Abstract:
It is possible to continuously obtain the function of FILO for plural sets of data while reducing regions for storing the data. A string of data input from a memory M0 is shifted to a memory M3, and is switched back and read from the memory M0. Consequently, the function of FILO can be obtained for the same string. On the other hand, a string of data input from the memory M3 is shifted to the memory M0, and is switched back and read from the memory M3. Consequently, the function of FILO can also be obtained for the same string. With these two data strings keeping a push-pull relationship each other, the data are shifted among the memories M0 to M3. Therefore, the number of elements of each data string is enough for that of required memories.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a storage, and more particularly to a bidirectional storage capable of continuously functioning of FILO (First In Last Out). 
     2. Description of the Background Art 
     A storage functioning of FILO (hereinafter such storage is also referred to as a &#34;FILO&#34;) sequentially inputs data to be output in order reverse to input order. 
     Since the FILO according to the prior art operates by pushing and popping, in the case where only one FILO is used, a second set of data cannot be processed of FILO immediately after a first set of data are processed of FILO. The reason is that the first set of data are destroyed if another set of data are input to the FILO before the first set of data are completely read. 
     FIG. 53 is a block diagram showing a structure in which two conventional FILOs are used to continuously obtain processings of FILO for first and second sets of data. Input data DIN is distributed for its each set and given to a FILO 1 and a FILO 2 by a demultiplexer DMUX0. Either of outputs from FILO 1 and FILO 2 is sent as output data DOUT by a multiplexer MUX0. 
     For example, when the input data DIN gives the first set of data to the demultiplexer DMUX0, the first set of data are sequentially input to the FILO1. When the input data DIN gives the second set of data to the demultiplexer DMUX0, the second set of data are sequentially input to the FILO2 and the data given to the FILO1 are output as the output data DOUT through the multiplexer MUX0 in reverse order. Then, when the data given to the FILO2 are output as the output data DOUT through the multiplexer MUX0 in reverse order, the input data DIN gives a third set of data to the demultiplexer DMUX0. 
     In the prior art, a pair of FILOs which are alternately selected should be used to continuously perform the processings of FILO for plural sets of data. Accordingly, a region for storing data should be twice as much as a set of data. 
     SUMMARY OF THE INVENTION 
     A first aspect of the present invention is directed to a bidirectional transfer type storage comprising 0th to (N-1)th storage units (N≧2), each of which stores data having a predetermined length, wherein input data form sets, each set having N input data, and are alternately input to the 0th and (N+K-1)th storage units, respective first input data of two of the sets are opposed to each other through the K storage units, and shift transfer is sequentially performed (N+K-1) times for the data stored in the 0th to (N+K-1)th storage units, and a direction of the shift transfer is alternately changed reversely so that the input data are switched back and output. 
     A second aspect of the present invention is directed to the bidirectional transfer type storage as defined in the first aspect of the present invention, wherein the input data are stored in the 0th storage unit if a control signal has a first value, and are stored in the Nth storage unit if the control signal has a second value, contents stored in the 0th storage unit to an (N-2)th storage unit are transferred to a 1st storage unit to the (N-1)th storage unit if the control signal has the first value, and contents stored in the 1st to (N-1)th storage units are transferred to the 0th to (N-2)th storage units if the control signal has the second value, the contents stored in the (N-1)th storage unit are employed as output data if the control signal has the first value, and the contents stored in the 0th storage unit are employed as the output data if the control signal has the second value, and the control signal is set to the second value corresponding to a first event that the input data are continuously written to the 0th storage unit N times, and is set to the first value corresponding to a second event that the input data are continuously written to the (N-1)th storage unit N times. 
     A third aspect of the present invention is directed to a bidirectional transfer type storage comprising a memory having 0th to (N+K-1)th storage units (N≧2, K≧0) which store data having a predetermined length and correspond to 0th to (N+K-1)th addresses, and operating on the basis of a clock, and an address generating section for generating read and write addresses of the memory, respectively, wherein input data form sets, each set having N input data, and are written following reading of contents stored in the storage units, and directions of movement of the read and write addresses are reversed corresponding to update of the sets. 
     A fourth aspect of the present invention is directed to the bidirectional transfer type storage as defined in the third aspect of the present invention, wherein the read address of the memory is set greater than the write address by the K and both the read address and the write address are increased by 1 in response to the clock if a control signal has a first value, the read address is set smaller than the write address by the K and both the read address and the write address are decreased by 1 in response to the clock if the control signal has a second value, the control signal is set to the second value corresponding to a first case where the write address specifies an (N-1)th storage unit, and is set to the first value corresponding to a second case where the write address specifies a Kth storage unit, the write address and the read address are set to the (N+K-1)th address and an (N-1)th address respectively after the write is performed for the (N-1)th storage unit, and the write address and the read address are set to the 0th address and a Kth address respectively after writing into the Kth storage unit. 
     A fifth aspect of the present invention is directed to the bidirectional transfer type storage as defined in the fourth aspect of the present invention, wherein write and read of the memory are performed in response to rise and fall of the clock respectively, and the write and read addresses are successively updated after the write of the memory. 
     A sixth aspect of the present invention is directed to a method for controlling input and output of a memory having 1st to (N+K)th storage units (N≧2, K&gt;0), each of which stores data having a predetermined length and correspond to 1st to (N+K)th addresses, and operating in response to a clock, comprising the steps of setting a read address of the memory greater than a write address by the K and increasing both the read address and the write address by 1 in response to the clock if a control signal has a first value, setting the read address smaller than the write address by the K and decreasing both the read address and the write address by 1 in response to the clock if the control signal has a second value, setting the control signal to the second value corresponding to a first case where the write address specifies an Nth storage unit, and to the first value corresponding to a second case where the write address specifies a (K+1)th storage unit, and setting the write address and the read address to a 0th address and a Kth address respectively after writing into the Kth storage unit. 
     According to the first aspect of the present invention, the directions in which a pair of input data are switched back and output are reverse to each other. By using (N+K) storage units, therefore, the function of FILO can continuously be displayed for a plurality of sets, each set having N data. 
     According to the second aspect of the present invention, a set of data A1, A2, . . . A(N-1) and AN are sequentially input to the 0th storage unit and are transferred to the 1st to (N-1)th storage units in this order if the control signal has the first value, for example. Then, the control signal is changed to have the second value so that the data AN, A(N-1), . . . , A2 and A1 are sequentially read from the 0th storage unit, and a next set of data B1, B2, . . . , B(N-1) and BN are sequentially input to an (N+K-1)th storage unit and are transferred to (N+K-2)th to Kth storage units in this order. Thereafter, the control signal is changed to have the first value so that the data BN, B(N-1), . . . , B2 and B1 are sequentially read from the (N+K-1)th storage unit. 
     Thus, directions of data transfer between the storage units are reversed to each other. Consequently, by using (N+K) storage units, the function of FILO can continuously be displayed for a plurality of sets, each set having N data. 
     According to the third aspect of the present invention, when the data are written to the storage units in one direction, they are read from the storage units in a reverse direction and new data are written following the read. Accordingly, the function of FILO can continuously be displayed for a plurality of sets, each set having N data. 
     According to the fourth aspect of the present invention, a set of data A1, A2, . . . , A(N-1) and AN are sequentially stored in the 0th to (N-1)th storage units if the control signal has the first value. Then, the control signal is changed to have the second value so that the data AN, A(N-1), . . . , A2 and A1 are sequentially read from the (N-1)th to 0th storage units, and a next set of data B1, B2, . . . , B(N-1) and BN are sequentially input to the (N+K-1)th to Kth storage units. Thereafter, the control signal is changed to have the first value so that the data BN, B(N-1), . . . , B2 and B1 are sequentially read from the Kth to (N+K-1)th storage units. 
     Thus, the data are input and output between the storage units by address operation in two directions reverse to each other. By utilizing an ordinary memory having (N+K) storage units, therefore, the function of FILO can continuously be displayed for a plurality of sets, each set having N data. 
     According to the fifth aspect of the present invention, processings of write, address modification and read are performed in this order. Therefore, the effects according to the third aspect of the present invention can particularly be obtained also in the case where K is equal to 0. Consequently, the storage units of the memory can be utilized most efficiently. 
     According to the sixth aspect of the present invention, a set of data A1, A2, . . . , A(N-1) and AN are sequentially stored in the 1st to Nth storage units if the control signal has the first value. Then, the control signal is changed to have the second value so that the data AN, A(N-1), . . . , A2 and A1 are sequentially read from the Nth to 1st storage units, and a next set of data B1, B2, . . . , B(N-1) and BN are sequentially input to the (N+K)th to (K+1)th storage units. Thereafter, the control signal is changed to have the first value so that the data BN, B(N-1), . . . , B2 and B1 are sequentially read from the (K+1)th to (N+K)th storage units. 
     Thus, the data are input and output between the storage units by address operation in two directions reverse to each other. By utilizing an ordinary memory having (N+K) storage units, therefore, the function of FILO can continuously be displayed for a plurality of sets, each set having N data. 
     In order to solve the above-mentioned problems, it is an object of the present invention to provide a bidirectional FILO in which data are transferred bidirectionally, that is, in a direction from top to bottom and a direction from bottom to top between a plurality of memory elements to reduce regions for storing the data and continuously display the function of FILO. 
     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 are flowcharts together showing operation of a bidirectional FILO according to a first embodiment; 
     FIG. 3 is a block diagram illustrating a structure of the bidirectional FILO according to the first embodiment; 
     FIGS. 4 to 16 are typical diagrams showing, in order, operation of the bidirectional FILO illustrated in FIG. 3; 
     FIG. 17 is a block diagram showing a structure of a bidirectional FILO generated on the basis of a list in Table 1; 
     FIGS. 18 to 27 are circuit diagrams together showing details of a structure of a logic circuit 110; 
     FIGS. 28 and 29 are flowcharts together showing operation of a bidirectional FILO according to a second embodiment; 
     FIG. 30 is a block diagram illustrating a structure of the bidirectional FILO according to the second embodiment; 
     FIGS. 31 to 43 are typical diagrams showing, in order, operation of the bidirectional FILO illustrated in FIG. 30; 
     FIGS. 44 to 52 are circuit diagrams together showing details of a structure of a bidirectional FILO generated on the basis of the contents of a HDL in Table 2; 
     FIG. 53 is a block diagram showing the back ground art. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     In a first embodiment, a shift register type bidirectional FILO will be described below. FIGS. 1 and 2 are flowcharts together showing operation of the bidirectional FILO according to the first embodiment. 
     FIGS. 1 and 2 are connected to each other by connectors J1 to J4. FIG. 3 is a block diagram illustrating a structure of the bidirectional FILO according to the first embodiment. FIGS. 4 to 16 are typical diagrams showing, in order, the operation of the bidirectional FILO in FIG. 3. 
     The bidirectional FILO shown in FIG. 3 comprises memories M0 to M3 forming a shift register, and control sections CNT1 to CNT3 for sequentially transferring data among the memories M0 to M3. More specifically, the control section CNT1 transfers data between the memories M0 and M1, the control section CNT2 transfers data between the memories M1 and M2, and the control section CNT3 transfers data between the memories M2 and M3. The control sections CNT1 to CNT3 operate synchronized with a clock CLK. The data is transferred in a direction from top to bottom (from M0 to M3) and a direction from bottom to top (from M3 to M0) in response to values &#34;0&#34; and &#34;1&#34; of a mode signal MODE, respectively. 
     A demultiplexer DMUX and a multiplexer MUX perform output and input on the basis of the value of the mode signal MODE, respectively. More specifically, the demultiplexer DMUX receives input data DIN, and outputs the input data DIN to output terminals having &#34;0&#34; and &#34;1&#34; appended thereto (hereinafter referred to as a &#34;0&#34;output terminal and a &#34;1&#34; output terminal) in response to the values 0 and &#34;1&#34; of the mode signal MODE, respectively. The multiplexer MUX outputs, as output data DOUT, data DOUT1 and DOUT2 input to input terminals having &#34;0&#34; and &#34;1&#34; appended thereto (hereinafter referred to as a &#34;0&#34; input terminal and a &#34;1&#34; input terminal) in response to the values &#34;0&#34; and &#34;1&#34; of the mode signal MODE. The data DOUT1 and DOUT2 are stored in the memories M3 and M0, respectively. 
     The mode signal MODE is generated by counters COUNT1 and COUNT2 and a comparator CMP. The counter COUNT1 counts a clock CLK to generate a mode control value MODE --  CNT. The comparator CMP compares the mode control value MODE --  CNT with a predetermined value, for example, a 2bit value &#34;11&#34; (&#34;3&#34; in a decimal number). The mode control value MODE --  CNT designates order of a memory for storing the input data DIN in a direction of movement of the data. In the case where the mode control value MODE --  CNT is coincident with a predetermined value (which means that the memory M3 stores the input data DIN if the mode signal MODE has the value &#34;0&#34; and the memory M0 stores the input data DIN if the mode signal MODE has the value &#34;1&#34;), the counter COUNT1 is reset to set the mode control value MODE --  CNT to &#34;0&#34; and to cause the counter COUNT2 to perform counting. The counter COUNT2 is a 1-bit counter which does not count the clock CLK but counts an output of the comparator CMP. A timing in which a value of the mode signal MODE output from the counter COUNT2 is changed is synchronized with the clock CLK. 
     The counter COUNT1 has a reset terminal R2 for performing reset in response to the output of the comparator CMP, and a reset terminal R for performing reset in response to a reset signal RST used for reset of the bidirectional FILO. Similarly, the counter COUNT2 also has a reset terminal R for performing reset in response to the reset signal RST. The memories M0 to M3 also have reset terminals R for performing reset in response to the reset signal RST. 
     In the flowchart shown in FIG. 1, step S101 is executed irrespective of the clock CLK. At step S101, it is decided whether a reset instruction is given or not. In conformity with FIG. 3, it is decided whether the reset signal RST falls or not. If the reset instruction is given (the reset signal RST falls), steps S113 to S115 shown in FIG. 2 are executed through the connector J2, and the routine returns to step S101 again through the connector J3. 
     At steps S113 to S115, processings of resetting the bidirectional FILO are executed. At step S113, the contents of 0th to (N-1)th memories, that is, those of all the memories are initialized. In conformity with FIG. 3, N is equal to 4. A transition from &#34;H&#34; to &#34;L&#34; is given to the reset terminal R so that the contents of the memories M0 to M3 are cleared. At step S114, an operation mode is set to a first mode. In conformity with FIG. 3, this indicates that the value of the mode signal MODE is set to &#34;0&#34; by giving the transition from &#34;H&#34; to &#34;L&#34; to the reset terminal R of the counter COUNT2. At step S115, similarly, the transition from &#34;H&#34; to &#34;L&#34; is given to the reset terminal R of the counter COUNT1 so that the mode control value MODE --  CNT is set to &#34;0&#34;. 
     Referring to steps S102 and S100 shown in FIG. 1, a corresponding mechanism is not particularly provided in the bidirectional FILO shown in FIG. 3. The memory is usually managed by using a wait instruction WT and an enable instruction EN, and the mechanism can also be implemented by a well-known technique. 
     At step S103, it is decided whether the operation mode is a first mode or a second mode. The first and second modes correspond to the values &#34;0&#34; and &#34;1&#34; of the mode signal MODE, respectively. Consequently, the control sections CNT1 to CNT3 transfer data in a direction from top to bottom and a direction from bottom to top. In the case where steps S113 to S115 are executed and step S103 is then executed for the first time, the mode signal MODE has the value &#34;0&#34; as a result of execution of step S114. Thereafter, the routine proceeds to step S104. 
     At step S104, the contents of the (N-1)th memory provided at the end are employed as output data. For this purpose, the multiplexer MUX employs the data DOUT1 as the output data DOUT corresponding to the value &#34;0&#34; of the mode signal MODE. FIG. 4 is a diagram showing this state. In the state where all the memories M0 to M3 are cleared to store &#34;0&#34;, the output data DOUT adopts the contents (data DOUT1) stored in the memory M3. 
     Steps S106, S108, S111 and S112 are executed synchronously with the clock CLK, for example, in response to a rise timing. At step S106, the contents of the 0th memory acting as a head memory to the (N-2)th memory acting as a second memory from the end are transferred to the 1st memory acting as a second memory from the head to the (N-1)th memory acting as an end memory. For this purpose, the control sections CNT1 to CNT3 transfer data in the direction from top to bottom among the memories M0 to M3. Accordingly, data &#34;0&#34;, &#34;0&#34; and &#34;0&#34; stored in the memories M0 to M2 in FIG. 4 are stored in the memories M1 to M3, respectively. At step S108, a value of input data is stored in the 0th memory acting as the head memory. For example, if A1 is given as a value of the input data DIN, data A1 is stored in the memory M0. FIG. 5 shows the contents stored in the memories M0 to M3 which are thus obtained. 
     At step S110, the mode control value MODE --  CNT is examined. In the state shown in FIG. 5, the value of the input data DIN is to be stored in the memory M0 and the mode control value MODE --  CNT is &#34;0&#34;. Consequently, the routine proceeds to step S112. At step S112, the mode control value MODE --  CNT is increased by &#34;1&#34;. This indicates that the memory for storing the input data DIN is sequentially moved. If the memory for storing the input data DIN corresponds to the head or end memory (that is, the mode control value MODE --  CNT equals to N-1), the routine proceeds from step S110 to step S111 where the value of the mode signal MODE is changed to modify the operation mode and set the mode control value MODE --  CNT to &#34;0&#34;. While the memory for storing the input data DIN does not correspond to the head or end memory, the routine returns to step S103 through the connector J1. 
     FIG. 6 shows the state in which step S104 is executed at a second time after step S114 is executed. In response to rise of the clock CLK, the data A1, &#34;0&#34; and &#34;0&#34; stored in the memories M0 to M2 in FIGS. 4 to 6 are stored in the memories M1 to M3 respectively (step S106), and a new value A2 of the input data DIN is stored in the memory M0 (step S108, FIG. 7). At step S112, the mode control value MODE --  CNT is increased, and is 2 in a decimal number and is not equal to N-1(=3). The routine returns from step S111 to step S104 through the connector J1 and step S103. FIG. 8 shows this state. 
     Such processings are repeated so that values A3 and A4 of the input data DIN which are sequentially updated are stored in the memory M0. At step S110 to be executed corresponding to the rise of the clock CLK when the data A4 is stored in the memory M0, the mode control value MODE --  CNT has already been set to &#34;3&#34;. Therefore, a processing of step S111 is executed. In other words, the mode control value MODE --  CNT is set to &#34;0&#34; and the mode signal MODE is set to &#34;1&#34;. For this purpose, in FIG. 3, the comparator CMP compares the mode control value MODE --  CNT with a binary number &#34;11&#34;. As a result, the counter COUNT1 is reset through the reset terminal R2, and the counter COUNT2 is caused to perform counting. FIG. 9 shows this state. 
     Immediately after the processing of step S111 is executed, it is decided that the operation mode is the second mode at step S103. The routine proceeds from step S103 to step S105. The contents of the 0th memory acting as the head memory are employed as the output data DOUT. For this purpose, the multiplexer MUX employs the data DOUT2 as the output data DOUT in response to the value &#34;1&#34; of the mode signal MODE. FIG. 10 shows this state. The output data DOUT adopts the data A4 stored in the memory M0. 
     Steps S107 and S109 are executed synchronously with the clock CLK, for example, in response to a rise timing. At step S107, the contents of the 1st memory acting as the second memory from the head to the (N-1)th memory acting as the end memory are transferred to the 0th memory acting as the head memory to the (N-2)th memory acting as the second memory from the end. For this purpose, the control sections CNT1 to CNT3 transfer data in the direction from bottom to top among the memories M0 to M3. Accordingly, data A3, A2 and A1 stored in the memories M1 to M3 in FIG. 10 are stored in the memories M0 to M2, respectively. At step S109, a value of input data is stored in the (N-1)th memory acting as the end memory. For example, if B1 is given as a value of the input data DIN, data B1 is stored in the memory M3. FIG. 11 shows the contents stored in the memories M0 to M3 which are thus obtained. 
     After step S109 is executed, steps S110 to S112 are executed in the same manner as in the first mode. Consequently, output data A3, A2 and A1 are sequentially obtained as shown in FIGS. 12 to 14. On the other hand, the contents of the memory M3 at the end are successively updated to new input data values B2, B3 and B4. 
     After the state shown in FIG. 14 is obtained by execution of step S109, step S111 is executed when the clock CLK rises. In the second mode, the value &#34;3&#34; is employed as the mode control value MODE --  CNT because it is supposed that the memory M0 is specified. The operation mode is set to the first mode and the mode control value MODE --  CNT is reset from &#34;3&#34; to &#34;0&#34;. As shown in FIG. 15, then, a new value of the input data DIN is updated to C1 and the data B4 stored in the memory M3 is output as the output data DOUT (step S104). The data B1, B2 and B3 stored in the memories M0 to M2 are stored in the memories M1 to M3, respectively (step S106). Data C1 is stored in the memory M0 (step S108, FIG. 16). 
     By sequentially employing, as the input data DIN, data (A1, A2, A3, A4), (B1, B2, B3, B4), . . . , each having a set of N values (N=4 in FIGS. 3 to 16), the output data DOUT (A4, A3, A2, A1), (B4, B3, B2, B1), . . . are obtained. In the bidirectional FILO comprising N memories, the function of FILO can continuously be displayed for a plurality of sets, each set having N data. The reason is that the data is transferred among N memories bidirectionally, that is, in the direction from top to bottom and the direction from bottom to top, and that the data is input and output in a so-called switchback manner. In the following, the set having N values of the data is sometimes handled in a unit of a packet. 
     While heads of different sets of data are adjacent to each other in the shift register type bidirectional FILO as shown in FIGS. 11 to 13, it is apparent that they may be opposed to each other with K(≧0) storage units interposed therebetween. In that case, (N+K) memory elements are necessary. A value of N at steps S104, S106, S107, S109 and S113 is changed to (N+K), the above-mentioned effects can be obtained. Also in this case, the value of N at step S110 is kept. 
     By properly selecting to transfer data in either direction among the memories M0 to M3 forming a part of a shift register, to store the input data in the head or end memory and to read the output data from the head or end memory, the bidirectional FILO can be implemented. Accordingly, it is not necessary to distinguish the control sections CNT1 to CNT3 from the demultiplexer DMUX. A circuit in which such a distinction is not made will be described below. A positive logic is employed (in which &#34;1&#34; and &#34;0&#34; correspond to &#34;H&#34; and &#34;L&#34;, respectively). 
     Table 1 shows a list using a hardware descriptive language (HDL) which describes the bidirectional FILO. By way of example, Verilog-HDL manufactured by Cadence Co., Ltd. has been used. 
     
                       TABLE 1______________________________________module bifilo1 (clk, rst, en, wt, din, dout);input           clk, rst, en, wt;input   [1:0]    din;output  [1:0]    dout;wire    [1:0]    dout1, dout2;reg     [1:0]    filo[0:3];reg             mode;reg     [1:0]    mode.sub.-- cnt;integer  i;always @(posedge clk or negedge rst) beginif(!rst) beginfor (i = 3; i &gt;= 0; i=i-1) begin      filo[i] &lt;= 2&#39;h0;endmode &lt;= 1&#39;b0;mode.sub.-- cnt &lt;= 2&#39;h0;end else beginif(!wt &amp;&amp; en) begin     case(mode)     1&#39;b0:begin      for (i = 3; i &gt; 0; i=i-1) begin          filo[i] &lt;= filo[i-1];      end      filo[0] &lt;= din;      mode.sub.-- cnt &lt;= mode.sub.-- cnt + 1&#39;b1;      if(mode.sub.-- cnt == 2&#39;d3) begin          mode &lt;= 1&#39;b1;          mode.sub.-- cnt &lt;= 2&#39;h0;      end     end     1&#39;b1:begin       for (i = 3; i &gt; 0; i=i-1) begin          filo[i-1] &lt;= filo[i];       end       filo[3] &lt;= din;       mode.sub.-- cnt &lt;= mode.sub.-- cnt + 1&#39;b1;       if(mode.sub.-- cnt == 2&#39;d3) begin          mode &lt;= 1&#39;b0;          mode.sub.-- cnt &lt;= 2&#39;h0;       end     end     endcaseend else if(!en) begin      mode.sub.-- cnt &lt;= 2&#39;h0;endendendassign dout1 = filo[3];assign dout2 = filo[0];assign dout = (mode == 1&#39;b0) ? dout1 : dout2;endmodule______________________________________ 
    
     In Table 1, a start of a module is shown on the 1st line of the list, and various declarations are made on the 2nd to 9th lines. Each packet has 4 data, and a bit width of each data is 2 bits. 
     FIG. 17 is a block diagram showing a structure of a bidirectional FILO generated on the basis of the list. In the list, variables written in small letters correspond to signals and data written in capital letters in the following drawings. 
     A logic circuit 110 inputs a clock CLK, 2-bit input data DIN&lt;1:0&gt;, a reset signal RST, a wait signal WT and an enable signal EN, and outputs a mode signal MODE, 2-bit data FILO &lt;0&gt;&lt;0&gt; and FILO &lt;0&gt;&lt;1&gt; (which form data DOUT2) corresponding to the data stored in the memory M0, and 2-bit data FILO &lt;3&gt;&lt;0&gt; and FILO &lt;3&gt;&lt;1&gt; (which form data DOUT1) corresponding to the data stored in the memory M3. 
     Inverters G1 and G2 and composite gates G3 and G4 correspond to the multiplexer MUX so that a pair of FILO &lt;3&gt;&lt;0&gt; and FILO &lt;3&gt;&lt;1&gt; and a pair of FILO &lt;0&gt;&lt;0&gt; and FILO &lt;0&gt;&lt;1&gt; are output as 2-bit output data DOUT&lt;1:0&gt; corresponding to the values &#34;0&#34; and &#34;1&#34; of the mode signal MODE, respectively. 
     Such a structure is defined by assign statements on the 48th to 50th lines of the list, and corresponds to steps S103 to S105 shown in FIG. 1. 
     The contents shown in the 10th to 47th lines of the list define the logic circuit 110, and FIGS. 18 to 27 are circuit diagrams together showing details of the structure of the logic circuit 110. 
     An &#34;always&#34; statement on the 10th line of the list indicates that processings on the 11th to 47th lines are performed when the clock CLK rises or the reset signal falls. Description on the 11th line which recognizes that the reset signal RST falls irrespective of the clock CLK is equivalent to step S101 shown in FIG. 1, and steps S113 to S115 (FIG. 2) of performing reset processings are equivalent to the 12th to 14th lines, the 15th line and the 16th line, respectively. 
     Step S102 shown in FIG. 1 is equivalent to the 18th line of the list. Steps S103 to S112 to be processed when decision of step S102 is &#34;Y&#34; are equivalent to the 19th to 42nd lines. A processing to be performed when the wait signal WT is &#34;1&#34; is not described on the 17th to 46th lines. Therefore, if the wait signal WT is &#34;1&#34;, all the processings of steps S103 to S112 are waited for. This corresponds to a processing in which the routine returns to step S101 through step S100 if the decision of step S102 is &#34;N&#34;. On the 43rd to 45th lines, it is defined that a 2-bit value &#34;00&#34; is given to the mode control value MODE --  CNT if the enable signal EN is &#34;L&#34;. This corresponds to step S100. 
     On the list, the 19th line corresponds to step S103, processings at and after step S106 are defined on the 20th to 30th lines, and processings at and after step S107 are defined on the 31st to 41st lines. 
     More specifically, if the operation mode is set to a first mode (a mode signal MODE=0), step S106 corresponds to the 21st to 23rd lines, step S108 corresponds to the 24th line, step S110 corresponds to the 26th line, step S111 corresponds to the 27th and 28th lines, and step S112 corresponds to the 25th line. A non-blocking assignment statement is used on the 25th and 28th lines where procedure assignment to the mode control value MODE --  CNT is performed. Accordingly, the flowchart shown in FIG. 2 corresponds to the 25th and 28th lines irrespective of order of description on the list. 
     Similarly, if the operation mode is set to a second mode (the mode signal MODE=1), step S107 corresponds to the 32nd to 34th lines, step S109 corresponds to the 35th line, step S110 corresponds to the 37th line, step S111 corresponds to the 38th and 39th lines, and step S112 corresponds to the 36th line. Also in this operation mode, the non-blocking assignment statement is used on the 36th and 39th lines where the procedure assignment to the mode control value MODE --  CNT is performed. Accordingly, the flowchart shown in FIG. 2 corresponds to the 36th and 39th lines irrespective of the order of description on the list. 
     FIG. 18 shows a portion for generating a value MODE --  CNT&lt;0&gt; of the 10th bit of the mode control value MODE --  CNT and a normal operation signal WTEN. The normal operation signal WTEN is set to &#34;H&#34; only when the wait signal WT is &#34;L&#34; and the enable signal EN is &#34;H&#34;, which corresponds to the decision of &#34;Y&#34; at step S102 according to the flowchart and corresponds to execution of the processings described on the 19th to 42nd lines according to the list. 
     A D flip-flop MDCT0 outputs the mode control value MODE --  CNT&lt;0&gt; of the 0th bit. The mode control value MODE --  CNT&lt;0&gt; is logically inverted to generate a signal X1. If the normal operation signal WTEN is &#34;H&#34;, the signal X1 is sent to a D input terminal of the D flip-flop MDCT0. Therefore, every time a clock CLK is sent to a clock terminal T of the D flip-flop MDCT0, &#34;0&#34; and &#34;1&#34; are alternately output as the mode control value MODE --  CNT&lt;0&gt;of the 0th bit. The D flip-flop MDCT0 is also provided with a reset terminal R for performing reset when a reset signal RST falls. 
     FIG. 19 shows a portion for generating a mode control value MODE --  CNT&lt;1&gt; of a 1st bit. A D flip-flop MDCT1 outputs the mode control value MODE --  CNT&lt;1&gt; of the 1st bit. In response to the mode control value MODE --  CNT&lt;1&gt; of &#34;0&#34; and &#34;1&#34;, a selector SEL logically inverts data sent to A and B input terminals, and outputs the logically inverted data to a D input terminal of the D flip-flop MDCT1. If the normal operation signal WTEN is &#34;H&#34;, logical inversion of the mode control value MODE --  CNT&lt;0&gt; of the 0th bit and logical inversion of the signal X1 (that is, the mode control value MODE --  CNT&lt;0&gt; of the 0th bit) are given to the A and B input terminals of the selector SEL, respectively. 
     As described above, the structures shown in FIGS. 18 and 19 almost correspond to the counter COUNT1 shown in FIG. 3. However, FIGS. 18 and 19 show quaternary counters which do not require feedback of the comparator CMP differently from the counter COUNT1. 
     FIG. 20 shows a portion for generating a mode signal MODE. If the normal operation signal WTEN is &#34;H&#34;, a logical product of the mode control values MODE --  NT&lt;0&gt; and MODE --  CNT&lt;1&gt; is obtained to decide whether or not the mode control value MODE --  CNT is coincident with a value &#34;3&#34; in a decimal number (this processing is equivalent to a processing of step S110). In order to perform a processing of step S111, furthermore, an exclusive-OR operation between the logical product of the mode control values MODE --  CNT&lt;0&gt; and MODE --  CNT&lt;1&gt; and the mode signal MODE is conducted to the D input terminal of the D flip-flop MD. The D flip-flop MD outputs the mode signal MODE. 
     FIG. 21 shows a structure including D flip-flops M00 and M01 corresponding to 0th and 1st bits of the memory M0. Outputs of composite gates G5 and G6 are given to respective D input terminals. Respective outputs FILO &lt;0&gt;&lt;0&gt; and FILO &lt;0&gt;&lt;1&gt; of the D flip-flops M00 and M01 are equivalent to 2-bit information stored in the memory M0. 
     2-bit input data DIN&lt;0&gt; and DIN&lt;1&gt; are logically inverted and input to the composite gates G6 and G5, respectively. The reason why the input data DIN&lt;0&gt; and DIN&lt;1&gt; are logically inverted is that final output stages of the composite gates G5 and G6 are NAND gates. 
     FIG. 22 shows a structure including a D flip-flop M11 corresponding to a 1st bit of the memory M1, and FIG. 23 shows a structure including a D flip-flop M10 corresponding to a 0th bit of the memory M1. Outputs of composite gates G7 and G8 are given to respective D input terminals of the D flip-flops M11 and M10. Respective outputs FILO &lt;1&gt;&lt;0&gt; and FILO &lt;1&gt;&lt;1&gt; of the D flip-flops M10 and M11 are equivalent to 2-bit information stored in the memory M1. 
     FIG. 24 shows a structure including a D flip-flop M21 corresponding to a 1st bit of the memory M2, and FIG. 25 shows a structure including a D flip-flop M20 corresponding to a 0th bit of the memory M2. Outputs of composite gates G9 and G10 are given to respective D input terminals of the D flip-flops M21 and M20. Respective outputs FILO &lt;2&gt;&lt;0&gt; and FILO &lt;2&gt;&lt;1&gt; of the D flip-flops M20 and M21 are equivalent to 2-bit information stored in the memory M2. 
     FIG. 26 shows a structure including a D flip-flop M31 corresponding to a 1st bit of the memory M3, and FIG. 27 shows a structure including a D flip-flop M30 corresponding to a 0th bit of the memory M3. Outputs of composite gates G11 and G12 are given to respective D input terminals of the D flip-flops M31 and M30. Respective outputs FILO &lt;3&gt;&lt;0&gt; and FILO &lt;3&gt;&lt;1&gt; of the D flip-flops M30 and M31 are equivalent to 2-bit information stored in the memory M3. Signals X13 and X12 are obtained by logically inverting the input data DIN&lt;0&gt; and DIN&lt;1&gt;, and are input to the composite gates G12 and G11, respectively. 
     The composite gates G5 to G12 implement the function of integrating the control sections CNT1 to CNT3 and the demultiplexer DMUX shown in FIG. 3 together with an inverter (including a NAND gate functioning as the inverter when the normal operation signal WTEN is set to &#34;H&#34;). 
     In the drawings, if the normal operation signal WTEN is &#34;H&#34;, signals X2 and X4 have the same logic as that of the mode signal MODE, respectively. Irrespective of the value of the normal operation signal WTEN, signals X5, X3, X9, X7, X11, X10, X8 and X6 are obtained by logically inverting FILO &lt;0&gt;&lt;0&gt;, FILO &lt;0&gt;&lt;1&gt;, FILO &lt;1&gt;&lt;0&gt;, FILO &lt;1&gt;&lt;1&gt;, FILO &lt;2&gt;&lt;0&gt;, FILO &lt;3&gt;&lt;0&gt; and FILO &lt;3&gt;&lt;1&gt;, respectively. 
     In the case where the wait signal WT is set to &#34;H&#34; to give a wait instruction, the normal operation signal WTEN is set to &#34;L&#34; so that none of the contents of the D flip-flops is changed by operation of the composite gates G5 to G12. Also in the case where the enable signal EN is set to &#34;L&#34; and is not brought into the enable state, the normal operation signal WTEN is set to &#34;L&#34; so that &#34;0&#34; is given to the D input terminals of the D flip-flops MDCT0 and MDCT1 to execute step S100. Furthermore, in the case where the reset signal RST falls to receive a reset instruction, the processings of steps S113 to S115 are performed. 
     Second Embodiment 
     In a second embodiment, an address pointer type bidirectional FILO will be described below. FIGS. 28 and 29 are flowcharts together showing operation of the bidirectional FILO according to the second embodiment. FIGS. 28 and 29 are connected to each other by connectors J5 and J7. FIG. 30 is a block diagram illustrating a structure of the bidirectional FILO according to the second embodiment. FIGS. 31 to 43 are typical diagrams showing, in order, operation of the bidirectional FILO in FIG. 30. 
     The bidirectional FILO shown in FIG. 30 comprises a memory R256 acting as a RAM, multiplexers MUX1 and MUX2, counters COUNT10, COUNT11 and COUNT2, and comparators CMP1 and CMP2. 
     The counters COUNT10 and COUNT11 are controlled such that counting directions are set to &#34;increase&#34; or &#34;decrease&#34; depending on whether a mode signal MODE generated by the counter COUNT2 has a value &#34;0&#34; or &#34;1&#34;, and generate 8-bit mode control values W --  MODE --  CNT and R --  MODE --  CNT. The mode control values W --  MODE --  CNT and R --  MODE --  CNT act as write and read addresses for the memory R256, respectively. 
     Each of the counters COUNT10, COUNT11 and COUNT2 has a clock terminal T for receiving a clock CLK. The counters COUNT10 and COUNT11 count rise of the clock CLK. On the other hand, the counter COUNT2 is a 1-bit counter which does not count the clock CLK but counts outputs of the comparators CMP1 and CMP2 in the same manner as in the first embodiment. A timing in which the value of the mode signal MODE output from the counter COUNT2 is changed synchronous with the clock CLK. A reset signal RST or an enable signal EN is sent to enable terminals E of the counters COUNT10, COUNT11 and COUNT2 to control operation. 
     Initial values of the counters COUNT10 and COUNT11 are obtained from outputs of the multiplexers MUX1 and MUX2, respectively. The multiplexer MUX1 inputs 8-bit signals having values of 0 and 204 in decimal numbers, and the multiplexer MUX2 inputs 8-bit signals having values of 203 and 1 in decimal numbers. Either of the two inputs received by each of the multiplexers MUX1 and MUX2 is output depending on the outputs of the comparators CMP1 and CMP2. 
     A value of 1 in a decimal number and the mode control value W --  MODE --  CNT are given to the comparator CMP1. If both values are equal to each other, the multiplexers MUX1 and MUX2 are controlled to output &#34;0&#34; and &#34;1&#34;, respectively. A value of 203 in a decimal number and the mode control value W --  MODE --  CNT are given to the comparator CMP2. If both values are equal to each other, the multiplexers MUX1 and MUX2 are controlled to output &#34;204&#34; and &#34;203&#34;, respectively. 
     The clock CLK is sent to a write clock terminal WT and a read clock terminal RT of the memory R256 in common to write and read data to and from the memory R256 synchronized with rise and fall of the clock CLK, respectively. 
     Input data DIN is given to a write terminal DI so that data of the memory R256 is read from a read terminal DO. In FIG. 30, a multiplexer MUX3 is further provided. When a reset signal RST is deactivated (&#34;1&#34;), the data read from the memory R256 is employed as output data DOUT of the bidirectional FILO. When the reset signal RST is activated (&#34;0&#34;), the value &#34;0&#34; in a decimal number is employed as the output data DOUT of the bidirectional FILO. The reason will be described below. 
     FIGS. 31 to 43 show storage units #0 to #N corresponding to (N+1) addresses 0 to N. The memory R256 has 256 storage units, and N=204 is set. As a matter of course, other values can be used if N is less than 256. In that case, values given to the multiplexers MUX1 and MUX2 are N and (N-1) in place of 204 and 203, respectively. 
     In a flowchart shown in FIG. 28, steps S201 to S204 are executed irrespective of the clock CLK. At step S201, it is decided whether a reset instruction is received or not (whether the reset signal RST is activated or deactivated). If the reset instruction is received, reset processings of steps S215 to S219 are executed and the routine returns to step S201 through a connector J8. If the reset instruction is not received, it is decided whether the enable state is set or not at step S202 (whether the enable signal EN is activated or deactivated). If the enable state is not set, the processings of steps S218 and S219 are executed and the routine returns to step S201. At step S203, it is decided whether a wait instruction is given or not (whether a wait signal WT is activated or deactivated). If the wait instruction is not given, the routine proceeds to step S204. 
     The reset processing will be described below. At step S215, it is decided whether or not an initializing instruction for the memory R256 acting as the RAM is received. If the initializing instruction is received, the RAM is initialized. If an initializing signal SRAM sent to an initializing terminal S of the memory R256 is activated (&#34;0&#34;), a specific value, for example, &#34;0&#34; is given to all addresses of the memory R256 synchronized with rise of the clock CLK. More specific description will be given later. At this time, the read data output from the memory R256 does not make sense as the output data DOUT. Therefore, a specific value (which is set to &#34;0&#34; in FIG. 30 and coincident with a value used for initialization by chance) is separately employed as the output data DOUT by the multiplexer MUX3 (step S217). 
     At step S218, an operation mode is initialized to a first mode. At step S219, a write address and a read address are initialized to &#34;0&#34; and &#34;1&#34;, respectively. The initial values of the counters COUNT10 and COUNT11 are set to &#34;0&#34; and &#34;1&#34; by activation of the reset signal RST or deactivation of the enable signal EN, respectively. Also in the cases where the reset instruction is given and where the enable state is not set, the processings of steps S218 and S219 are executed. As far as the write and read addresses are concerned, the processings of steps S218 and S219 are identical. Therefore, the reset signal RST and the enable signal EN are used for the signal to the enable terminals E of the counters COUNT2, COUNT10 and COUNT11 in common. 
     FIG. 31 shows the state of the storage units #0 to #N obtained immediately after reset is performed. &#34;0&#34; is stored in all the storage units #0 to #N. 
     At step S204, it is decided whether the operation mode is a first mode or a second mode. In response to respective cases, the routine branches into step S205 or step S210 through the connector J5. 
     The first mode is equivalent to a case where the mode signal MODE is &#34;0&#34;. At step S205, the input data DIN is employed as the write data to the RAM. Write operation is performed synchronously with the rise of the clock CLK. 
     At step S206, it is decided whether the write address is (N-1) or not. This processing is executed by the comparator CMP2 for inputting the mode control value W --  MODE --  CNT and 203 (=204-1). As long as the write address is not (N-1), both the write address and the read address are increased by 1 at step S208. This processing is executed by the counters COUNT10 and COUNT11. While the processing of step S208 is also executed in response to the rise of the clock CLK, it is performed after step S205. 
     FIG. 32 shows processings for the storage units #0 to #N to be executed when the clock CLK first rises after the reset processing is performed. Since the write address has been set to &#34;0&#34; at step S219, a value A1 of the input data DIN is first stored in the storage unit #0 in accordance with step S205. Then, the read and write addresses are increased by 1 in accordance with step S208 respectively so that values &#34;1&#34; and &#34;2&#34; are taken. A broken line in FIG. 32 indicates the write operation of step S205, and a white arrow indicates operation of step S208 in which the address is changed in response to the rise of the clock CLK. 
     At step S209, the read data of the RAM is employed as output data synchronously with the fall of the clock CLK. Since the read address is set to &#34;2&#34; at step S208, the value &#34;0&#34; obtained after reset which has been stored in the storage unit #2 is read. FIG. 33 shows this state. Although the write address has the value &#34;1&#34;, the write operation is not performed in response to the fall of the clock CLK. At this time, consequently, the value stored in the storage unit #1 is not changed. 
     It is assumed that steps S206, S208 and S209 are repeated through steps S201 to S205 and the write and read addresses are set to (N-1) and N at step S208, respectively. (N-1) values A1 to A(N-1) have sequentially been stored in the storage units #0 to #(N-1) at step S205. FIG. 34 shows operation obtained when step S209 is executed in this state. At this time, the clock CLK has fallen just before, and new input data DIN has not been stored in the address #(N-1). On the other hand, the output data DOUT outputs the value &#34;0&#34; obtained after reset. 
     Then, step S205 is executed through a connector J6 and steps S201 to S204, and a value AN is written to the storage unit #(N-1). This state is shown by a broken line in FIG. 35. Since the write address is (N-1), the routine proceeds from step S206 to step S207 where the operation mode is changed to the second mode and the write and read addresses are set to N and (N-1), respectively. A solid line and a white arrow in FIG. 35 show this address modification. 
     Such address setting corresponds to a fact that the comparator CMP2 shown in FIG. 30 causes the multiplexers MUX1 and MUX2 to output 204 and 203, respectively. Consequently, the initial values of the counters COUNT10 and COUNT11 are reset. 
     In the same manner as step S208, step S207 is also executed after step S205 in response to the rise of the clock CLK. As shown in FIG. 35, the N data A1 to AN have sequentially been stored in the storage units #0 to #(N-1). 
     At step S209, the value stored in the storage unit #(N-1) is read. Accordingly, the value AN is obtained as a first value after &#34;0&#34; is continuously obtained as the output data DOUT (FIG. 36). 
     Furthermore, when the routine reaches step S204 through the connector J6 and steps S201 to S203, the operation mode is changed to the second operation mode. Therefore, step S210 is executed. Since the write address is not changed before step S210 is reached, a value B1 of the input data DIN is written to the storage unit #N. This state is shown by a broken line in FIG. 37. 
     Then, the routine proceeds to step S211 where it is decided whether the write address is 1 or not. This processing is performed by the comparator CMP1 for inputting the mode control value W --  MODE --  CNT and &#34;1&#34;. As long as the write address is not 1, the write and read addresses are decreased by 1 at step S213. This processing is performed by the counters COUNT10 and COUNT11. Step S211 is also executed in response to the rise of the clock CLK after step S210. Since the write address is N at step S211, the write and read addresses are set to (N-1) and (N-2) at step S213, respectively (see a white arrow and a solid line in FIG. 37). 
     Then, the routine proceeds to step S214 where the output data DOUT is obtained in the same manner as step S209. FIG. 38 shows the state in which a value A(N-1) is read from the storage unit #(N-2) synchronously with the fall of the clock CLK. 
     Thus, steps S210 to S213 and S214 are repeated through the connector J7 and steps S201 to S204, and the output data DOUT sequentially outputs the values AN, A(N-1), . . . . 
     It is assumed that the write and read addresses are set to 1 and 0 respectively at step S213. When the clock CLK then falls and step S214 is executed, the value A1 is obtained as the output data DOUT as shown in FIG. 39. The value stored in the storage unit #1, however, is still set to A2 and is not updated. 
     Then, step S210 is executed through the connector J7 and steps S201 and S204, and a value BN is written to the storage unit #1 (see a broken line in FIG. 40). Since the write address is 1, the routine proceeds from step S211 to step S212) where the operation mode is changed to the first mode and the write and read addresses are set to 0 and 1, respectively (see a white arrow and a solid line in FIG. 40). Such address setting corresponds to a fact that the comparator CMP1 shown in FIG. 30 causes the multiplexers MUX1 and MUX2 to output 0 and 1, respectively. Consequently, the initial values of the counters COUNT10 and COUNT11 are reset. 
     In the same manner as step S213, step S212 is also executed after step S210 in response to the rise of the clock CLK. FIG. 40 shows the state of the storage units #0 to #N obtained at this time. N data B1 to BN have sequentially been stored in the storage units #N to #1. 
     Then, the values stored in the storage unit #1 are read at step S214. Accordingly, the value BN is obtained as the output data DOUT after the values AN, A(N-1), . . . , A1 (FIG. 41). 
     Furthermore, when the routine proceeds to step S204 through the connector J7 and steps S201 to S203, the operation mode is changed to the first operation mode. Consequently, step S205 is executed. The write address is not changed before step S205 is reached. Therefore, a value C1 of the input data DIN is written to the storage unit #0. This state is shown by a broken line in FIG. 42. 
     Then, the routine proceeds from step S206 to step S208 where the write and read addresses are set to 1 and 2, respectively. A white arrow in FIG. 42 shows operation of step S208. 
     If a set of N (204 in FIG. 30) values (A1, A2, . . . , AN), (B1, B2, . . . , BN), . . . , are sequentially input as the input data DIN by using (N+1) memory elements as described above, (AN, . . . , A2, A1), (BN, . . . , B2, B1), . . . , are obtained as the output data DOUT. More specifically, the function of the FILO can continuously be displayed for plural sets of N data by using the RAM having at least (N+1) addresses. 
     The reason is that the input data is written following the reading of the contents stored in the storage units, and directions of movement of the read and write addresses are changed to those from top to bottom and from bottom to top corresponding to update of the set of input data. 
     According to the second embodiment described above, the bidirectional FILO can be implemented by utilizing the conventional RAM. 
     While the mode control value W --  MODE --  CNT and R --  MODE --  CNT have been shifted by 1 in the present embodiment, it is apparent that they may be shifted by K (an integer equal to or more than 0). In that case, the necessary memory elements are (N+K). At step S207, the write address is set to (N+K-1). At step S211, it is decided whether the write address is K or not. At steps S212 and S219, the read address is set to K. Even if K=0, write and read are performed in response to the rise and fall of the clock CLK, respectively. Therefore, the above-mentioned effects can be obtained. 
     Table 2 shows a list using a hardware descriptive language (HDL) which describes the bidirectional FILO according to the second embodiment. In the same manner as in Table 1, the Verilog-HDL manufactured by Cadence Co., Ltd. has been used. N=204 and K=1 are set. 
     
                       TABLE 2______________________________________module bifilo1 (clk, rst, en, wt, din, dout, sram);input            clk, rst, en, wt, sram;input    [7:0]    din;output   [7:0]    dout;wire     [7:0]    dout1, dout2, dout3, memo;reg      [7:0]   filo[0:203];reg              mode;reg      [7:0]   w.sub.-- mode.sub.-- cnt, r.sub.-- mode.sub.-- cnt;reg      [7:0]   dout;integer  i;always @(posedge clk) beginif(!rst) begin  mode &lt;= 1&#39;b0;  w.sub.-- mode.sub.-- cnt &lt;= 8&#39;h0;  r.sub.-- mode.sub.-- cnt &lt;= 8&#39;h1;end else begin   if(!wt &amp;&amp; en) begin   case(mode)   1&#39;b0:begin      w.sub.-- mode.sub.-- cnt &lt;= w.sub.-- mode.sub.-- cnt + 1&#39;b1;      r.sub.-- mode.sub.-- cnt &lt;= r.sub.-- mode.sub.-- cnt + 1&#39;b1;      if(w.sub.-- mode.sub.-- cnt == 8&#39;d203) begin         mode &lt;= 1&#39;b1;         w.sub.-- mode.sub.-- cnt &lt;= 8&#39;d204;         r.sub.-- mode.sub.-- cnt &lt;= 8&#39;d203;      end   end   1&#39;b1:begin      w.sub.-- mode.sub.-- cnt &lt;= w.sub.-- mode.sub.-- cnt - 1&#39;b1;      r.sub.-- mode.sub.-- cnt &lt;= r.sub.-- mode.sub.-- cnt - 1&#39;b1;      if(w.sub.-- mode.sub.-- cnt == 8&#39;d1) begin         mode &lt;= 1&#39;b0;         w.sub.-- mode.sub.-- cnt &lt;= 8&#39;1h0;         r.sub.-- mode.sub.-- cnt &lt;= 8&#39;d1;      end   end   endcase   end else if(!en) begin     w.sub.-- mode.sub.-- cnt &lt;= 8&#39;h0;     r.sub.-- mode.sub.-- cnt &lt;= 8&#39;h1;     mode &lt;= 1&#39;b0;   endendendalways @(posedge clk) beginif(!rst) begin  dout &lt;= 8&#39;h0;end else begin  if(!wt) begin     dout &lt;= memo;  endendendRAM256 mem256 (.T0(clk), .T1(clk), .A0(w.sub.-- mode.sub.-- cnt),.A1(r.sub.-- mode.sub.-- cnt),.DI0(din), .D01(memo), .rst(rst), .sram(sram));endmodule module RAM256 (T0, T1, A0, A1, DI0, D01, rst, sram);  input   T0;       // WRITE CLOCK  input   T1;       // READ CLOCK  input[7:0]          A0;        // WRITE ADDRESS  input[7:0]          A1;        // READ ADDRESS  input[7:0]          DI0;       // WRITE DATA  input   rst, sram;  output[7:0]          D01;       // READ DATA  reg[7:0]          WP, RP;always @(posedge T0) begin if(!rst &amp;&amp; sram) begin   {WP,RP} &lt;= 16&#39;h0;end else if(!sram) begin   RP &lt;= 8&#39;h0;   if(WP == 8&#39;d255) begin   WP &lt;= 8&#39;h0;   end else begin   WP &lt;= WP+1&#39;b1;   endendendwire[7:0] RPRAM = (sram == 1&#39;b0) ? RP : A1;wire[7:0] WPRAM = (sram == 1&#39;b0) ? WP : A0; RAM256C r256(T0, T1, WPRAM, RPRAM, DI0, D01);endmodulemodule RAM256C (T0, T1, A0, A1, DI0, D01);  input   T0;       // WRITE CL0CK  input   T1;       // READ CL0CK  input[7:0]          A0;       // WRITE ADDRESS  input[7:0]          A1;       // READ ADDRESS  input[7:0]          DI0;      // WRITE DATA  output[7:0]          D01;      // READ DATA  reg[7:0]          D01;  reg[7:0]  MEM0RY[255:0];  always @(negedge T1)   D01 &lt;= MEM0RY[A1];  always @(posedge T0)   MEM0RY[A0] &lt;= DIO[7:0];endmodule______________________________________ 
    
     The 1st to 56th lines of the list describe &#34;module bifilol&#34;, the 57th to 81st lines of the list describe &#34;module RAM256&#34;, and the 82nd to 95th lines of the list describe &#34;module RAM256C&#34;. 
     A start of a module is shown on the 1st line of the list, and various declarations are made on the 2nd to 10th lines. A start of a module is shown on the 57th line of the list, and various declarations are made on the 58th to 65th lines. A start of a module is shown on the 82nd line of the list, and various declarations are made on the 83rd to 90th lines. 
     The &#34;module RAM256C&#34; defines a core section of the RAM (including a portion corresponding to the storage unit shown in FIGS. 31 to 43) in which write and read are performed in response to the rise and fall of the clock CLK, respectively. The 66th to 79th lines define a part of a reset processing and an initialization processing. 
     Steps S215 and S216 to be executed through step S201 are equivalent to processings of an &#34;if-else&#34; statement shown on the 69th to 76th lines of the list, and to the 78th to 80th lines of the list for performing initialization through read and write addresses specified by variables RP and WP, respectively. 
     Referring to the reset processing, furthermore, step S217 is equivalent to the 47th line of the list, and steps S218 and S219 to be executed through step S201 are equivalent to the 12th to 15th lines of the list. 
     Steps S218 and S219 to be executed through step S202 are equivalent to the 38th to 41st lines of the list. If decision is &#34;Y&#34; at step S203, the output data is not updated corresponding to the 49th to 51st lines of the list. If the decision is &#34;N&#34; at step S203, processings defined by &#34;case&#34; statements on the 18th to 37th lines of the list are performed. 
     Referring to &#34;case&#34; statements on the 18th to 37th lines of the list, the 18th line of the list is equivalent to step S204, the 19th to 27th lines of the list are equivalent to steps S206 to S208, and the 28th to 36th lines of the list are equivalent to steps S211 to S213. Read and write from and to the RAM at steps S205, S209, S210 and S214 are defined by assignment statements on the 78th and 79th lines of the list and the &#34;module RAM256C&#34; shown in lines 82nd to 95th to be referred to on the 80th line. 
     Referring to the 19th to 27th lines of the list, step S206 is equivalent to the 22nd line of the list, step S207 is equivalent to the 23rd to 25th lines of the list, and step S208 is equivalent to the 20th and 21st lines of the list. 
     Referring to the 28th to 36th lines of the list, step S211 is equivalent to the 31st line of the list, step S212 is equivalent to the 32nd to 34th lines of the list, and step S213 is equivalent to the 29th and 30th lines of the list. 
     FIGS. 44 to 52 are circuit diagrams showing details of a structure of a bidirectional FILO generated on the basis of the contents of the HDL in Table 2. 
     A circuit shown in FIG. 44 inputs an enable signal EN, a reset signal RST, a wait signal WT, an 8-bit mode control value W --  MODE --  CNT&lt;0:7&gt;, and a signal Y3 generated by a circuit shown in FIG. 45, and outputs signals Y1, Y2, Y5, Y6 and W1 to W8. The signal W2 has a value complementary to the reset signal RST, the signal W4 has a value complementary to W --  MODE --  CNT&lt;3&gt;, the signal W5 has a value complementary to W --  MODE --  CNT&lt;1&gt;, the signal W6 has a value complementary to W --  MODE --  CNT&lt;6&gt;, the signal W7 has a value complementary to W --  MODE --  CNT&lt;7&gt;, the signal W8 has a value complementary to W --  MODE --  CNT&lt;0&gt;, the signal Y1 has &#34;1&#34; and &#34;/RST+/WT (/ and + denote logical inversion and logical sum, respectively)&#34; corresponding to the values &#34;0&#34; and &#34;1&#34; of the enable signal EN respectively, and the signal Y2 has &#34;0&#34; and &#34;RST•/WT&#34; (• denotes logical product) corresponding to the values &#34;0&#34; and &#34;1&#34; of the enable signal EN, respectively. 
     The circuit shown in FIG. 45 inputs the signals Y1, Y2, Y5, and W3 to W7 generated by the circuit shown in FIG. 44, the 8-bit mode control value W --  MODE --  CNT&lt;0:7&gt; and the clock CLK, and outputs signals Y3, Y4, Y7 and Y8, 8-bit data DT1&lt;0:7&gt; and a mode signal MODE. 
     In FIG. 45, a D flip-flop MD outputs the mode signal MODE corresponding to the COUNT2 in FIG. 30. An adder-subtracter 201 is controlled to add 1 to or subtract 1 from the mode control value W --  MODE --  CNT depending on the mode signal MODE. The signal Y7 has a value complementary to the mode signal MODE. 
     A circuit shown in FIG. 46 inputs the signal Y6 generated by the circuit shown in FIG. 44, the signals Y4 and Y7 and the mode signal MODE generated by the circuit shown in FIG. 45, and an 8-bit mode control value R --  MODE --  CNT&lt;0:7&gt;, and outputs a signal Y11 and 8-bit data DT2&lt;0:7&gt;. In FIG. 46, an adder-subtracter 202 is controlled to add 1 to or subtract 1 from the mode control value R --  MODE --  CNT depending on the mode signal MODE. 
     A circuit shown in FIG. 47 inputs the signals Y1, Y2, Y6 to Y9, and W4 to W8, a mode control value W --  MODE --  CNT&lt;3:5&gt; and the data DT1&lt;0:7&gt;, and outputs signals Y9, Y10 and N397 to N404. A circuit shown in FIG. 48 inputs the signals Y1, Y2 and Y9 to Y11, the mode control value R --  MODE --  CNT&lt;0:7&gt;and data DT2&lt;0:7&gt;, and outputs signals N405 to N412. 
     A circuit shown in FIG. 49 comprises a memory R256 acting as a RAM, a circuit W --  CNT for generating a mode control value W --  MODE --  CNT from the signals N397 to N404, and a circuit R --  CNT for generating a mode control value R --  MODE --  CNT from the signals N405 to N412. The mode control values W --  MODE --  CNT and R --  MODE --  CNT are given to a write address terminal WA and a read address terminal RA of the memory R256, respectively. The 8-bit input data DIN&lt;7:0&gt; is given to a write terminal DI and an 8-bit signal MEMO&lt;7:0&gt; is read from a read terminal DO. The clock CLK is sent to the circuits R --  CNT and W --  CNT, and write and read clock terminals WT and RT of the memory R256. The reset signal RST and the initializing signal SRAM are sent to a reset terminal R and an initializing terminal S of the R256, respectively. 
     A circuit shown in FIG. 50 is equivalent to the multiplexer MUX3 shown in FIG. 30, and comprises D flip-flops F0 to F7 for sending output data DOUT&lt;7:0&gt;. The D flip-flops F0 to F7 are controlled by the signal MEM0&lt;7:0&gt; obtained from the memory R256, the clock CLK, the wait signal WT and the signal W2. 
     FIG. 51 shows a structure of a circuit W --  CNT comprising D flip-flops W0 to W7. A D flip-flop Wp is controlled by the clock CLK, and outputs a mode control value W --  MODE --  CNT&lt;p&gt; (p=0 to 7) on receipt of a signal N (404-p). 
     FIG. 52 shows a structure of a circuit R --  CNT comprising D flip-flops R0 to R7. A D flip-flop Rq is controlled by the clock CLK, and outputs a mode control value R --  MODE--CNT&lt;q&gt; (q=0 to 7) on receipt of a signal N (412-q). 
     While the invention has been described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is understood that numerous other modifications and variations can be devised without departing from the scope of the invention.