Abstract:
An apparatus and method is disclosed for selecting data in a FIFO memory array made up of a plurality of memory cells arranged in rows and columns, where each row of cells has an associated number of word lines selectively addressable by an associated row address, and each column of cells has an associated bit line that provides access to the memory cells of the associated column as enabled by the respective word lines; and the memory array includes an address decoder having an address input for receiving an input address for selecting word lines in accordance with the input address, and a programmable-width vertical pointer for providing read and write input addresses to the address input of the address decoder during associated read and write operations of the memory array, where the programmable-width vertical pointer modifies the read and write addresses during operations of the memory array and provides a FIFO memory functionality.

Description:
BACKGROUND  
         [0001]    There are a variety of ways to organize and access data in integrated memories. Such memory access techniques include Random Access, Last In First Out (“LIFO”), and First In First Out (“FIFO”). In FIFO memories, the generally older data (first in) is released (first out) before the generally newer data is released. Such FIFO memories are generally used in data communications and digital processing systems.  
           [0002]    FIFO integrated memories generally have a fixed width chosen in accordance with a given data width requirement. Accordingly, different applications having different data widths typically require different FIFO memories for matching the different data width requirements.  
         SUMMARY  
         [0003]    This disclosure presents an apparatus and method for selecting data in a FIFO memory array made up of a plurality of memory cells arranged in rows and columns, where each row of cells has an associated number of word lines selectively addressable by an associated row address, and each column of cells has an associated bit line that provides access to the memory cells of the associated column as enabled by the respective word lines; and the memory array includes an address decoder having an address input for receiving an input address for selecting word lines in accordance with the input address, and a programmable-width vertical pointer for providing read and write input addresses to the address input of the address decoder during associated read and write operations of the memory array, where the programmable-width vertical pointer modifies the read and write addresses during operations of the memory array and provides a FIFO memory functionality. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]    The present disclosure will be better understood, and its numerous features and advantages will become apparent to those skilled in the pertinent art by referencing the accompanying drawings, wherein like elements are numbered alike in the several figures, in which:  
         [0005]    [0005]FIG. 1 is a schematic block diagram of a 2-D FIFO integrated memory device;  
         [0006]    [0006]FIG. 2 is a schematic block diagram of an input redirection unit of the 2-D FIFO integrated memory device of FIG. 1;  
         [0007]    [0007]FIG. 3 is a schematic block diagram of an output redirection unit of the 2-D FIFO integrated memory device of FIG. 1;  
         [0008]    [0008]FIG. 4 is a schematic block diagram of a 2-D FIFO integrated memory device;  
         [0009]    [0009]FIG. 5 is a schematic block diagram of an input redirection unit of the 2-D FIFO integrated memory device of FIG. 4; and  
         [0010]    [0010]FIG. 6 is a schematic block diagram of an output redirection unit of the 2-D FIFO integrated memory device of FIG. 4. 
     
    
     DETAILED DESCRIPTION  
       [0011]    As shown in FIG. 1, reference numeral  10  generally designates a two-dimensional FIFO integrated memory device. The 2-D FIFO integrated memory device  10  includes an input register  12  for receiving input data, which is coupled to an input redirection unit  14  for redirecting the received data to the appropriate location in a memory array  16 . The memory array  16  is coupled, in turn, to an output redirection unit  18 , which is itself coupled to an output register  20  for providing the output data.  
         [0012]    The memory array  16  includes a number (N) of banks  22  of single-dimensional memory, arranged as the individual rows of the memory array  16 . Each bank is a regular single-dimensional FIFO having a user-defined size where D is the width of the data signal, W is the number of write word enables, and R is the number of read word enables. Each input also includes parity bits associated with the data and word enable signals.  
         [0013]    Turning now to FIG. 2, a portion of the input redirection unit  14  of FIG. 1 is shown in greater detail. The portion shown is repeated for each bank, and includes a multiplexer (“MUX”)  24  for receiving the input data and sending the bits of the data to the associated bank  22  in accordance with the MUX select signals in the following tables and equations:  
                                                                                                                                       SELECT (0:S − 1, S = log 2  N)                0   1   2   . . .   S − 3   S − 2   S − 1                        BANK N − 1   D N−1     D N−2     D N−3     . . .   D2   D1   D0       BANK N − 2   D N−2     D N−3     D N−4     . . .   D1   D0   D N−1         BANK N − 3   D N−3     D N−4     D N−5     . . .   D0   D N−1     D N−2         .   .   .   .       .   .   .       .   .   .   .       .   .   .       .   .   .   .       .   .   .       BANK 2   D2   D1   D0   . . .   D5   D4   D3       BANK 1   D1   D0   D N−1     . . .   D4   D3   D2       BANK 0   D0   D N−1     D N−2     . . .   D3   D2   D1                    WR_DATA_SEL &gt;= WR_EXTRA_OFFSET + WR_VERT_PTR       RD_BEN_DATA_SEL &lt;= RD_BEN_EXTRA_OFFSET +       RD_VERT_PTR       WR_VERT_PTR IS INCREMENTED BY THE NUMBER OF WORDS       WRITTEN IN THE FIFO       •EACH “IN DATA X” CONSISTS OF A CORRESPONDING DATA       W OF PARITY BIT, RDBEnX AND WRBENx            WR_EXTRA_OFFSET   POSITION OF FIRST 1 IN WR_BEn               0   0       N − 1   1       N − 2   2       N − 3   3       3   N − 3       2   N − 2       1   N − 1               RD_BEN_EXTRA_OFFSET   POSITION OF FIRST 1 IN RD_BEn               0   0       N − 1   1       N − 2   2       N − 3   3       3   N − 3       2   N − 2       1   N − 1                  
 
         [0014]    With reference to FIG. 3, a portion of the output redirection unit  18  of FIG. 1 is shown in greater detail. The portion shown is repeated for each bank, and includes a MUX  26  for receiving the output data from the associated bank  22  and sending the bits of the data to the output register  20  of FIG. 1 in accordance with the MUX select signals in the following tables and equations:  
                                                                                             SIGNIFICANT   SELECT (0:S − 1, S = log 2  N)            RANGE   0   1   2   . . .   S − 3   S − 2   S − 1               N − 1   D N−1     D N−2     D N−3     . . .   D2   D1   D0       N − 2   D N−2     D N−3     D N−4     . . .   D1   D0   D N−1         N − 3   D N−3     D N−4     D N−5     . . .   D0   D N−1     D N−2         .   .   .   .       .   .   .       .   .   .   .       .   .   .       .   .   .   .       .   .   .       2   D2   D1   D0   . . .   D5   D4   D3       1   D1   D0   D N − 1     . . .   D4   D3   D2       0   D0   D N−1     D N−2     . . .   D3   D2   D1                    RD_DATA_SEL &lt;= RD_VERT_PTR − RD —         EXTRA_OFFSET RD_VERT_PTR IS INCREMENTED       BY THE NUMBER OF WORDS READ FROM THE FIFO                        RD_EXTRA_OFFSET   POSITION OF FIRST 1 IN RD_BEn                       0   0           1   1           2   2           3   3           .   .           .   .           .   .           N − 3   N − 3           N − 2   N − 2           N − 1   N − 1                      
 
         [0015]    In operation, 2-D FIFO memories according to FIGS. 1 through 3 combine the features of regular FIFOs with an ability to store and read a selectable number of k-bit  
         [0016]    The PCI bus standard, which supports both 32 and 64 bit bus widths, transfers 4 or 8 byte enables, respectively, with every 32 or 64 bits of data, respectively. These byte enables are used to determine which incoming bytes are valid. When using DMA to transfer data to or from system memory and the actual valid data is not aligned on 32 or 64-bit boundaries, respectively, the associated data transfer contains non-valid bytes. Current market implementation of PCI bridge chips are either forcing the system CPU to realign the data on a 64-bit boundary, for example, before starting the DMA transfer or they store in the receiving FIFO the actual value of the byte enable signals. The first case is causing a degradation of the system performance while the second case wastes memory space and discards non-valid bytes.  
         [0017]    2-D FIFOs according to the present disclosure may also have immediate application to bus size matching, allowing the connection of two different width data busses. For example, since 2-D FIFOs can be filled one or two bytes at a time, without penalty, and emptied 4 bytes at a time, they provide a solution for efficiently loading a 32-bit target device using an 8-bit bus or a 16-bit bus. Data is loaded on one of three possible 16-bit boundaries and corresponding byte enables are activated. Data would then be read 32 bits at a time.  
         [0018]    In addition, 2-D FIFOs according to the present disclosure may have immediate application to serial to parallel FIFO applications. If the size of the banks is set to 2 N ×1, then byte enables become bit enables and can be used to read and write individual bits. Serial data is connected to the input and only the bit enable line corresponding to the input signal is activated. A 2-D FIFO would then shift data internally without any need for user interaction. Data would then be read out of the 2D FIFO one word (or part of it) at a time.  
         [0019]    2-D FIFOs according to the present disclosure may also have immediate application to parallel to serial FIFOs. This FIFO can be implemented in a manner similar to the previous serial to parallel example. Parallel data is loaded into the 2-D FIFO in the usual manner. Serial data is then read out 1 bit at a time, by activating only one bit enable line at a time. Unlike standard FIFOs, 2-D FIFOs cover both serial-to-parallel and parallel-to-serial versions with the same hardware.  
         [0020]    Additionally, 2-D FIFOs according to the present disclosure may have immediate application to buffering the output of variable bit width devices. Certain devices, such as compression engines, generate outputs having variable quantities of bits. 2-D FIFOs may be used to assemble these outputs onto a desired fixed bus width.  
         [0021]    As shown in FIG. 4, reference numeral  110  generally designates a two-dimensional FIFO integrated memory device. The 2-D FIFO integrated memory device  110  includes an input register  112  for receiving input data, which is coupled to an input redirection unit  114  for redirecting the received data to the appropriate location in a memory array  116 . The memory array  116  is coupled, in turn, to an output redirection unit  118 , which is itself coupled to an output register  120  for holding the output data.  
         [0022]    The memory array  116  includes four banks  122  of single-dimensional memory, arranged as the individual rows of the memory array  116 . Each bank is a regular single-dimensional FIFO having a depth of 128 bytes and a width of 9 bits (8 data bits and a corresponding parity bit). The input data signal has a width of D=36 bits, a number of write word enables of W=4, and a number of read word enables of R=4.  
         [0023]    Turning now to FIG. 5, the input redirection unit  114  of FIG. 4 is shown in greater detail. The input redirection unit  114  includes a multiplexer (“MUX”)  24  for receiving the input data and sending the bits of the data to the associated bank  22  in accordance with the MUX select signals in the following equations and tables:  
         [0024]    WR_DATA_SEL&lt;=WR_EXTRA_OFFSET+WR_VERT_PTR  
         [0025]    RD_BEN_DATA_SEL&lt;=RD_BEN_EXTRA_OFFSET+RD_VERT_PTR WR_VERT_PTR IS INCREMENTED BY THE NUMBER OF 1&#39;s IN WR (3.0) ON EVERY CLOCK EDGE  
                                                                                                 WR_EXTRA_OFFSET   POSITION OF FIRST 1 IN WR_BEn                            0   0           3   1           2   2           1   3                            RD_BEN —     POSITION OF FIRST 1 IN RD_BEn           EXTRA_OFFSET                            0   0           3   1           2   2           1   3                                  
 
         [0026]    With reference to FIG. 6, the output redirection unit  118  of FIG. 4 is shown in greater detail. The output redirection unit  118  includes a MUX  126  for receiving the output data from the associated bank  122  and sending the bits of the data to the output register  120  of FIG. 1 in accordance with the MUX select signals in the following equations and table:  
         [0027]    RD_DATA_SEL&lt;=RD_VERT_PTR-RD_EXTRA_OFFSET RD_VERT_PTR IS INCREMENTED BY THE NUMBER OF 1&#39;s IN RD_(3.0) ON EVERY CLOCK EDGE  
                                                                 RD_EXTRA_OFFSET   POSITION OF FIRST 1 IN RD_BEn                                        0   0           1   1           2   2           3   3                      
 
         [0028]    In operation, the 2-D FIFO according to FIGS. 4 through 6 contains 4 memory banks and each memory bank is in itself a 128×9 mini FIFO. Data is written to a mini FIFO by activating the WRITE line corresponding to the particular mini FIFO. Similarly, data is read from the mini FIFO by activating the READ line. Each mini FIFO has EMPTY, NEAR_EMPTY, FULL and NEAR_FULL output lines that are used in the main 2-D FIFO.  
         [0029]    A 2-bit write vertical pointer (WR_VERT_PTR) points to the memory bank that will receive the next lowest significant byte from the input. Its value is incremented by the number of bytes written to the FIFO in the previous write sequence.  
         [0030]    A 2-bit read vertical pointer (RD_VERT_PTR) points to the memory bank that contains the byte that will be output as the lowest significant byte. Its value is incremented by the number of bytes read from the FIFO in the previous read sequence.  
         [0031]    The 2-D FIFO is able to redirect any byte lane to any memory bank in order to behave according to a design specification. This is achieved by using three banks of four 4:1 multiplexers for the following signals:  
         [0032]    Input data (including the parity bit): Data is broken up in four groups of 9 bits and applied to the multiplexer inputs according to the block diagram. Each output is applied to a memory bank. A select signal (WR_DATA_SEL) for this MUX is calculated by adding the value of the write vertical pointer (WR_VERT_PTR) to an extra offset (WR_EXTRA_OFFSET), calculated according to the table defined above.  
         [0033]    Write byte enables (WRBEn): Four write byte enables are applied to the multiplexer inputs according to the block diagram. Each output is applied to a memory bank. The select signal is WR_DATA_SELECT, described above.  
         [0034]    Read byte enables (RDBEn): Four read byte enables are applied to the multiplexer inputs according to the block diagram. Each output is applied to a memory bank. Select (RD_BEN_DATA_SEL) for this MUX is calculated by adding the value of the read vertical pointer (RD_VERT_PTR) with an offset (RD_BEN_EXTRA_OFFSET), calculated according to the table defined above.  
         [0035]    In addition to input redirection, the 2-D FIFO  110  is able to redirect the output of memory banks to appropriate byte lanes. This is achieved by the bank of four 4:1 multiplexers. Outputs of the memory banks are applied to the multiplexer inputs according to the block diagram. Multiplexer outputs are then combined into a main FIFO output at the output register. Select (RD_DATA_SEL) for this MUX is calculated by adding the value of the read vertical pointer (RD_VERT_PTR) to an extra offset (RD_EXTRA_OFFSET), calculated according to the table defined above.  
         [0036]    The above-described features are generally transparent to the user, which makes such 2-D FIFOs fully back compatible with standard FIFOs. The greater flexibility and simplicity of the implementation makes the 2-D FIFO a more flexible and desirable choice for many applications, such as, for example, data buffering and data re-alignment on industrial standard busses like PCI-X and PCI, bus size matching, and serial-to-parallel or parallel-to-serial FIFO applications.  
         [0037]    An advantage of the above-described method is that clock cycles maybe saved when accessing the FIFO memory whenever the actual data width is less than or equal to the width of the FIFO memory.  
         [0038]    While exemplary embodiments have been shown and described, those of ordinary skill in the pertinent art may make various modifications and substitutions thereto without departing from the spirit and scope of the present disclosure. Accordingly, it is to be understood that the present disclosure has been made by way of illustration only, and such illustrations and embodiments as have been disclosed herein are not to be construed as limiting to the claims.