Abstract:
A direct memory access control system supplies the respective status signals indicating timings of the read data effective state or writable state between the input/output interface and memory interface, both interfaces maintain the read data effective state and writable state of the input/output memory and synchronous memory under control until the later timing comes up. Consequently, it is possible to match the read data effective timing and writable timing of the synchronous memory and input/output memory, thus making possible flyby transfer of data between both memories.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a DMA control system enabling flyby transfer between synchronous memory such as SDRAM (synchronous dynamic random access memory) and an external device. 
     2. Description of the Related Art 
     A microprocessor contains a CPU core, and a direct memory access controller (DMAC) internally; the DMAC controls access. to an externally connected memory. For example, data transfer between the external memory and an external device is performed in that data stored in external memory is read and transferred to the external device, or data is read from the external device and transferred to the external memory. 
     In a typical method for such data transfer, data is read from the external memory, the read data is stored temporarily in an internal buffer of the microprocessor, and then the data stored in the internal buffer is transferred to the external device. Further, a data reading request is made to the external device, the read data is stored temporarily in the internal buffer, and then that data is written to the external memory. 
     FIG. 1 is a diagram showing the relationship between the microprocessor containing the DMAC, the external memory, and the external device. The microprocessor  10  is connected via a universal bus  22  to synchronous memory (SDRAM)  24  and an external device control unit  26 . The external device control unit  26  includes an input/output memory  28  and controls access to the external device  30  such as a hard disk or communications device, or the like. The microprocessor  10  includes a CPU core  14 , DMAC  16 , and an I/O memory interface  18  for controlling the input/output memory  28 , and an SDRAM interface  14  for controlling the SDRAM  24 . These are each connected via the internal bus  12 . Also, the microprocessor  10  includes an I/O internal buffer memory  20  for temporarily storing the data read from the I/O memory  28  in the external device control unit  26 , and an SDRAM internal buffer  16  for temporarily storing data read from the SDRAM  24 . 
     The CPU core  14  in the microprocessor  10  sends data transfer requests to the DMAC, first to read data from the SDRAM  24 , which is the external memory, and supply the data to the external device  30 , and secondly to take data from the external device  30  and write the data to the external memory SDRAM  24 . 
     In the abovementioned first case, the DMAC  16  issues a read command to the SDRAM interface  14 ; the SDRAM interface  14  reads data by supplying the read command to the SDRAM  24 , and stores the data temporarily in the SDRAM internal buffer  16 . Thereafter, DMAC  16  issues an I/O write command to the I/O memory interface  18 . The I/O memory interface  18  supplies the write command to I/O memory  28  and writes the data stored in the SDRAM internal buffer  16 . 
     In the abovementioned second case, conversely, the DMAC  16  causes the I/O memory interface  18  to execute an I/O read, reads data in the I/O memory  28 , and stores the data temporarily in the I/O internal buffer  20 . Thereafter, a write command is supplied from the SDRAM interface  14  to the SDRAM  24  and the stored data is thus written thereto. 
     Consequently, for data transfer between SDRAM  24  which is the external memory and the I/O memory  28 , operations to access each memory are necessary; a total of two access operation cycles are required. Moreover, this causes the system processing to become less efficient because the universal bus  22  is monopolized in each of the operation cycles. 
     Flyby transfer between the I/O memory  28  of the external device  30  and the SDRAM  24  is desirable. In flyby transfer, data is transferred between the device  24  and  28  directly via the universal bus  22 . However, although the external device  30  can control the data effective period to a certain extent by asserting read/write commands, the cycle wherein the input/output data is effective in synchronous memory such as SDRAM is only one cycle at a predetermined timing within the command cycle because of the high-speed operations of the memory. It is therefore difficult to control data transfer through conforming to that input/output data effective cycle. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a DMA control system which enables flyby transfer for directly transferring data between a synchronous memory and an I/O memory of an external device. 
     In order to achieve the abovementioned object, a first aspect of the present invention is a direct memory access control system including: a direct memory access controller for issuing a flyby transfer command; a memory interface for receiving the flyby transfer command and supplying a first access command to an external synchronous memory; and an input/output interface for supplying a second access command to an external input/output memory. The input/output interface supplies a first status-signal indicating a writable timing or a read data effective timing of the input/output memory to the memory interface. The memory interface supplies a second status signal, that indicates a read data effective timing or a writable timing corresponding to the access command supplied to the synchronous memory, to the input/output interface. Until the later of the timings indicated by the first and second status signals, the memory interface and the input/output interface maintain a read data effective state and a writable state respectively. 
     According to the abovementioned invention, while supplying the respective status signals indicating timings of the read data effective state or writable state between the input/output interface and memory interface, both interfaces maintain the read data effective state and writable state of the input/output memory and synchronous memory under control until the later timing comes up. Consequently, it is possible to match the read data effective timing and writable timing of the synchronous memory and input/output memory, thus making possible flyby transfer of data between both memories. 
     In a preferred embodiment of the abovementioned invention, the memory interface extends the read data effective state by negating a clock enable signal for controlling a clock input to the synchronous memory. In effect, the memory interface extends the read data effective state by maintaining the state of negating the clock enable signal until the timing of the first status signal. Also, the memory interface supplies a write command, for instructing writing to the synchronous memory, to the synchronous memory in accordance with the timing of the first status signal. 
     In a preferred embodiment of the abovementioned invention, the input/output interface maintains the state of asserting the read signal to the input/output memory until the timing of the second status signal. Also, the input/output interface maintains the state of asserting the write signal to the input/output memory until the timing of the second status signal. 
     Furthermore, in a preferred embodiment of the abovementioned invention, the input/output interface counts the access cycles to the external device under control and outputs the first status signal when the count ends, or supplies the count value to the memory interface. Or, the input/output interface outputs the first status signal in response to a ready signal from the external device under control. 
     Furthermore, in a preferred embodiment of the abovementioned invention, the memory interface counts the number of cycles, until the read data effective state or the writable state, of the synchronous memory under control and outputs the second status signal when that count ends, or supplies the count value to the input/output memory interface. 
     Furthermore, in a preferred embodiment of the abovementioned invention, at a time of burst transfer for successively transferring data, the memory interface negates a clock enable signal for controlling clock input to the synchronous memory to maintain a state of completed setup for data input/output, and asserts the clock enable signal at the timing of the first status signal, to initiate the next input/output setup. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing the relationship between the microprocessor containing the DMAC, external memory, and external device; 
     FIG. 2 is a diagram of the configuration of the DMA control system relating to the present embodiment; 
     FIG. 3 is a flowchart for the operations of the I/O sequencer; 
     FIG. 4 is a flowchart for the SDRAM sequencer; 
     FIG. 5 is a timing chart for conventional transfer operations from SDRAM to I/O memory; 
     FIG. 6 is a timing chart for flyby transfer from SDRAM to I/O memory according to the present embodiment; 
     FIG. 7 is a timing chart for conventional transfer operations from SDRAM to I/O memory; 
     FIG. 8 is a timing chart for flyby transfer from SDRAM to I/O memory according to the present embodiment; 
     FIG. 9 is a timing chart for conventional transfer operations from I/O memory to SDRAM; 
     FIG. 10 is a timing chart for flyby transfer from I/O memory to SDRAM according to the present embodiment; 
     FIG. 11 is a timing chart for conventional transfer operations from I/O memory to SDRAM; 
     FIG. 12 is a timing chart for flyby transfer from I/O memory to SDRAM according to the present embodiment; 
     FIG. 13 is a timing chart for burst transfer using flyby transfer from SDRAM to I/O memory, according to the present embodiment; 
     FIG. 14 is a timing chart for burst transfer using flyby transfer from I/O memory to SDRAM, according to the present embodiment; 
     FIG. 15 is a diagram of the configuration of a modified example of the DMA control system relating to the present embodiment; 
     FIG. 16 is a timing chart for burst transfer operations in flyby transfer from SDRAM to I/O memory using the system of the modified example; and 
     FIG. 17 is a timing chart of burst transfer operations in flyby transfer from I/O memory to SDRAM using the system of the modified example. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention are explained below with reference to the drawings. However, the scope of the present invention is not limited by the following embodiment examples and extends to the inventions appearing in the claims and to items equivalent thereto. 
     FIG. 2 is a diagram of the configuration of the DMA control system relating to the present embodiment. The microprocessor  10  includes a CPU core, not shown, a direct memory access controller (DMAC)  16 , SDRAM interface  14 , and I/O interface  18 . The SDRAM interface  14  controls the SDRAM  24  which is an external synchronous memory. Also, the I/O interface  18  controls the I/O memory  28  included in the control circuit, or the like, of an external device, not shown. The SDRAM  24  and I/O memory  28  are connected via an external universal bus  22 . 
     The I/O sequencer  180  in the I/O interface  18  asserts the I/O read command (or I/O read signal) /IORD to the I/O memory  28  and reads the stored data to the universal bus  22 ; and asserts the I/O write command (I/O write signal) /IOWR to the I/O memory  28  and writes the data on the universal bus  22 . The I/O memory  28  receives the requested data from the external device and stores the data in the I/O memory, and asserts the ready signal READY 0  at the stage of output to the universal bus  22 . In response thereto, the I/O sequencer  180  negates the I/O read command /IORD. Furthermore, at the stage where requested data can be fetched from the universal bus  22 , the I/O memory  28  asserts the ready signal READY 0  and in response thereto, the I/O sequencer  180  negates the I/O write command /IOWR. 
     Also, when the access cycle of the external device is fixed, the I/O sequencer  180  sets that access cycle in advance in the I/O register  182 . When accessing the I/O memory  28 , the I/O sequencer sets the set value S 2  of the register  182  to the I/O counter  184  using the signal S 1  and negates the I/O read command /IORD or the I/O write command /IOWR in response to the count end signal S 3 . 
     However, in the case where the flyby transfer command FLYBYi is put out from the DMAC  16 , if the I/O sequencer  180  receives the ready signal READY 0  from the I/O memory  28  without receiving the second ready signal READY 2  from the SDRAM interface  14 , the I/O sequencer  180  maintains the assertion state without negating the I/O read command /IORD or the I/O write command /IOWR until receiving the second ready signal READY 2 . When the second ready signal READY 2  has already been received, the I/O sequencer  180  negates the I/O read command /IORD or the I/O write command /IOWR as usual in response to the reception of the ready signal READY 0  or the count end signal S 3 . 
     FIG. 3 is a flowchart for the operations of the I/O sequencer. Upon receiving the I/O access request from the DMAC  16  (S 20 ), the I/O sequencer  180  asserts the I/O read or I/O write command /IORD, /IOWR and waits for the end of the countdown, or waits to receive the ready signal READY 0  from the I/O memory  28  (S 21 , S 22 ). Once the waiting state has ended, in the case of a flyby transfer command (S 23 ), if the I/O sequencer receives the second ready signal READY 2  indicating that SDRAM setup is complete (S 24 ), the sequencer immediately negates the I/O read or I/O write command /IORD, /IOWR (S 26 ). If the I/O sequencer has not received the second ready signal READY 2 , the I/O sequencer maintains the command assertion state until that signal is received (S 25 ). In response to receiving the second ready signal, the I/O sequencer negates the I/O read or I/O write command /IORD, /IOWR (S 26 ). 
     When there is a request to access the SDRAM, the sequencer  140  in the SDRAM interface  14  supplies commands CMD, comprising a combination of /RAS, /CAS, /CE, and /WE, to the SDRAM  24  and performs a read operation or write operation. In the case of SDRAM, the read operation and the write operation in an access operation are carried out with a combination of the following: “active” for activating a word line, “read” for reading data from a memory cell, “write” for writing data to a memory cell, and “precharge” for resetting a bit line. Consequently, the commands CMD include an active command, read command, write command, precharge command, and so forth. The SDRAM sequencer  140  supplies those commands in order and carries out the read operation and write operation. Also, the sequencer  140  supplies the clock enable CKE, for enabling and disabling the supply of the clock CLK into the SDRAM, to the SDRAM  24 . 
     The SDRAM sequencer  140  controls the operation cycles of each command and supplies the abovementioned commands CMD in a predetermined order. For that reason, the number of operation cycles of each command is set in the register  142  and the operation cycle data S 12  corresponding to a command is loaded to a counter  144  in response to an instruction signal S 11 . When the counter  144  ends the countdown, the count end signal S 13  is supplied to the sequencer  140 . 
     In the case of a read operation, the SDRAM  24  outputs the read data for a period of one clock cycle following CAS latency once the read command has been supplied. CAS latency is preset in the register  142  and the number of clock cycles from the read command until data output is fixed. However, when the clock enable CKE is negated, the internal operation of the synchronous memory can be stopped for the negation period without the clock being supplied therein. 
     Consequently, the SDRAM sequencer  140  can extend the time of the read data output effective state by appropriately negating the clock enable CKE. The SDRAM sequencer  140  therefore negates the clock enable CKE from before one clock cycle when read data is output, and in response to the first ready signal READY 1  that is output when the I/O memory  28  completes input setup, asserts the clock enable CKE. Accordingly, during flyby transfer, the SDRAM sequencer  140  can output read data from the SDRAM  24  to the universal bus  22  at the timing corresponding to the I/O memory  28  operation speed. 
     Furthermore, in the write operation, the SDRAM  24  inputs the write data in the same clock cycle as the write command. Consequently, at the time of flyby transfer, the SDRAM sequencer  140  issues the write command in response to the first ready signal READY 1 , and can thereby match the fetch timing of the write data with the operation of the I/O memory  28 . 
     FIG. 4 is a flowchart for the SDRAM sequencer. When SDRAM access is issued (S 30 ), the SDRAM sequencer  140  issues a precharge command PRE and an active command ACTV depending on the state of the SDRAM  24 , and then issues the write command or read command. If the SDRAM  24  makes a page miss in the bank active state, it is necessary to perform reading or writing after the precharge PRE and active ACTV. Also, if the SDRAM  24  makes a page miss in the bank inactive state, it is necessary to perform reading or writing after the active ACTV. 
     In the case where precharging is necessary (S 32 ), the SDRAM sequencer  140  issues the precharge command PRE and after waiting for the precharge clock cycle tRP (RAS precharge) (S 34 ), issues the active command ACTV. Also, the SDRAM sequencer  140  issues the active command ACTV in the case where active is necessary (S 36 ) or after the precharge, and waits for the active clock cycle tRCD (S 38 ). Each clock cycle wait operation is performed by loading the set values in the register  142  to the counter  144  and initiating countdown. 
     In the case where the access request is a read operation, the SDRAM sequencer  140  issues the read command READ. In response thereto, the SDRAM  24  outputs the read data after the CAS latency CL clock cycle. However, in the case of flyby transfer (S 44 ), the SDRAM sequencer  140  negates the clock enable CKE from one clock before the CAS latency CL clock cycle ends and waits until the I/O memory  28  setup ends (S 46 , S 48 ). Upon receiving the first ready signal READY 1  from the I/O sequencer  180  (S 48 ), the SDRAM sequencer asserts the clock enable CKE in response thereto (S 50 ), renews clock supply, and causes output of the read data to the universal bus  22  (S 52 ). 
     In the case where the access request is a write operation, the SDRAM sequencer  140  issues a write command WRIT and causes the SDRAM  24  to fetch the write data in the universal bus  22  (S 60 ). However, in the case of flyby transfer (S 54 ), the sequencer waits until I/O memory  28  setup ends (S 56 , S 58 ), and then issues the write command WRIT. 
     Next, the flyby transfer operation is explained for the cases where I/O memory is slower than SDRAM and where I/O memory is faster than SDRAM. Flyby transfer includes transfer from SDRAM to I/O memory and from I/O memory to SDRAM. 
     Flyby Transfer from SDRAM to I/O Memory (I/O Memory is Slower) 
     FIG. 5 is a timing chart for a conventional operation for transfer from SDRAM to I/O memory. Because flyby transfer could not be performed conventionally as discussed above, the DMAC  16  initially causes the SDRAM interface  14  to perform SDRAM read, temporarily stores the read data in an internal buffer, and then causes the I/O interface  18  to perform I/O write. The SDRAM sequencer  140  issues a read command READ, the SDRAM  24  fetches same at clock cycle C 2  and outputs read data D at clock cycle C 3  after CAS latency CL=1. As a result, the read data is stored in an internal buffer. Next, the I/O sequencer  180  asserts the I/O write signal /IOWR from clock cycle C 4  and outputs a write command to the I/O memory  28 . In response thereto, the I/O memory  28  asserts the ready signal READY 0  at clock cycle C 6 , when the write setup is complete, whereupon I/O sequencer  180  detects this at clock cycle C 7  and negates the I/O write signal /IOWR. In order to fetch data, the I/O memory  28  requires the write data D and address A during the I/O hold period of one clock cycle from the I/O write negation. 
     FIG. 6 is a timing chart of flyby transfer from SDRAM to I/O memory relating to the present embodiment. In this example, CAS latency CL is 1 and the SDRAM can operate from the read command. Initially, the DMAC  16  issues the flyby signals FLYBYi, FLYBYs to the I/O interface  18  and SDRAM interface  14  respectively. These flyby signals also include the direction of flyby transfer. In response thereto, the SDRAM sequencer  140  issues the read command READ to the SDRAM  24 , and the I/O sequencer  180  asserts the I/O write signal /IOWR to I/O memory at clock cycle C 1 . 
     Since the CAS latency CL=1 in the example in FIG. 6, the SDRAM sequencer  140  negates the clock enable CKE at clock cycle C 1  and exercises control so as to extend the read data output state. Shortly afterwards, when the I/O memory  28  setup is complete at clock cycle C 3  and the I/O sequencer  180  asserts the first ready signal READY 1 , the SDRAM sequencer  140  asserts the clock enable CKE in response thereto. As a result, the SDRAM  24  maintains read data output during the one clock cycle from the rising edge of clock cycle C 4 . Consequently, the I/O hold period is established automatically and the I/O memory  28  can fetch the write data correctly. 
     Compared to FIG. 5, flyby transfer in FIG. 6 is completed in three clock cycles shorter than FIG.  5 . Moreover, by using the clock enable CKE to extend the effective state of read data output for the SDRAM  24 , a data hold is effected for one clock cycle automatically when the clock enable CKE is asserted in response to the negation of the I/O write signal/IOWR. 
     Moreover, the I/O memory  28  having lower-speed operation requires the data hold period after the negation of the I/O write signal /IOWR. In the case of I/O memory  28  having higher-speed operation, this data hold period is not necessary. Consequently, the I/O interface  18  contains a register of whether data hold is necessary or not and notifies the SDRAM interface  14  via the DMAC. 
     Flyby Transfer from SDRAM to I/O Memory (SDRAM is Slower) 
     FIG. 7 is a timing chart of a conventional transfer operation from SDRAM to I/O memory in the case of slow SDRAM. The order of operations is the same as in the example in FIG. 5, but in the example in FIG. 7, the operation of the I/O memory is complete in two clock cycles while the CAS latency CL of the SDRAM is long,  3 . Otherwise, the operations are the same as in FIG. 5 and a detailed explanation of the operations is omitted. 
     FIG. 8 is a timing chart of flyby transfer from SDRAM to I/O memory relating to the present embodiment. In this example, because the read operation of the SDRAM  24  is long, read data is output for a period of only two clock cycles without extending the read data output as in FIG.  6 . Initially, the SDRAM sequencer  140  issues a read command READ at clock cycle C 1  in response to a flyby request and the I/O sequencer  180  asserts the I/O write signal /IOWR. At clock cycle C 3 , when the write setup for the I/O memory  28  is complete, the I/O sequencer  180  asserts the first ready signal READY 1 . Thereafter, at clock cycle C 4  corresponding to CAS latency CL=3, the SDRAM sequencer  140  asserts the second ready signal READY 2  to indicate that the read data setup for the SDRAM  24  is complete, whereupon the I/O sequencer  180  negates the I/O write signal /IOWR at clock cycle C 5  and causes the I/O memory  28  to write data during the I/O hold period. 
     Corresponding to CAS latency CL=3, at clock cycle C 3  that is one clock before read data output the SDRAM sequencer  140  negates the clock enable CKE, but because the first ready signal READY 1  is already asserted, the sequencer  140  asserts the clock enable CKE at clock cycle C 4  and effectively outputs the read data D for a period of two clock cycles. 
     In the example in FIG. 6, the SDRAM sequencer  140  maintains the output of read data for a period of three clock cycles because the I/O memory setup is long, but in the example in FIG. 8, this becomes a period of two clock cycles because the I/O memory setup is short. In the case of FIG. 8, flyby transfer is complete in a smaller number of clock cycles than in the example in FIG.  7 . 
     Flyby Transfer from I/O Memory to SDRAM (I/O Memory is Slower) 
     FIG. 9 is a timing chart of a conventional transfer operation from I/O memory to SDRAM. The DMAC  16  sends an I/O read request to the I/O interface  18 , and the I/O sequencer  180  reads data from the I/O memory  28  and temporarily stores same in an internal buffer. Next, DMAC  16  sends an SDRAM write operation request to the SDRAM interface  14  and the SDRAM sequencer  140  issues an active command ACTV and write command WRIT to the SDRAM  24  and controls the writing of data. In the example in FIG. 9, since the active operation requires two clock cycles, the write command WRIT is issued at C 7 , two clock cycles after the issuing of the active command ACTV. 
     FIG. 10 is a timing chart of flyby transfer from I/O memory to SDRAM relating to the present embodiment. The DMAC  16  issues the flyby signals FLYBYi, FLYBYs and sends a flyby transfer request to both interfaces  18 ,  14 . At clock cycle C 1 , the I/O sequencer  180  asserts the I/O read signal /IORD and requests a read operation of the I/O memory  28 . Also, the SDRAM sequencer  140  starts the write operation transaction from the same cycle C 1 . In the example in FIG. 10, the active command ACTV is issued and then the write command WRIT is issued. 
     When the read data output setup is complete, the I/O memory  28  asserts the ready signal READY 0  at clock cycle C 2  (not shown) and the I/O sequencer  180  asserts the first ready signal READY 1  at clock cycle C 3 . In response to this assertion, the SDRAM sequencer  140  issues the write command WRIT and the SDRAM  24  fetches the read data D on the universal bus and writes to a memory cell at clock cycle C 4 . 
     In the case where the SDRAM  24  does not require the active operation ACTV, the SDRAM sequencer  140  can issue the write command WRIT without issuing the active command ACTV at clock cycle C 1 . Even in that case, however, the first ready signal READY 1 , that indicates that I/O memory  28  setup is complete, is not asserted, and therefore the write command WRIT is not issued; the issuing of the write command is waited until clock cycle C 3 . 
     In this case also, the flyby transfer operation is completed in fewer clock cycles than in the example in FIG.  9 . 
     Flyby Transfer from I/O Memory to SDRAM (SDRAM is Slower) 
     FIG. 11 is a timing chart of a conventional transfer operation from I/O memory to SDRAM. With the same operations as in FIG. 9, the SDRAM  24  writes data after reading same from the I/O memory  28 . In the example in FIG. 11, however, the IO read operation of the I/O memory takes two clock cycles and is shorter compared with that of FIG.  9 . 
     FIG. 12 is a timing chart of flyby transfer from I/O memory to SDRAM relating to the present embodiment. In this example, the read operation in I/O memory  28  is complete in one clock cycle and the first ready signal READY 1  is already asserted at clock cycle C 2 . Consequently, the SDRAM sequencer  140  issues the active command ACTV and write command WRIT in order and asserts the second ready signal READY 2  at the same time as issuing the write command. As a result, data is written to the SDRAM  24  at clock cycle C 4  and the I/O sequencer  180  negates the I/O read signal /IORD thereafter. Since the operation of the SDRAM is slow in this example, the period during which the I/O memory  28  outputs read data is three clock cycles, longer than the two clock cycles in the case of FIG.  10 . 
     Burst Transfer 
     FIG. 13 is a timing chart of burst transfer using flyby transfer from SDRAM to I/O memory relating to the present embodiment. This example is an operation with a burst length of 4, in the case where the CAS latency CL of the SDRAM  24  is short, 2, and the I/O memory  28  is slow and requires an I/O hold period of one clock cycle after the negation of the I/O write command /IOWR. Also, the I/O sequencer  180  and SDRAM sequencer  140  indicate that setup is complete in each case by transmitting the first and second ready signals. Moreover, burst transfer includes a flyby request signal, for example, and a request for burst transfer is sent to each interface from the DMAC. 
     In response to a flyby transfer request, not shown, the SDRAM sequencer  140  issues a read command READ at clock cycle C 1  and the I/O sequencer  180  asserts the I/O write signal /IOWR. Since the CAS latency is CL=2, the SDRAM sequencer  140  asserts the second ready signal READY 2  at clock cycle C 3  to indicate that the read data setup is prepared. Also, the SDRAM sequencer  140  negates the clock enable CKE at clock cycle C 2 , which is one clock cycle before data output, and waits to receive the first ready signal READY 1 . 
     When the I/O memory write setup is prepared at clock cycle C 4 , the ready signal READY 0  is asserted and at the same time the I/O sequencer  180  asserts the first ready signal READY 1 . In response thereto, the SDRAM sequencer  140  asserts the clock enable CKE and performs the next read data output control. Also, the I/O sequencer  180  negates the I/O write signal /IOWR at clock cycle C 5 . 
     In burst mode also, the SDRAM sequencer must maintain read data output until the first ready signal READY 1 , which indicates that I/O memory setup is prepared, is asserted. Consequently, the SDRAM sequencer  140  immediately negates the clock enable CKE at clock cycle C 5  and waits for the first ready signal READY 1 . 
     Once the I/O hold period ends at clock cycle C 5  is complete, the I/O sequencer  180  asserts the I/O write signal /IOWR at clock cycle C 6  in order to write the next data. Because the SDRAM  24  already has the output setup prepared for burst mode for the second read data output, the second ready signal READY 2  remains asserted. Once the setup for the I/O memory  28  is prepared, the I/O sequencer  180  asserts the first ready signal READY 1  at clock cycle C 7 . In response thereto, the SDRAM sequencer  140  also asserts the clock enable CKE and carries out the next read data output control. 
     The third and fourth data flyby transfers are the same as the abovementioned second data flyby transfer. In effect, at clock cycle C 3 , two clock cycles corresponding to CAS latency CL=2 after issuing the read command READ, the SDRAM sequencer  140  asserts the second ready signal READY 2  and then maintains that asserted state. The SDRAM sequencer  140  negates the clock enable CKE at each time to maintain read data output, and then waits for the first ready signal READY 1  from the I/O sequencer  180  to assert the clock enable CKE, so that the SDRAM sequencer performs control of the next read data output. Therefore, the SDRAM sequencer  140  can even handle flyby transfer in burst mode by using the clock enable CKE. 
     FIG. 14 is a timing chart for burst transfer using flyby transfer from the I/O memory to SDRAM relating to the present embodiment. This example concerns the case of burst transfer of three data. 
     In response to a request for flyby transfer, not shown, the SDRAM sequencer  140  issues the active command ACTV at clock cycle C 1  for a write operation, asserts the second ready signal READY 2  indicating the completion of write setup at clock cycle C 3 , and waits for the first ready signal READY 1 . However, in the example in FIG. 14, the read output setup of the I/O memory  28  is completed at the same time and the first ready signal READY 1  is asserted at clock cycle C 3 . In response thereto, the SDRAM sequencer  140  issues the write command WRIT. At the same time that the write command is issued, the read data on the universal bus are written to the SDRAM  24  at C 4 . 
     Consequently, in the case where the SDRAM  24  does not require the active operation, the second ready signal READY 2  is asserted sooner than in FIG.  14 . However, in this case also, the issuing of the write command WRIT is not performed until the first ready signal READY 1  is asserted. 
     When the write operation for the initial data is performed, the I/O sequencer  180  asserts the I/O read signal /IORD once more, at clock cycle C 5 , to output the next data. At this time, because the SDRAM  24  is in a writable state, the second ready signal READY 2  remains asserted. Once the setup for the next data output is prepared in the I/O memory  28 , the first ready signal READY 1  is asserted at clock cycle C 7 , the SDRAM sequencer  140  issues the write command WRIT, and causes a write operation to the SDRAM  24 . 
     The operations for the next data are the same as the flyby transfer for the second data. In this way, with burst transfer in flyby transfer from I/O memory to SDRAM, subsequent writing of data can be performed promptly with the issuing of a write command once the initial write setup is prepared for the SDRAM  24 . Consequently, the SDRAM sequencer  140  may simply continue to wait for the first ready signal READY 1  and then issue the write command WRIT. In effect, this is burst transfer, but the SDRAM sequencer  140  only repeats single write control, rather than burst write control, for the burst length. 
     Modified Example 
     FIG. 15 is a diagram of the configuration of a modified example of the DMA control system relating to the present embodiment. The same reference numbers are used for portions that are the same as in FIG.  2 . The configuration differs from FIG. 2 as follows: the count value IOCOUNT of the I/O counter  184  in the I/O interface  18  is supplied to the SDRAM sequencer  140  side instead of receiving a first ready signal, the SDRAM sequencer  140  monitors the I/O count value IOCOUNT and detects whether I/O memory setup is prepared. Likewise, instead of receiving the second ready signal READY 2  from the SDRAM sequencer  140 , the I/O sequencer  180  receives the count value SDCOUNT until the read or write operation setup is prepared. The SDRAM sequencer  140  counts down the counter  144  to manage the number of clock cycles necessary for each type of operation of the SDRAM  24 . The SDRAM sequencer  140  converts from that count value S 13  to the number of clock cycles SDCOUNT until the read or write setup is prepared and thus supplies SDCOUNT to the I/O sequencer  180 . 
     Consequently, in the DMA control system in FIG. 15, the completion of the SDRAM setup is determined according to the SDRAM count value SDCOUNT in the course of the process S 24  in the flowchart of the I/O sequencer operations shown in FIG.  3 . Furthermore, the completion of the I/O memory setup is determined according to the I/O count value IOCOUNT in the course of the processes S 48 , S 58  in the flowchart of the SDRAM sequencer operations shown in FIG.  4 . 
     FIG. 16 is a timing chart of the burst transfer operation with flyby transfer from SDRAM to I/O memory according to the system in the abovementioned modified example. In this example, CAS latency CL is 3 and the I/O memory engages in high-speed operations that do not require the I/O hold. As shown, the I/O count value IOCOUNT and SDRAM count value SDCOUNT are monitored instead of the first and second ready signals. As in FIG. 13, the output of the read data D in the SDRAM  24  is held by the negation of the clock enable CKE; The clock enable CKE is asserted by predicting the cycle at which the write operation is completed in the I/O memory according to the change in the I/O count value so that the next read data is output. 
     In response to a flyby transfer request, not shown, the SDRAM sequencer  140  issues the read command READ at clock cycle C 1  and supplies to the I/O sequencer  180 , the number of clock cycles “3”, showing cycles until read data is output, as the SDRAM count value S 144 . When it is necessary for the, SDRAM  24  to execute from active operation, this SDRAM count value SDCOUNT becomes a value that is equal to the CAS latency added to the clock cycle for active operation. Also, the I/O sequencer  180  supplies to the SDRAM sequencer  140  the number of clock cycles “1” showing cycles until the write operation setup is prepared, as the I/O count value IOCOUNT. 
     Because the I/O count value IOCOUNT becomes “0” at clock cycle C 2 , the SDRAM sequencer  140  detects that I/O memory setup is complete and does not negate the clock enable CKE even if this is one clock cycle before the output of the read data D. Then, the sequencer  140  negates the clock enable CKE at clock cycle C 4  in order to hold the next read data output. 
     The I/O sequencer  180  detects the completion of the writing of the effective read data when the SDRAM count value SDCOUNT becomes “0”, temporarily negates the I/O write signal /IOWR at clock cycle C 5 , and then asserts that signal once more at the next clock cycle C 6 . Accordingly, the I/O memory starts write setup for the next flyby transfer. The I/O sequencer sets the I/O count value IOCOUNT to “1” with the assertion of the I/O write signal /IOWR at clock cycle C 6 . The SDRAM sequencer sees this I/O count value and detects that the I/O memory setup for writing the second data is prepared after one clock cycle. The sequencer  140  asserts the clock enable CKE in order to output the third read data. At the next clock cycle C 7  following the assertion of the clock enable CKE, the clock enable CKE is negated and the SDRAM is controlled so as to hold the third read data. 
     Subsequent to clock cycle C 5 , the SDRAM is set up for read data output and therefore that count value SDCOUNT remains “0”. Consequently, control of the second and subsequent read data by the SDRAM sequencer  140  is based on the I/O count value IOCOUNT and is controlled by the negation and assertion of the clock enable CKE. 
     FIG. 17 is a timing chart of the burst transfer operation with flyby transfer from I/O memory to SDRAM according to the system in the abovementioned modified example. In this example, the SDRAM sequencer supplies a burst write instruction to the SDRAM, delays the time of the write operation by negating the clock enable CKE, monitors the time at which the read data in I/O memory becomes effective according to the I/O count value IOCOUNT, asserts the clock enable CKE and gives an instruction for the write operation. 
     Initially, when flyby transfer is ordered at clock cycle C 1 , the SDRAM sequencer  140  issues the active command ACTV and sets the SDRAM count value SDCOUNT to “3”. Meanwhile, the I/O sequencer  180  asserts the I/O read command /IORD and sets the I/O count value IOCOUNT to “1”. In the initial data flyby transfer, the read data setup of I/O memory is prepared first. Accordingly, the SDRAM sequencer can issue a write command at the shortest clock cycle C 4  and the data is written at clock cycle C 5 . At that time, the SDRAM sequencer  140  issues the write command WRIT, while negating the clock enable CKE and controls the next data writing in a hold state. 
     In response to the SDRAM count value SDCOUNT becoming “0”, the I/O sequencer  180  negates the I/O read command /IORD at clock cycle C 5  and asserts the command once more at the next cycle C 6 . At this assertion, the I/O count value IOCOUNT is set to “1”. In response thereto, the SDRAM sequencer  140  asserts the clock enable CKE and controls the writing operation in clock cycle C 8 . The SDRAM sequencer  140  negates the asserted clock enable CKE once more at the next cycle C 7  and controls the third data writing operation in a hold state. 
     Subsequently, the SDRAM  24  remains in a writable state and therefore the SDRAM sequencer  140  maintains the SDRAM count value SDCOUNT at “0” and then asserts and negates the clock enable CKE for a period of one clock cycle once the I/O count value IOCOUNT is set to “1”. The I/O count value IOCOUNT is set to “1” by the I/O sequencer  180  at the assertion of the I/O read command /IORD to start the read data setup of the I/O memory. 
     As above, the SDRAM sequencer and I/O sequencer provide each other with the number of clock cycles showing cycles until setup is complete as count values, and thereby allow the other to make predictions according to that count value and control the corresponding controlling device. In the case of the burst transfer discussed above, the SDRAM sequencer regulates the timing for writing a plurality of data by toggling the clock enable CKE during burst write control and makes possible flyby transfer. 
     As above, the present invention makes it possible to perform data transfer directly via a universal bus between synchronous memory and input/output memory.