notation (“00010000” in the binary number notation), while the parameter MIMatch may be set as “0x10”. 
     The logical AND circuit  83  may obtain a logical AND of a value of each bit of the identifier MI and a value of a corresponding bit of the parameter MIMask (in this example, “0x10”), to generate an 8-bit parameter. In other words, in this example, the logical AND circuit  83  may mask the bits other than the bit b 4  of the identifier MI. In one specific example, the logical AND circuit  83  may generate a parameter, a value of a bit b 4  of which is a value of the bit b 4  of the identifier MI, and values of bits b 0  to b 3  and b 5  to b 7  of which are “0”. The logical AND circuit  83  may output the parameter thus generated. 
     The comparison unit  84  may compare the parameter outputted by the logical AND circuit  83  with the 8-bit parameter MIMatch (in this example, “0x10”). The comparison unit  84  may output a comparison result as a parameter DPW. In one specific example, the comparison unit  84  may allow the parameter DPW to be “1” upon coincidence of these parameters. The comparison unit  84  may allow the parameter DPW to be “0” upon non-coincidence of these parameters. 
       FIG. 4  illustrates relation between the identifier MI and the parameter DPW. In the memory controller  40 , the parameter DPW may be “1” when the information processing unit  9  makes the write request, whereas the parameter DPW may be “0” when the DMA controller  20  makes the write request. In other words, when the information processing unit  9  makes the write request, the parameter DPW may be “1” because the bit b 4  of the identifier MI of the information processing unit  9  is “1”. When the DMA controller  20  makes the write request, the parameter DPW may be “0” because the bit b 4  of the identifier MI of the DMA controller  20  is “0”. Thus, in the response controller  80 , the parameter DPW may be generated on the basis of the value of the bit b 4  of the identifier MI. 
     The logical AND circuit  86  may obtain a logical AND of a reverse value of the parameter DPW and the value of the transfer mode information MODE. The logical AND circuit  86  may output a result thus obtained, as the parameter PW. The parameter PW having a value of “1” may indicate the posted transfer. The parameter PW having the value of “0” may indicate the non-posted transfer. 
     With this configuration, the response controller  80  may allow the parameter PW to be “0”, because the parameter DPW is “1” as summarized in  FIG. 4 , when the information processing unit  9  makes the write request. Moreover, the response controller  80  may output the value of the transfer mode information MODE as the parameter PW, because the parameter DPW is “0” as summarized in  FIG. 4 , when the DMA controller  20  makes the write request. 
     The response signal generator  43  may generate the response signal RE, on the basis of the parameter PW and a control signal supplied from the controller  49 . In one specific example, upon receipt of the parameter PW having the value of “1” from the response controller  80 , the response signal generator  43  may generate the response signal RE at timing of the receipt. In other words, the response signal generator  43  may generate the response signal RE at the timing at which the data writing has been performed to the write buffer  44 . That is, in this case, the posted transfer may be performed. Moreover, upon the receipt of the parameter PW having the value of “0”, the response signal generator  43  may generate the response signal RE at timing of receipt, from the controller  49 , of a control signal that indicates the end of the data writing to the memory  50 . That is, in this case, the non-posted transfer may be performed. 
     The read interface  48  may be an interface related to read access. The read interface  48  may perform predetermined processing on the basis of the read address RdADD supplied from the interconnect unit  30 , and transfer the read address RdADD to the controller  49 . The read interface  48  may perform predetermined processing on the basis of the read data RdDATA supplied from the controller  49 , and transfer the read data RdDATA to the interconnect unit  30 . 
     The controller  49  may control the operation of the memory  50  on the basis of instructions from the write interface  41  and the read interface  48 . In one specific example, the controller  49  may generate the control command COM such as a write command and a read command, on the basis of the write address WrADD and the read address RdADD. The controller  49  may supply the control command COM to the memory  50 . Moreover, the controller  49  may generate the address ADD, on the basis of the write address WrADD and the read address RdADD. The controller  49  may supply the address ADD to the memory  50 . Further, the controller  49  may supply the memory  50  with the write data WrDATA supplied from the write interface  41 , as the data DATA. Furthermore, the controller  49  may supply the read interface  48  with the data DATA supplied from the memory  50 , as the read data RdDATA. 
     [Memory Controller  60 ] 
       FIG. 5  illustrates one example of a configuration of the memory controller  60 . The memory controller  60  may include a read interface  68 , a write interface  61 , and a controller  69 . 
     The read interface  68  may be an interface related to read access, as with the read interface  48 . Unlike the read interface  48 , the read interface  68  may have the snoop mechanism. In one specific example, upon receipt of the read request for data stored in the write buffer  64  (described later) of the write interface  61 , the read interface  68  may read the data from the write buffer  64 . The read interface  68  may output the data thus read, as the read data RdDATA. Hence, in the memory controller  60  and the memory  70 , it is possible to suppress the coherency-related disadvantage. 
     The write interface  61  may be an interface related to write access, as with the write interface  41 . The write interface  61  may include the write buffer  64 , a response controller  90 , and a response signal generator  63 . 
     The write buffer  64  may be a buffer memory that temporarily stores the write address WrADD and the write data WrDATA, as with the write buffer  44 . Moreover, the write buffer  64  may supply the read interface  68  with the write data WrDATA stored in the write buffer  64 , on the basis of an instruction from the read interface  68 . 
     The response controller  90  may generate the parameter PW on the basis of the write address WrADD, as with the response controller  80 . A configuration of the response controller  90  may be same as that of the response controller  80  ( FIG. 3 ). Here, in the response controller  90 , the parameter MIMask may be set as “0x00”. Accordingly, in the memory controller  60 , the parameter DPW may be “0”, regardless of whether the information processing unit  9  or the DMA controller  20  makes the write request to the memory  70 , as summarized in  FIG. 4 . Therefore, the response controller  90  may output the value of the transfer mode information MODE, as the parameter PW. 
     The response signal generator  63  may generate the response signal RE, on the basis of the parameter PW and a control signal supplied from the controller  69 , as with the response signal generator  43 . The controller  69  may control the operation of the memory  70 , on the basis of instructions from the write interface  61  and the read interface  68 , as with the controller  49 . 
     Here, the cash  10 , the cores  11  and  12 , and the DMA controller  20  each correspond to one specific example of a “master device” in the disclosure. The memory controller  40  corresponds to one specific example of a “slave device” in the disclosure. The memory  50  corresponds to one specific example of a “device to be accessed” in the disclosure. The write request corresponds to one specific example of a “request for access” in the disclosure. The transfer mode information MODE corresponds to one specific example of “request information” in the disclosure. 
     [Operation and Workings] 
     Description is given next of operation and workings of the bus system  1  according to the embodiment. 
     [Outline of Overall Operation] 
     First, an outline of overall operation of the bus system  1  is described with reference to  FIG. 1  and other figures. The information processing unit  9  may perform the operation processing. The DMA controller  20  may control the DMA transfer. The interconnect unit  30  may arbitrate the access to the memories  50  and  70  by the information processing unit  9  and the DMA controller  20 . The memory controller  40  may control the operation of the memory  50 , and generate the response signal RE. At this occasion, the memory controller  40  may decide the timing of the generation of the response signal RE, on the basis of the identifier MI and the transfer mode information MODE. The memory controller  60  may control the operation of the memory  70 , and generate the response signal RE. At this occasion, the memory controller  60  may decide the timing of the generation of the response signal RE, on the basis of the transfer mode information MODE. The memories  50  and  70  may store the data. 
     Next, detailed operation of the bus system  1  is described with reference to three operation examples. 
     Operation Example 1 
     First, an operation example 1 is described in which the cash  10  of the information processing unit  9  makes the write request by the posted transfer to the memory  50 . 
       FIG. 6  illustrates the operation example 1 of the bus system  1 . In  FIG. 6 , a signal denoted by a thick solid line denotes a signal that becomes active prior to the timing of the generation of the response signal RE. A signal denoted by a thick broken line denotes a signal that becomes active after the timing of the generation of the response signal RE. 
     First, the cash  10  of the information processing unit  9  may make the write request by the posted transfer to the memory  50 . In one specific example, the information processing unit  9  may generate the write address WrADD and the write data WrDATA. The information processing unit  9  may supply the write address WrADD and the write data WrDATA to the interconnect unit  30 . The write address WrADD may include the identifier MI of the cash  10  (“0x10”) and the transfer mode information MODE having the value of “1” (MODE=1). In other words, because the cash  10  requests for the posted transfer, the transfer mode information MODE may be set as “1”. Moreover, the interconnect unit  30  may transfer, to the memory controller  40 , the write address WrADD and the write data WrDATA supplied from the information processing unit  9 . Further, the memory controller  40  may receive the write address WrADD and the write data WrDATA from the interconnect unit  30 . The memory controller  40  may allow the write buffer  44  to temporarily store the write address WrADD and the write data WrDATA. Thereafter, the memory controller  40  may perform the data writing to the memory  50 , on the basis of the information stored in the write buffer  44 . 
     The response controller  80  of the memory controller  40  may generate the parameter PW having the value of “0”, on the basis of the write address WrADD. In one specific example, because the identifier MI included in the write address WrADD is “0x10”, the parameter DPW may be “1”, and the parameter PW may be “0”. Accordingly, the response signal generator  43  may generate the response signal RE at the timing of the end of the data writing to the memory  50 . Thus, in the bus system  1 , the non-posted transfer may be performed. 
     In some cases, the information processing unit  9  may make the write request by the posted transfer, and thereafter, make the read request to the address to which the write request has been made. To be specific, there may be such a case in performing, for example, so-called eviction operation. For example, in performing the eviction operation, with the information processing unit  9  constituted with use of the processor Cortex-A9 MPCore and the level 2 cash controller L2C-310 available from ARM Ltd., the transfer mode information MODE (AWCACHE [0]) may be set as “1”. In other words, in this case, the information processing unit  9  may make the write request by the posted transfer. In such a case, as described above, the memory controller  40  may determine that the non-posted transfer ought to be performed, and generate the response signal RE at the timing of the end of the data writing to the memory  50 . This makes it possible to reduce the possibility that old data before re-writing be read from the memory  50 , in the read access after the write access. Hence, it is possible to suppress the coherency-related disadvantage. 
     As described, in the bus system  1 , the memory controller  40  may determine that the non-posted transfer ought to be performed, when the information processing unit  9  makes the write request by the posted transfer to the memory  50 . Hence, in the bus system  1 , it is possible to suppress the coherency-related disadvantage. 
     Operation Example 2 
     Next, an operation example 2 is described in which the DMA controller  20  makes the write request by the posted transfer to the memory  50 . 
       FIG. 7  illustrates the operation example 2 of the bus system  1 . First, the DMA controller  20  may make the write request by the posted transfer to the memory  50 . In one specific example, the DMA controller  20  may generate the write address WrADD and the write data WrDATA. The DMA controller  20  may supply the write address WrADD and the write data WrDATA to the interconnect unit  30 . The write address WrADD may include the identifier MI of the DMA controller  20  (“0x20”) and the transfer mode information MODE having the value of “1” (MODE=1). The memory controller  40  may receive the write address WrADD and the write data WrDATA, through the interconnect unit  30 . Moreover, the memory controller  40  may allow the write buffer  44  to temporarily store the write address WrADD and the write data WrDATA. 
     The response controller  80  of the memory controller  40  may generate the parameter PW having the value of “1”, on the basis of the write address WrADD. In one specific example, because the identifier MI included in the write address WrADD is “0x20”, the parameter DPW may become “0”. Accordingly, the parameter PW may become “1”, i.e., the same value as the value of the transfer mode information MODE. The response signal generator  43  may, therefore, generate the response signal RE, at the timing at which the data writing has been performed to the write buffer  44 . Moreover, the memory controller  40  may perform the data writing to the memory  50 , on the basis of the information stored in the write buffer  44 . Thus, in the bus system  1 , the posted transfer may be performed. 
     The DMA controller  20  may often make a series of write transfer through a plurality of write requests. In what follows, description is made with reference to an exemplary case in which 1 kilobyte (=4×256 bytes) transfer is performed through four write requests. In such a case, for example, the DMA controller  20  may make the write request by the posted transfer three times, and thereafter, make the write request by the non-posted transfer once. At this occasion, as requested, the memory controller  40  may determine that the posted transfer ought to be performed with respect to first three write requests, and determine that the non-posted transfer ought to be performed with respect to the one write request afterward. Accordingly, in the bus system  1 , it is possible to make responses, at earlier timing, to the first three write requests. This leads to enhancement in transfer performance. Moreover, with respect to the one write request afterward, the response signal RE may be generated at the timing of the end of the data writing to the memory  50 . Hence, it is possible to suppress the coherency-related disadvantage. 
     As described, in the bus system  1 , when the DMA controller  20  makes the write request to the memory  50 , the memory controller  40  may perform the posted transfer or the non-posted transfer, as requested. In other words, because the DMA controller  20  makes the write requests meticulously, the memory controller  40  may grant the write requests. Hence, in the bus system  1 , it is possible to suppress the coherency-related disadvantage, and to enhance the transfer performance. 
     Operation Example 3 
     Next, an operation example 3 is described in which the cash  10  of the information processing unit  9  makes the write request by the posted transfer to the memory  70 . 
       FIG. 8  illustrates the operation example 3 of the bus system  1 . First, the cash  10  of the information processing unit  9  may make the write request by the posted transfer to the memory  70 . In one specific example, the information processing unit  9  may generate the write address WrADD and the write data WrDATA. The information processing unit  9  may supply the write address WrADD and the write data WrDATA to the interconnect unit  30 . The write address WrADD may include the identifier MI of the cash  10  (“0x10”) and the transfer mode information MODE having the value of “1” (MODE=1). The memory controller  60  may receive the write address WrADD and the write data WrDATA, through the interconnect unit  30 . Moreover, the memory controller  60  may allow the write buffer  64  to temporarily store the write address WrADD and the write data WrDATA. 
     The response controller  90  of the memory controller  60  may generate the parameter PW having the value of “1”, on the basis of the write address WrADD. In one specific example, in the memory controller  60 , because the parameter DPW is “0” ( FIG. 4 ), the parameter PW may become “1”, i.e., the same value as the value of the transfer mode information MODE. Accordingly, the response signal generator  43  may generate the response signal RE at the timing at which the data writing has been performed to the write buffer  64 . Moreover, the memory controller  60  may perform the data writing to the memory  70 , on the basis of the information stored in the buffer memory  64 . Thus, in the bus system  1 , the posted transfer may be performed. Furthermore, thereafter, when the memory controller  60  receives the read request for the data stored in the write buffer  64 , the memory controller  60  may read the data from the write buffer  64 . The memory controller  60  may output the data thus read, as the read data RdDATA. 
     As described, in the bus system  1 , when the information processing unit  9  makes the write request by the posted transfer to the memory  70 , the memory controller  60  may determine that the posted transfer ought to be performed, as requested. Accordingly, in the bus system  1 , it is possible to make the response at the earlier timing, leading to the enhancement in the transfer performance. Hence, in the bus system  1 , it is possible to increase a processing speed in the whole system. In particular, in the posted transfer, it is possible to effectively utilize the snoop mechanism of the memory controller  60 . Hence, it is possible to suppress the coherency-related disadvantage, and to enhance the transfer performance. 
     Next, description is given on workings of the embodiment, in comparison with some comparative examples. 
     Comparative Example 1 
       FIG. 9  illustrates one example of a bus system  1 R according to a comparative example 1. The bus system  1 R may include an information processing unit  9 R, a DMA controller  20 R, the interconnect unit  30 , a memory controller  40 R, and a memory  50 . The information processing unit  9 R and the DMA controller  20 R may be devoid of the assignment of the identifiers MI. The memory controller  40 R may have the snoop mechanism. Moreover, the memory controller  40 R may decide the timing of the generation of the response signal RE, on the basis of the transfer mode information MODE. 
     In the example in  FIG. 9 , the information processing unit  9 R may make the write request by the posted transfer to the memory  50 . In one specific example, first, the information processing unit  9 R may generate the write address WrADD and the write data WrDATA. The information processing unit  9 R may supply the write address WrADD and the write data WrDATA to the interconnect unit  30 . The write address WrADD may include the transfer mode information MODE having the value of “1” (MODE=1). Moreover, the memory controller  40  may receive the write address WrADD and the write data WrDATA, through the interconnect unit  30 . The memory controller  40 R may allow the write buffer incorporated to temporarily store the write address WrADD and the write data WrDATA. 
     The memory controller  40 R may generate the response signal RE at the timing at which the data writing has been performed to the write buffer incorporated, because the value of the transfer mode information MODE is “1”. Moreover, the memory controller  40 R may perform the data writing to the memory  50 , on the basis of the information stored in the write buffer. Thus, in the bus system  1 R, the posted transfer may be performed. Moreover, thereafter, when the memory controller  40 R receives the read request for the data stored in the write buffer, the memory controller  40 R may read the data from the write buffer. The memory controller  40 R may output the data thus read, as the read data RdDATA. 
     In the bus system  1 R according to the comparative example 1, the memory controller  40 R may include the snoop mechanism. This makes it possible to suppress the coherency-related disadvantage even in performing the posted transfer. However, the snoop mechanism may include a large scale of circuit, causing possibility of an increase in costs. 
     In contrast, in the bus system  1  according to the embodiment, when the information processing unit  9  makes the write request by the posted transfer, the memory controller  40  may determine that the non-posted transfer ought to be performed. This makes it possible to suppress the coherency-related disadvantage without providing the snoop mechanism. Hence, it is possible to decrease a circuit scale, and suppress the increase in the costs. 
     Comparative Example 2 
       FIGS. 10 and 11  illustrate one example of a bus system  1 S according to a comparative example 2. The bus system  15  may include the information processing unit  9 R, the DMA controller  20 R, the interconnect unit  30 , a memory controller  40 S, and the memory  50 . The memory controller  40 S may be devoid of the snoop mechanism. Moreover, the memory controller  40 S may constantly determine that the non-posted transfer ought to be performed, regardless of the transfer mode information MODE received. 
     In the example in  FIG. 10 , the information processing unit  9 R may make the write request by the posted transfer to the memory  50 . In one specific example, first, the information processing unit  9 R may generate the write address WrADD and the write data WrDATA. The information processing unit  9 R may supply the write address WrADD and the write data WrDATA to the interconnect unit  30 . The write address WrADD may include the transfer mode information MODE having the value of “1” (MODE=1). Moreover, the memory controller  40 S may receive the write address WrADD and the write data WrDATA, through the interconnect unit  30 . The memory controller  40 S may allow the write buffer incorporated to temporarily store the write address WrADD and the write data WrDATA. Thereafter, the memory controller  40 S may perform the data writing to the memory  50 , on the basis of the information stored in the write buffer. The memory controller  40 S may generate the response signal RE at the timing of the end of the data writing to the memory  50 . Thus, in the bus system  1 S, the non-posted transfer may be performed. 
     In the example in  FIG. 11 , the DMA controller  20 R may make the write request by the posted transfer to the memory  50 . In one specific example, first, the DMA controller  20 R may generate the write address WrADD and the write data WrDATA. The DMA controller  20 R may supply the write address WrADD and the write data WrDATA to the interconnect unit  30 . The write address WrADD may include the transfer mode information MODE having the value of “1” (MODE=1). Moreover, the memory controller  40 S may receive the write address WrADD and the write data WrDATA, through the interconnect unit  30 . The memory controller  40 S may allow the write buffer incorporated to temporarily store the write address WrADD and the write data WrDATA. Thereafter, the memory controller  40 S may perform the data writing to the memory  50 , on the basis of the information stored in the write buffer. The memory controller  40 S may generate the response signal RE at the timing of the end of the data writing to the memory  50 . Thus, in the bus system  15 , the non-posted transfer may be performed. 
     In the bus system  1 S according to the comparative example 2, the memory controller  40 S may constantly determine that the non-posted transfer ought to be performed, regardless of the transfer mode information MODE received. This makes it possible to suppress the coherency-related disadvantage. However, the non-posted transfer may be performed even when the DMA controller  20 R makes the write request by the posted transfer. This leads to possibility of a lowered processing speed in the whole system. In other words, there is low possibility that the DMA controller  20 R makes the write request, and immediately thereafter, makes the read request to the same address. Accordingly, making the response at delayed timing in this way causes a lowered transfer speed. This results in possibility of the lowered processing speed in the whole system. 
     In contrast, in the bus system  1  according to the embodiment, the memory controller  40  may determine, on the basis of the identifier MI, whether the posted transfer or the non-posted transfer ought to be performed. Accordingly, for example, the non-posted transfer may be performed when the information processing unit  9  makes the write request by the posted transfer, whereas the posted transfer may be performed when the DMA controller  20  makes the write request by the posted transfer. Thus, in the bus system  1 , different types of transfer may be performed depending on the device that makes the write request. Hence, it is possible to enhance the transfer performance while suppressing the coherency-related disadvantage. 
     Comparative Example 3 
       FIGS. 12 and 13  illustrate one example of a bus system  1 T according to a comparative example 3. The bus system  1 T may include an information processing unit  9 T, the DMA controller  20 R, the interconnect unit  30 , memory controllers  40 T and  60 T, and the memories  50  and  70 . The information processing unit  9 T may constantly make the write request by the non-posted transfer to the memories  50  and  70 . The memory controller  40 T may be devoid of the snoop mechanism. The memory controller  60 T may include the snoop mechanism. The memory controllers  40 T and  60 T may decide the timing of the generation of the response signal RE, on the basis of the transfer mode information MODE. 
     In the example in  FIG. 12 , the information processing unit  9 T may make the write request by the non-posted transfer to the memory  50 . In one specific example, first, the information processing unit  9 T may generate the write address WrADD and the write data WrDATA. The information processing unit  9 T may supply the write address WrADD and the write data WrDATA to the interconnect unit  30 . The write address WrADD may include the transfer mode information MODE having the value of “0” (MODE=0). Moreover, the memory controller  40 T may receive the write address WrADD and the write data WrDATA, through the interconnect unit  30 . The memory controller  40 T may allow the write buffer incorporated to temporarily store the write address WrADD and the write data WrDATA. Thereafter, the memory controller  40 T may perform the data writing, on the basis of the information stored in the write buffer. Furthermore, the memory controller  40 T may generate the response signal RE at the timing of the end of the data writing to the memory  50 . Thus, in the bus system  1 T, the non-posted transfer may be performed. 
     In the example in  FIG. 13 , the information processing unit  9 T may make the write request by the non-posted transfer to the memory  70 . In one specific example, first, the information processing unit  9 T may generate the write address WrADD and the write data WrDATA. The information processing unit  9 T may supply the write address WrADD and the write data WrDATA to the interconnect unit  30 . The write address WrADD may include the transfer mode information MODE having the value of “0” (MODE=0). Moreover, the memory controller  60 T may receive the write address WrADD and the write data WrDATA, through the interconnect unit  30 . The memory controller  60 T may allow the write buffer incorporated to temporarily store the write address WrADD and the write data WrDATA. Thereafter, the memory controller  60 T may perform the data writing to the memory  70 , on the basis of the information stored in the write buffer. Furthermore, the memory controller  60 T may generate the response signal RE at the timing of the end of the data writing to the memory  70 . Thus, in the bus system  1 T, the non-posted transfer may be performed. Moreover, thereafter, when the memory controller  60 T receives the read request for the data stored in the write buffer, the memory controller  60 T may read the data from the write buffer. The memory controller  60 T may output the data thus read, as the read data RdDATA. 
     In the bus system  1 T according to the comparative example 3, the information processing unit  9 T may constantly make the write request by the non-posted transfer. This makes it possible to suppress the coherency-related disadvantage. However, constantly performing the non-posted transfer in spite of the snoop mechanism of the memory controller  60 T keeps the snoop mechanism from working effectively and exhibiting its performance. 
     In contrast, in the bus system  1  according to the embodiment, the memory controllers  40  and  60  may each determine independently whether the posted transfer or the non-posted transfer ought to be performed. Accordingly, for example, when the information processing unit  9  makes the write request by the posted transfer, the memory controller  40  devoid of the snoop mechanism may select the non-posted transfer, allowing for the suppression of the coherency-related disadvantage. Meanwhile, the memory controller  60  including the snoop mechanism may select the posted transfer, allowing for the enhancement of the transfer performance. Hence, it is possible to suppress the coherency-related disadvantage, and to enhance the transfer performance, even in a case of a mixture of the device devoid of the snoop mechanism and the device including the snoop mechanism. 
     [Effects] 
     As described, in this embodiment, determination may be made, on the basis of the identifier, on whether posted transfer or non-posted transfer ought to be performed. Accordingly, for example, when an information processing unit makes a write request by the posted transfer, selecting the non-posted transfer makes it possible to suppress a coherency-related disadvantage. When a DMA controller makes the write request by the posted transfer, selecting the posted transfer makes it possible to enhance transfer performance. 
     In this embodiment, a plurality of memory controllers may each determine independently whether the posted transfer or the non-posted transfer ought to be performed. Accordingly, for example, a memory controller devoid of a snoop mechanism may select the non-posted transfer, allowing for the suppression of the coherency-related disadvantage. A memory controller including the snoop mechanism may select the posted transfer, allowing for the enhancement in the transfer performance. 
     Modification Example 1 
     In the forgoing embodiment, in the response controller  80 , the pair of parameters MIMask and MIMatch may be used, but this is non-limiting. Instead, a plurality of pairs of parameters MIMask and MIMatch may be used.  FIG. 14  illustrates an exemplary case with use of two pairs of parameters MIMask and MIMatch. A response controller  80 A may include a register  82 A, logical AND operating units  831  and  832 , comparison units  841  and  842 , and a logical OR circuit  87 A. The register  82 A may store parameters MIMask 1  and MIMatch 1 , and parameters MIMask 2  and MIMatch 2 . The logical AND operating unit  831  may obtain a logical AND of a value of each bit of the identifier MI and a value of a corresponding bit of the parameter MIMask 1 , to generate an 8-bit parameter. The comparison unit  841  may compare the parameter outputted by the logical AND operating unit  831  with the 8-bit parameter MIMatch 1 . The comparison unit  841  may output a comparison result. The logical AND operating unit  832  may obtain a logical AND of a value of each bit of the identifier MI and a value of a corresponding bit of the parameter MIMask 2 , to generate an 8-bit parameter. The comparison unit  842  may compare the parameter outputted by the logical AND operating unit  832  with the 8-bit parameter MIMatch 2 . The comparison unit  842  may output a comparison result. The logical OR circuit  87 A may obtain a logical OR of the comparison result in the comparison unit  841  and the comparison result in the comparison unit  842 . The logical OR circuit  87 A may output the logical OR thus obtained, as the parameter DPW. Thus, using the plurality of pairs of parameters MIMask and MIMatch makes it possible to control the write access with a higher degree of freedom. 
     Modification Example 2 
     In the forgoing embodiment, the memory controller  40  may identify the device that has made the write request, on the basis of the identifier MI, but this is non-limiting. In the following, this modification example is described in details. 
       FIGS. 15 and 16  illustrate one example of a bus system  1 B according to this modification example. The bus system  1 B may include an information processing unit  9 B, a DMA controller  20 B, the interconnect unit  30 , memory controllers  40 B and  60 B, and the memories  50  and  70 .  FIG. 17  illustrates one example of a configuration of a response controller  80 B related to the memory controllers  40 B and  60 B. 
     The information processing unit  9 B and the DMA controller  20 B may assign a transfer identifier ID, for each transfer (access). In this example, the transfer identifier ID assigned by the information processing unit  9 B may range from 0 to 127 both inclusive. The transfer identifier ID assigned by the DMA controller  20 B may range from 128 to 255 both inclusive. Moreover, the information processing unit  9 B and the DMA controller  20 B may generate the write address WrADD including the transfer identifier ID. The memory controllers  40 B and  60 B are able to identify, with use of the transfer identifier ID included in the write address WrADD, the device that has made the write request or the read request. In other words, in the forgoing embodiment, the memory controllers  40  and  60  may identify, with use of the identifier MI, the device that has made the write request or the read request. Meanwhile, the memory controllers  40 B and  60 B may identify, with use of the transfer identifier ID, the device that has made the write request or the read request. 
     The response controller  80 B may include a transfer identifier acquiring unit  81 B and a register  82 B. The transfer identifier acquiring unit  81 B may acquire the transfer identifier ID from the write address WrADD. The register  82 B may store parameters IDMask and IDMatch. The parameter IDMask may be set as “0x80” in the hexadecimal number notation (“10000000” in the binary number notation). The parameter IDMatch may be set as “0x00”. 
     In the example in  FIG. 15 , the information processing unit  9 B may make the write request by the posted transfer to the memory  50 . In one specific example, first, the information processing unit  9 B may generate the write address WrADD and the write data WrDATA. The information processing unit  9 B may supply the write address WrADD and the write data WrDATA to the interconnect unit  30 . The write address WrADD may include the transfer identifier ID that ranges from 0 to 127 both inclusive, and the transfer mode information MODE having the value of “1” (MODE=1). Moreover, the memory controller  40 B may receive the write address WrADD and the write data WrDATA, through the interconnect unit  30 . The memory controller  40 B may allow the write buffer  44  incorporated to temporarily store the write address WrADD and the write data WrDATA. Thereafter, the memory controller  40 B may perform the data writing to the memory  50 , on the basis of the information stored in the write buffer  44 . 
     The response controller  80 B of the memory controller  40 B may generate the parameter PW having the value of “0”, on the basis of the write address WrADD. In other words, because the transfer identifier ID included in the write address WrADD ranges from 0 to 127 both inclusive, the parameter DPW may become “1”, and the parameter PW may become “0”. Accordingly, the memory controller  40 B may generate the response signal RE at the timing of the end of the data writing to the memory  50 . Thus, in the bus system  1 B, the non-posted transfer may be performed. 
     In the example in  FIG. 16 , the DMA controller  20 B may make the write request by the posted transfer to the memory  50 . In one specific example, first, the DMA controller  20 B may generate the write address WrADD and the write data WrDATA. The DMA controller  20 B may supply the write address WrADD and the write data WrDATA to the interconnect unit  30 . The write address WrADD may include the transfer identifier ID that ranges from 128 to 255 both inclusive, and the transfer mode information MODE having the value of “1” (MODE=1). Moreover, the memory controller  40 B may receive the write address WrADD and the write data WrDATA, through the interconnect unit  30 . The memory controller  40 B may allow the write buffer  44  incorporated to temporarily store the write address WrADD and the write data WrDATA. 
     The response controller  80 B of the memory controller  40 B may generate the parameter PW having the value of “1”, on the basis of the write address WrADD. In other words, because the transfer identifier ID included in the write address WrADD ranges from 128 to 255 both inclusive, the parameter DPW may become “0”. Accordingly, the parameter PW may become “1”, i.e., the same value as the value of the transfer mode information MODE. The memory controller  40 B may, therefore, generate the response signal RE at the timing at which the data writing has been performed to the write buffer  44 . Thus, in the bus system  1 B, the posted transfer may be performed. Moreover, the memory controller  40 B may perform the data writing to the memory  50 , on the basis of the information stored in the write buffer  44 . 
     With this configuration as well, it is possible to produce similar effects to those of the forgoing embodiment. It is to be noted that this is non-limiting. For example, memory regions (address regions) may be separately provided for access by an information processing unit and access by a DMA controller. A memory controller may identify the device that has made the write request, on the basis of address information. 
     Modification Example 3 
     In the forgoing embodiment, the information processing unit  9  and the DMA controller  20  may generate the write address WrADD including the identifier MI, but this is non-limiting. Instead, for example, as in a bus system  1 C illustrated in  FIG. 18 , an interconnect unit may generate the write address WrADD including the identifier MI. The bus system  1 C may include an information processing unit  9 C, a DMA controller  20 C, an interconnect unit  30 C, the memory controllers  40  and  60 , and the memories  50  and  70 . The information processing unit  9 C and the DMA controller  20 C may generate the write address WrADD that is devoid of the assignment of the identifier MI. The interconnect unit  30 C may include an identifier generator  31 C and an identifier generator  32 C. The identifier generator  31 C may add the identifier MI of the information processing unit  9 C (e.g., “0x10”) to the write address WrADD supplied from the information processing unit  9 C. The identifier generator  32 C may add the identifier MI of the DMA controller  20 C (e.g., “0x20”) to the write address WrADD supplied from the DMA controller  20 C. With this configuration as well, it is possible to produce similar effects to those of the forgoing embodiment. 
     Modification Example 4 
     In the forgoing embodiment, the memory controllers  40  and  60  may control the timing of the generation of the response signal RE, but this is non-limiting. Instead, for example, an interconnect unit may control the timing of the generation of the response signal RE. In the following, detailed description is given of an interconnect unit  30 D according to this modification example. 
       FIG. 19  illustrates one example of a configuration of the interconnect unit  30 D. The interconnect unit  30 D may include response controllers  33 D and  35 D, response signal generators  34 D and  36 D, and a processor  37 D. The response controller  33 D may generate the parameter PW, on the basis of the write address WrADD supplied from the information processing unit  9 C through the bus B 1 , as with the response controller  80  ( FIG. 3 ). The response signal generator  34 D may generate the response signal RE, on the basis of the parameter PW generated by the response controller  33 D and a control signal supplied from the processor  37 D, as with the response signal generator  43 . The response signal generator  34 D may supply the response signal RE to the information processing unit  9 C through the bus B 1 , as with the response signal generator  43 . The response controller  35 D may generate the parameter PW, on the basis of the write address WrADD supplied from the DMA controller  20  through the bus B 2 , as with the response controller  80  ( FIG. 3 ). The response signal generator  36 D may generate the response signal RE, on the basis of the parameter PW generated by the response controller  35 D and a control signal supplied from the processor  37 D, as with the response signal generator  43 . The response signal generator  36 D may supply the response signal RE to the DMA controller  20  through the bus B 2 . The processor  37 D may arbitrate the access to the memories  50  and  70  by the information processing unit  9  and the DMA controller  20 . With this configuration as well, it is possible to produce similar effects to those of the forgoing embodiment. 
     Although description has been made by giving the embodiment and the modifications as mentioned above, the contents of the technology are not limited to the above-mentioned example embodiments and may be modified in a variety of ways. 
     For example, in the forgoing example embodiments, one interconnect unit may be provided, but this is non-limiting. Instead, as in a bus system  2  illustrated in  FIG. 20 , a plurality of interconnect units (in this example, two interconnect units  131  and  132 ) may be provided. In this example, the information processing unit  9  and the DMA controller  20  may access the memories  50  and  70  through the two interconnect units  131  and  132 . 
     Moreover, for example, in the forgoing example embodiments, the technology is applied to memory access, but this is non-limiting. The technology may have various applications that involve making a response to a request. 
     It is to be noted that effects described herein are merely exemplified and not limitative, and effects of the disclosure may be other effects or may further include other effects. 
     It is to be noted that the technology may have the following configuration. 
     (1) An access control method, including: 
     allowing one master device of a plurality of master devices to generate a request for access to a device to be accessed; 
     allowing a slave device to identify, on a basis of the request for the access, the one master device that has generated the request for the access; and 
     allowing the slave device to make a response to the one master device, at response timing that corresponds to the one master device identified. 
     (2) The access control method according to (1), wherein 
     the response timing is first timing or second timing, the first timing being timing at which the slave device receives the request for the access, and the second timing being timing at which the device to be accessed makes an end of processing based on the request for the access. 
     (3) The access control method according to (2), wherein 
     the request for the access includes request information that indicates a request of the one master device regarding the response timing, and 
     the allowing the slave device to make the response includes allowing the slave device to decide the response timing, on a basis of the one master device identified and the request information. 
     (4) The access control method according to (3), wherein 
     the allowing the slave device to decide the response timing includes allowing the slave device to decide the response timing, on the basis of the one master device identified, on a condition that the request information indicates the first timing. 
     (5) The access control method according to (3) or (4), wherein 
     the allowing the slave device to decide the response timing includes taking the second timing as the response timing, on a condition that the request information indicates the second timing. 
     (6) The access control method according to any one of (2) to (5), wherein 
     the slave device includes a buffer memory, and 
     the first timing is timing at which the request for the access has been stored in the buffer memory. 
     (7) The access control method according to any one of (1) to (6), further including 
     assigning an identifier to each of the plurality of master devices, the identifier being different for each of the plurality of master devices, wherein 
     the allowing the slave device to identify the one master device includes allowing the slave device to identify the one master device on a basis of the identifier. 
     (8) The access control method according to (7), wherein 
     the allowing the one master device to generate the request for the access includes allowing the one master device to perform processing to allow the request for the access to include the identifier assigned to the one master device. 
     (9) The access control method according to (7), further including 
     allowing a device on a signal path between the one master device and the slave device to perform processing to allow the request for the access to include the identifier assigned to the one master device. 
     (10) The access control method according to any one of (1) to (9), wherein 
     the slave device controls the device to be accessed. 
     (11) The access control method according to any one of (1) to (10), wherein 
     the device to be accessed is a memory, and 
     the request for the access is a write request. 
     (12) A bus system, including: 
     a plurality of master devices that are able to generate a request for access to a device to be accessed; and 
     a slave device that makes, on a basis of the request for the access, a response to one master device of the plurality of master devices that has generated the request for the access, at response timing that corresponds to the one master device. 
     (13) A semiconductor device, including: 
     a plurality of master devices that are able to generate a request for access to a device to be accessed; and 
     a slave device that makes, on a basis of the request for the access, a response to one master device of the plurality of master devices that has generated the request for the access, at response timing that corresponds to the one master device. 
     This application claims the benefit of Japanese Priority Patent Application JP2014-183777 filed Sep. 10, 2014, the entire contents of which are incorporated herein by reference. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.