Abstract:
Interface circuitry is disclosed for interfacing between an operational circuit, a microprocessor, for example, and data storage circuitry, for example direct Rambus memory. The interface circuitry comprises buffer circuitry coupled between the operational circuitry and the data storage circuitry which is arranged to store data access requests received from the operational circuitry and to store data retrieved from the data storage circuitry. The buffer circuitry comprises an output for supplying the data access request signals to the data storage circuitry and to supply the stored data from the data storage circuitry to the operational circuitry. 
     In use, the number of stored data access request signals decreases as the amount of stored data from the data storage circuitry increases. Similarly, the number of stored data access request signals increases as the amount of stored data from the data storage circuitry decreases. The overall effect is that in use, the sum of this stored data access request signals and stored data in the buffer circuitry remains substantially constant.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to controlling the flow of data between an operational circuit and a data storage circuit. In particular, but not exclusively, the operational circuit may be a device which uses the data storage circuit and the data storage circuit may be a memory. 
     BACKGROUND OF THE INVENTION 
     In known systems using integrated circuits one of the areas which restricts the overall system performance is the interface between the memory device and the internal bus of an operational circuit which accesses that memory. A number of memory devices have been recently introduced which have improved data transfer rates in comparison to conventional memory devices. For example, conventional SDRAM (synchronous dynamic random access memory) typically has a data transfer rate of 32 bits at 100 MHz. An improvement to this is double data rate (DDR) SDRAM which is capable of transferring double the data rate than a conventional SDRAM and hence has a data transfer rate of 32 bits at 200 MHz. There are also available memory devices known as Direct Rambus memories (RDRAM Rambus Dynamic Random Access Memory) which have a transfer rate of 16 bits at 800 MHz. ‘Rambus’ and ‘Direct Rambus’ are trade marks of Rambus Inc. 
     Present conventional integrated circuits typically have an internal system bus with a data transfer rate of 32 bits at 100 MHz. FIG. 1 schematically illustrates a conventional integrated circuit  1  with an internal system bus  5  and three known memory devices, a conventional SDRAM  2 , a double data rate SDRAM  3  and a Direct Rambus memory  4 . (In practice only one of the three memory devices is provided). Each of the memory devices  2 ,  3  and  4  has an output bus which in use is coupled to the internal system bus  5  of the integrated circuit. The output bus  6  of the conventional SDRAM  2  has a data transfer rate of 32 bits at 100 MHz and is therefore entirely compatible with the internal bus  5  of the integrated circuit  1 , which as shown, also has a data transfer rate of 32 bits at 100 MHz. However, the output bus  7  of the DDR SDRAM  3  has a data transfer rate of 32 bits at 200 MHz and the output bus  8  of the Direct Rambus memory  4  has a data transfer rate of 16 bits at 800 MHz. Accordingly the output buses  7  and  8  of the DDR SDRAM  3  and the Direct Rambus memory  4  are not compatible with the internal system bus  5  of the integrated circuit  1  in terms of data rate. Accordingly, with the existing conventional internal bus system of the integrated circuit, the higher data transfer rate of the DDR SDRAM and the Direct Rambus cannot be readily used. 
     To exploit the increased transfer rate of the faster memory devices, the width of the internal bus of the operational integrated circuit could be increased. For example, for a Direct Rambus memory with a transfer rate of 16 bits at 800 MHz, the internal bus of the operational integrated circuit would have to be increased to a 128 bit bus operating at 100 MHz. As this is four times the present conventional bus width the resulting integrated circuit would be much more complex and require increased effort in designing the layout of the interconnects within the integrated circuit and would also consume a much larger area of silicon. This is disadvantageous. FIG. 2 illustrates an example of a Direct Rambus  4  connected to an integrated circuit  1 , the integrated circuit having an internal system bus  5  with a transfer rate of 128 bits at 100 MHz. At the interface between the Direct Rambus memory  4  and the integrated circuit  1 , a demultiplexer  10  would be required to spread the short 16 bit words from the Direct Rambus onto the 128 bit wide internal bus of the integrated circuit. The addition of a demultiplexer  10  further increases the complexity and required silicon area of the integrated circuit. 
     The speed of the internal bus of the integrated circuit could be increased to match that of the memory device connected to it. However, this would require redesigning the integrated circuit and in practice, the internal buses of integrated circuits which represent the current state of the art already typically operate at a speed close to the current maximum possible speed. 
     SUMMARY OF THE INVENTION 
     It is an aim of embodiments of the present invention to provide an improved interface between data storage devices with a relatively high data transfer rate and the internal bus system of an integrated circuit operating with a relatively low data transfer which overcomes or at least mitigates against the problems described hereinbefore. 
     According to one aspect of the present invention there is provided circuitry for controlling a flow of data comprising: an operational circuit arranged to generate at least one data access request; data storage circuitry arranged to output or store data in response to receiving said at least one data access request signal; and buffer circuitry coupled between the operational circuitry and the data storage circuitry arranged to store said at least one data access request from the operational circuitry and store data from the data storage circuitry, wherein in use the number of stored data access request signals decreases as the amount of stored data from the data storage circuitry increases and the number of stored data access request signals increases as the amount of stored data from the data storage circuitry decreases. 
     According to a second aspect of the present invention there is provided a method for controlling flow of data comprising the steps: receiving data access request signals at the input of buffer circuitry; storing said data request signals in the buffer circuitry; receiving data at said input of the buffer circuitry; and storing said data in the buffer circuitry. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention and as to how the same may be carried into effect, reference will now be made by way of example only to the accompanying drawings in which: 
     FIG. 1 schematically shows a conventional integrated circuit device and three data storage devices to which the integrated circuit device may be connected; 
     FIG. 2 shows an arrangement for connecting a Direct Rambus memory to a conventional integrated circuit; 
     FIG. 3 shows the structure of a conventional DRAM device; 
     FIG. 4 shows instantaneous and average data transfer rates for a Direct Rambus memory; 
     FIG. 5 shows a Direct Rambus connected to an integrated circuit via an interface according to an embodiment of the present invention; 
     FIG. 6 schematically illustrates how the contents of the interface of FIG. 5 vary with time; 
     FIG. 7 shows a Direct Rambus connected to a number of integrated circuits via a number of interfaces according to a further embodiment of the present invention; 
     FIG. 8 illustrates the timing of the DRAM of FIG. 3; 
     FIG. 9 illustrates an example of the controller shown in FIG. 5, connected to a DRAM device; 
     FIG. 10 illustrates the timing of the DRAM device of FIG. 9; and 
     FIG. 11 shows a modified version of the controller and DRAM shown in FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made to FIG. 3 which shows a dynamic random access memory (DRAM) device. The actual memory cells  36 ,  37  which store the data are arranged in one or more banks  31 ,  32  consisting of a number of rows  40  and columns  42 . In each of the two banks shown, there are m rows and n columns. Each row  40  of a bank  31 ,  32  is known as a page. To access the data (referred to as a word) stored at a particular cell within a memory bank  31 ,  32  it is necessary to activate all of the memory cells which are in the same row  40  as the memory cell  36 ,  37  which it is desired to access. The act of activating a complete row of a memory array is referred to as opening a page. A particular cell  36 ,  37  is accessed by activating the associated column. Data output from the cell  36 ,  37  is read by an output decoder  38 , which transfers the data to the output bus  39 . To carry out this function one or more control signals  34 ,  35  are produced by an internal decoder  30 . 
     A page must be opened before data can be accessed from a memory cell contained within that page. Therefore there is a delay of several cycles introduced between the memory receiving a request to access a memory cell  36 ,  37  and that memory cell becoming available to transfer data. A further delay of several cycles is introduced as it is necessary to close any previously opened pages before the next page can be opened. For example, in a conventional SDRAM, to retrieve 8 words of data, it takes 3 clock cycles to close a previous page, 6 cycles to open the next page and retrieve the first-word of data and a further 7 clock cycles to transfer the requested data from the memory. In a Direct Rambus memory device, the delay necessary to close a page and open a subsequent page is 9 cycles and the time taken to transfer the remaining data from the memory is a further 1 cycle. 
     Because of the delay required to close and open pages, the instantaneous data transfer from the memory device is not constant. FIG. 4 shows the variation of instantaneous data transfer from a Direct Rambus device with respect to time. The instantaneous data transfer rate is shown by the line I and it can be seen that the memory only in fact outputs data at a high transfer rate for short periods of time or bursts. One such period is referenced db in FIG.  4 . The average data transfer rate over a longer period of time is shown by line II and is much lower than the peak data transfer rate of the memory device. 
     As stated hereinbefore, a Direct Rambus memory device requires a total of 10 cycles to output data from the memory which has been requested by the integrated circuit  (3  cycles to close a page and 6 cycles to open a new page and retrieve the first data word and 1 cycle to retrieve the remaining data words). As the transfer rate of a Direct Rambus memory device is 16 bits at 800 MHz, the average rate of data transfer is 320 M bytes per second. An internal bus of an integrated circuit operating at 32 bits at 100 MHz is capable of sustaining a data transfer rate of 400 M bytes per second. 
     Reference is now made to FIG. 5 which shows a Direct Rambus  102  connected to an integrated circuit  103  via an interface  101 ,  106 . To take advantage of the higher average data transfer rate of a conventional internal bus of a integrated circuit in comparison to the average data transfer rate of a Direct Rambus memory device, the interface  101 ,  106  is provided between the Direct Rambus  102  and the integrated circuit  103 . This interface  101 ,  106  is capable of smoothing out the peaks in the instantaneous data transfer rate of the Direct Rambus and providing an output to the internal bus of the integrated circuit  103  which operates at least at the average data transfer rate of the Direct Rambus  102 . It is preferred that the average data transfer rate of the Direct Rambus  102  be the same as the internal bus of the integrated circuit  103 . 
     In FIG. 5, the Direct Rambus memory device  102  is connected to the internal bus of integrated circuit  103  via the interface  101 ,  106  which consists of a buffer  101  and a controller  106 . Connected between the Direct Rambus  102  and the integrated circuit  103  is the buffer  101  and the interface controller  106 . The buffer  101  has an input  110  and an output  112 . Connected between the input  110  of the buffer  101  and the integrated circuit  103  is a first data bus  105   a  which has a transfer rate equal to that of the internal bus of the integrated circuit  103 , i.e. 32 bits at 100 MHz. Also connected between the input  110  of the buffer  101  and the integrated circuit  103  is a first control line  118 . Connected from the output  112  of the buffer  101  to the integrated circuit  103  is a second data bus  105   b  which has a transfer rate equal to the first data bus  105   a . It will be appreciated that first and second data buses  105   a  and  105   b  in fact comprise the same data bus and are shown separately in FIG. 5 merely for the sake of convenience. Also connected between the output  112  of the buffer  101  and the integrated circuit  103  is a second control line  120 . 
     Connected between the Direct Rambus  102  and the interface controller  106  is a third data bus  104 . The third data bus  104  has a data transfer rate of 128 bits at 100 MHz which is equal to the peak instantaneous data transfer rate of the Direct Rambus. Also connected between the Direct Rambus  102  and the interface controller  106  is a third Direct Rambus control line  122 . The interface controller  106  is connected to the buffer  101  by an interface bus  108 . The interface bus  108  comprises a plurality of individual data transfer lines  1081   l    108   i  . . . ,  108   n . There are n data transfer lines provided where n is the number of storage locations within buffer  101 . In practice, the buffer  101  may be a FIFO (first-in-first-out) register which passes data from its input to the output. 
     The operation of the circuit shown in FIG. 5 will now be described. Beginning from the initial conditions of the buffer  101  being empty and the Direct Rambus  102  having all its pages closed, the integrated circuit  103  loads a memory request, MEM REQ, into the buffer  101  via the first data bus  105   a . The memory request M-REQ may be a request to access (read) data stored in the Direct Rambus memory  102  or it may be a request to write data to the Direct Rambus memory  102 . If the memory request is a request to write data to the Direct Rambus memory  102 , the data to be written is also loaded into the buffer  101  via the data bus  105   a . Control signals for controlling the operation of the buffer  101  are also output from the integrated circuit  103  via the first control line  118 . 
     The interface controller  106  scans the storage locations of the buffer  101  in turn, starting from the nth data storage location and when a data storage location is scanned which contains a memory request the memory request is output from the buffer  101  via the corresponding data transfer line  108   n  to the interface controller  106 . The interface controller  106  scans a number of data storage locations within buffer  101  and multiplexes the memory request signals and any corresponding data onto the third data bus  104  such that the memory requests are input to the Direct Rambus  102 . The Direct Rambus  102  now begins the action of opening a page in the memory array in order to supply the requested data or to write the supplied data in the relevant memory location. As previously discussed, there is a delay of 6 cycles while the page is opened before any data can be output from the Direct Rambus  102 . During this delay, the integrated circuit  103  may be outputting further memory requests to the buffer  101 . These further memory requests are stored in the buffer  101  during the delay period which occurs while the Direct Rambus  102  is opening the page associated with the first memory request. 
     When the Direct Rambus  102  has opened the page associated with the first memory request, if that memory request is a request to access data from the Direct Rambus, the requested data is then transferred from the Direct Rambus  102  via the third data bus  104  to the interface controller  106 . Control signals associated with controlling the input and output from the Direct Rambus  102  are passed between the Direct Rambus  102  and the interface controller  106  by the third control line  122 . The interface controller  106  demultiplexes the data received from the Direct Rambus  102  and inputs it via data transfer lines  108   i  to empty data storage locations within buffer  101 . The interface controller  106  then scans the data storage locations within the buffer  101  for the next memory requests which are to be served. The data accessed from the Direct Rambus  102  in response to a memory request received from the integrated circuit  103  via the interface controller  106  and buffer  101 , is passed through the buffer  101  and output at output  112  to the integrated circuit  103  via the second data bus  105   b , together with associated control signals via the second control line  120 . 
     The buffer  101  serves two functions. Firstly, it is able to buffer the memory requests from the integrated circuit  103  to the Direct Rambus  102 , allowing the integrated circuit to output a number of memory requests without having to wait for each of those requests to be served by the Direct Rambus  102  before outputting subsequent requests. Secondly, the buffer  101  is able to buffer the data supplied from the Direct Rambus  102  before it is transmitted to the integrated circuit  103 . Preferably, the buffer  101  should always have space available to store the accessed data from the Direct Rambus  102  thus enabling the Direct Rambus  102  to always output data at its maximum speed of 16 bits at 800 MHz. 
     To calculate the required size of the buffer it is assumed that the integrated circuit  103  will output ICY consecutive memory requests. The Direct Rambus  102  will see these as MemY accesses, as MemY=ICY*(integrated circuit internal bus width÷Direct Rambus internal bus width). It is assumed that all of the memory requests are in the same page in the Direct Rambus  102 . The Direct Rambus  102  can process MemY−1 memory requests in MemY−1 cycles. During the same time, the integrated circuit  103  can issue X memory requests, where 
     
       
           X= ( MemY −1)*(Direct Rambus bus width÷ ic  bus width) 
       
     
     Approximating MemY−1 to MemY we have buffer size (in memory word units) 
     =MemY−X 
     =MemY(1−(icclk/memclk)*(icwd/memwd)) 
     where icclk=integrated circuit clock speed, memclk=Direct Rambus clock speed, icwd=integrated circuit internal bus width, and memwd=Direct Rambus internal bus width. 
     FIG. 6 demonstrates the behaviour of the buffer when the size of the buffer has been correctly chosen. Line a represents the number of memory requests stored in the buffer which are yet to be processed by the memory and line b represents the number of memory requests which have been processed by the memory with the associated data being stored in the buffer. The distance between lines a and b represents the total amount of data stored in the buffer. Initially line a rises sharply over the time period t op , as the buffer stores an increasing number of memory requests from the integrated circuit. This initial sharp rise occurs during the delay caused by the memory opening the required page of the first memory request. Once the required page has been opened, the memory is able to output the requested data to the buffer and begin to process the next memory request stored in the buffer. If subsequent memory requests require the same page which is currently open, a number of bursts of requested data may be output to the buffer. Thus, line a begins to fall as the number of memory requests in the buffer falls, and line b begins to fall, representing an increase in the amount of data stored in the buffer. This is indicated on FIG. 5 by the period t serv . 
     At a subsequent point in time, point c, it will be necessary to close the currently open page and open a new page in the memory, causing the delay in outputting data from the memory as previously discussed. The memory is said, at this point, to be ‘stalled’. Whilst the memory is stalled lines a and b rise again as the number of memory requests stored in the buffer once again rises and the amount of data stored in the buffer from the memory decreases. This is shown by the period t stall . This action continues over time with lines a and b rising and falling together. It can be seen that the distance between lines a and b which represents the total amount of data contained within the buffer remains approximately constant. 
     Embodiments of the present invention operate particularly advantageously when memory requests are issued by the integrated circuit  103  in short bursts. When this occurs data stored in the buffer in response to memory requests can always be output to the integrated circuit  103  at the same time that the Direct Rambus  102  is stalled during the opening of a further page in the memory. If the number of memory requests issued by the integrated circuit  103  at any one time is too large then when those requests are served by the Direct Rambus  102  the average rate of data transfer will increase to a value which is in excess of the transfer rate of the internal bus within the integrated circuit  103  and the Direct Rambus  102  will be forced to wait for previously served memory requests to be delivered to the integrated circuit before the Direct Rambus can output any further data. However, this occasional loss in performance may not be fatal in some embodiments of the invention to the operation of the integrated circuit, it will be more cost effective in terms of design effort and silicon area consumed to use the buffering arrangement of embodiments of the present invention. 
     A further embodiment of the present invention is shown in FIG.  7 . In this embodiment a number of buffers  201   a ,  201   b ,  201   c  are connected via a demultiplexer  130  to the output of a Direct Rambus  202 . Each of the buffers  201   a ,  201   b  and  201   c  are of the same type as the buffer  101  shown in FIG.  5  and described hereinbefore. Each of the integrated circuits  203   a ,  203   b ,  203   c  is of the same type as integrated circuit  103  shown in FIG.  5 . Each buffer is connected to the internal bus of an integrated circuit  203   a ,  203   b ,  203   c , each of the internal buses having a lower peak data transfer rate than that of the Direct Rambus  202 . Each of the integrated circuits may have a different function from each other and may thus make different memory requests to the Direct Rambus  202  at different times. The Direct Rambus  202  provides the data in response to these requests and outputs the data to the demultiplexer  130  which is arranged to route the data to whichever of the buffers  201   a ,  201   b  or  201   c  issued the memory requests. In this embodiment, by providing a number of buffers connected to the Direct Rambus  202  a higher maximum average data transfer rate from the Direct Rambus can be achieved as the data output from the memory is stored in a number of different buffers. If, as shown, the number of buffers provided is three, then this allows three times the maximum average data transfer rate from the Direct Rambus before the Direct Rambus is forced to wait for the slower internal buses of the integrated circuit to retrieve the stored memory data from the relevant buffers. 
     It will be appreciated that although embodiments of this invention have been described with reference to memory devices, the present invention is equally applicable to any data storage device which outputs data at a high data transfer rate, but in short, relatively infrequent bursts. An example of such an alternative system is a sensor apparatus where sensed information is stored by the sensor and is output to further system elements in periodic bursts of data. 
     As described hereinbefore, a typical DRAM device comprises a number of banks of memory cells and that delays occur in accessing different pages within a bank. 
     Reference will now be made to FIG. 8 which shows a sequence of steps which occurs when eight words from a first selected row R in a first memory bank  31  of a DRAM device as shown in FIG. 3 are read and then eight words from a second selected row in the second memory bank  32  are read. As can be seen, the first six cycles A are used to open the first selected row R and read the first required word in that row of the first memory bank  31 . The next seven cycles B are used to read the remaining required seven words in the opened row R. The next three cycles D are required to close the first selected row R in the first bank  31 . The next six cycles E are used to open the second selected row R in the second memory bank  32  and read the first required word from that row. The next seven cycles F are required to read the other seven required words in the second selected row. The last three cycles G are required to close the second selected row R in the second memory bank  32 . Thus, in order to read eight words from a given row in a memory bank requires 16 cycles even though the reading operation itself only requires 8 cycles. This therefore reduces the efficiency of the memory device and increases the time required in order to complete read and write operations. 
     It would therefore be advantageous to further reduce the delays in performing read and write operations in respect of a memory having a plurality of memory regions. Circuitry for enabling this function may be provided within the controller  106  of the present invention. It is possible for one memory bank to be accessed whilst the other memory bank was being selected or deselected, such that the number of cycles required to access a burst may be reduced as compared to the prior art described in relation to FIG.  3 . 
     Reference will now be made to FIG. 9 which illustrates a controller  106  embodying suitable circuitry in conjunction with a SDRAM  20  and the buffer  101 . As with the DRAM shown in FIG. 3, the SDRAM  50  comprises a first and a second memory bank  51   a  and  52   b . Each memory bank  51   a  and  51   b  has a plurality of rows and columns which define memory locations. Each memory bank  51   a  and  51   b  is also provided with a row control unit  90  and a column control unit  88 . 
     The controller  106  receives requests to access particular locations in the SDRAM from the IC  103  as described with reference to FIG.  5 . Each request will identify the memory bank of the SDRAM  50 , the row (page) and the column which are to be accessed. The requests output from the IC  103  are input to the buffer  101  where they are stored in the order in which they are received. The requests are output by the buffer  101  in the same order in which they are received. 
     Each location of the buffer  101  which stores an address provides an output  58  to a detection circuit  60 . In the example shown in FIG. 9, the buffer  101  is able to store nine different requests and accordingly the buffer has nine outputs  58  which are connected to the detection circuit  60 . The detection circuit  60  is arranged to detect which row (page) in the first bank  51   a  of the SDRAM  50  is next to be accessed as well as the next page which is to be accessed in the second bank  51   b  of the SDRAM  50 . In the example illustrated in FIG. 9, page N is the next page to be accessed in the second bank  51   b  whilst page J is the next page to be accessed in the first bank  51   a . The detection circuit  60  will generally be a combinatorial logic circuit but can take any other suitable form. 
     The detection circuit  60  provides two outputs  62  and  64 . The first output  62  contains information as to the next page to be accessed in the first bank  51   a . The second output  64  contains information as to the next page to be accessed in second bank  51   b . The first and second outputs  62  and  64  of the detection circuit  60  are connected to respective bank managers  66  and  68 . The first bank manager  66 , which receives the first output  62  from the detection circuit  60  also receives a second input  71  from a memory or buffer  70  which stores the current page which is currently open or selected in the first bank  51   a . The first bank manager  66  thus compares the page of the first bank  51   a  which is currently open with the next page of the first bank  51   a  which is to be accessed. The first bank manager  66  thus determines the next action for the first bank  51   a . If the current page which is open and the next page which is to be accessed are the same, the first bank manager  66  will indicate that the next action for the first bank  51   a  will be the accessing of the required location on the open page or alternatively provide no output. If the current page which is open and the page which is next to be accessed are not the same, the first bank manager  66  will provide an output  74  which indicates that the next action for the first bank  51   a  will be to close the page which is currently open and then to open the page which is next to be accessed. 
     In some circumstances, there will be no page which is currently open. The first bank manager  66  will still output the next action which is required at the first bank  51   a . In a preferred embodiment of the present invention the first bank manager  66  is arranged to output one action at a time. For example, the first bank manager  66  is arranged to provide an output which indicates that a page is to be closed. The first bank manager  66  is arranged to provide a subsequent output indicating which page in the first bank  51   a  is to be opened. This subsequent instruction may, but not necessarily be provided when the previous instruction has been completed or is likely to be completed shortly. In one arrangement of the illustrated example each instruction from the first bank manager  66  is output once. In other arrangements, each instruction is output until that instruction is acknowledged. 
     The second bank manager  68  operates in a similar manner to the first bank manager  66 . In particular, the second bank manager  68  receives the second output  64  from the detection circuit and an output  71  from a second memory  72  which stores information as to the page which is currently open in the second bank  51   b . As with the first bank manager  66 , the second bank manager  68  provides an output  76  which determines the next action for the second bank  51   b.    
     The two outputs  74  and  76  from the first and second bank managers  66  and  68  are input to a multiplexer  78  along with the next request which has the next address which is to be accessed. The next request which is to be accessed is output from the buffer  101  to the multiplexer  78 . This next request is the oldest request stored in the buffer  101 . The operation of the multiplexer  78  is controlled by a multiplexer controller  80  which arbitrates between the outputs  74  and  76  of the first and second bank managers  66  and  68  and the request output from the buffer  101 . The multiplexer controller  80  receives an input from the buffer  101  which provides the next request, an input from the first memory  70  as to which, if any, page is open in the first bank  51   a  and an input from the second memory  72  as to which, if any, page is currently open in the second bank  51   b . Based on this information, the multiplexer controller  80  decides what action should be carried out in the next clock cycle and controls the multiplexer  78 . Effectively the multiplexer controller  80  acts as an arbiter and provides a series of commands. Usually, but not necessarily, one command may be provided per clock cycle. An open or close page command may take priority over a read command so that one bank may have a page being opened/closed whilst the other bank is being read. However some commands may include the open and/or close page instruction within a single command with the read/write instructions. In other words, the multiplexer  78  provides a suitable multiplexed output of commands so that, for example, reading of one bank may take place whilst the other bank has a row which is being opened or closed. 
     The output of the multiplexer  78  is input to a memory input device  86 . The output of the multiplexer  78  constitutes the outputs of the memory interface  52 . Based on the input, the memory logic device  86  will cause a row to be opened or closed or a location to be accessed (read or written to). The memory logic device  86  controls the row and column control units  88  and  90  in accordance with the output from the controller  106 . The location can either be read or written to. The memory logic  86  acts as a, command decoder and also generates the control logic. 
     Because of the detection circuit  60 , it is possible to ensure that a row in one bank can be opened or closed at the same time that a row in the other bank is being read. This significantly reduces the number of cycles taken to complete operations. The column control units  88  controls the accessing of columns of the respective banks. The row control units  90  control the selection and deselection of the rows of the first and second banks  51   a  and  51   b.    
     The multiplexed output from the multiplexer  78  is received by the memory logic device  86 . As discussed hereinbefore, the output of the multiplexer  78  consists of a series of commands which are received by the memory logic device  86 . 
     Reference is made to FIG. 10 which shows an example of the timing where two eight word bursts are accessed. Each burst comprises eight words which are located in the same row of the same memory bank. Each burst may be accessed by a single command. The single command may also include the close page instruction. The first and second bursts are located in different banks. For the purposes of illustration, it will be assumed that the first burst requires access to the first bank  51   a  and the second burst requires access to the second bank  51   b . The first six cycles H are required in order to open the required page in the first memory bank  51   a  for the first word in the first burst. The first word and the first burst is also accessed at the same time. In the next seven cycles I, the remaining seven words of the first burst are accessed. At the same time that the row in the first bank  51   a  is being opened and subsequently read, the required row in the second bank  51   b  is also being opened, for example during the five cycles marked J. 
     Accordingly, when the first burst has been completely accessed, the required row of the second bank  51   b  can be immediately accessed in order to access the eight words of the second burst, this occurring in the eight cycles marked L. At the same that the second burst is being accessed, the row of the first bank  51   a  from which the first word was accessed can be closed in the three cycles marked M. In the six cycles marked N, which occur at the same time as five of the cycles marked L in which the words of the second burst are read, the next required row in the first memory bank  51   a  is opened and the first word of a third burst is read. 
     Thus, the first two eight word bursts can be read in 21 cycles. This compares favourably with the 29 cycles required with the known DRAM described in relation to FIG.  3 . Additionally, any subsequent burst in this mode will only require eight cycles. With the conventional DRAM described in relation to FIG. 3, any subsequent burst requires 16 cycles. 
     In the described circuitry of FIG. 9, the two banks cannot to read from or written to at the same time. However reading or writing in one bank can take place at the same time that the other bank is having a page opened or closed. 
     Reference is made to FIG. 11 which shows a modified version of the circuitry described hereinbefore. Two requesters  500  and  502  in the form of CPU 1  and CPU 2  are provided. The requests from the requesters  500  and  502  are input into respective first and second buffers  504  and  506 . These buffers are the same as buffer  101  of FIG.  9 . The output of first and second buffers  504  and  506  are input to a common unit  508  which includes the remaining circuitry of the controller  106  of FIG.  9 . The detection circuit (not shown), will thus look at the contents of both the first and second buffers  504  and  506  but will otherwise operate in a similar manner to that of FIG.  9 . The first and second buffers  504  and  506  and the common unit  508  define a memory interface unit  510 . The output of the common unit is input to the memory logic for a SDRAM  50 . 
     In preferred embodiments of the controller  106 , a different format is used for addressing the SDRAM. In typical SDRAMs the address is defined by bank, row and column, with the bank part being the most important part of the address and the column being the least important part of the address. In other words, one bank will have locations 0 to n−1 whilst the second bank will have locations n to 2n−1. However, in the controller  106  described hereinbefore, an address format of row, bank and column can be used with the row being the most important part of the address. In other words, the first row of the first bank will contain addresses 0 to M−1 (where M is the number of columns) and the first row of the second bank will have address locations M to 2M−1. The next rows in the first and second banks will have locations 2M to 4M−1 and so on. This means that when data is being written into the memory, the two banks are more likely to be equally used so that the advantages of the controller  106  can be achieved. 
     The input to the buffer is arranged to receive an input from a plurality of requesters. It is preferred that a respective register be provided for storing the requests from each requester. 
     In the example described hereinbefore, two banks are shown. However, it should be appreciated that any other suitable number of banks may be provided, for example four. 
     In the illustrated example, the buffer  101  can have any suitable number of locations. 
     In one modification to the hereinbefore described circuitry, the detection circuit  60  may be arranged to check that the buffer  101  does not contain any more requests for an open page before closing that page. This may involve reordering of the requests and accordingly additional storage capacity may be required in order to ensure that the information read out of the memory banks is ultimately output in the same order in which the requests are made. The additional memory may be required to receive the output of the memory banks. 
     It should be appreciated that whilst embodiments of the controller  106  have been described in relation to an SDRAM, the methodology is applicable to any other type of memory which has two separate banks or regions which cannot be accessed at the same time, for example, other types of DRAM.