Abstract:
An N-port memory architecture is disclosed that stores multi-dimensional arrays so that: (1) N contiguous elements in a row can be accessed without blocking, (2) N contiguous elements in a column can be accessed without blocking, (3) some N-element two-dimensional sub-arrays can be accessed without blocking, and (4) all N/2-element two-dimensional sub-arrays can be accessed without blocking. Second, the architecture has been modified so that the above can happen and that any element can be accessed on any data port. The architecture is particularly advantageous for loading and unloading data into the vector registers of a single-instruction, multiple-data processor, such as that used for video decoding.

Description:
FIELD OF THE INVENTION 
   The present invention relates to digital systems design in general, and, more particularly, to the architecture of a multi-port memory. 
   BACKGROUND OF THE INVENTION 
     FIG. 1  depicts a block diagram of a multi-processor and a multi-port memory. In general, the fact that the multi-processor comprises a plurality of execution units causes it to actually or virtually access more than one word within the memory at a time. There are three well-known memory architectures in the prior art for doing so. 
   In accordance with the first architecture, a full N-port design is employed that allows any N memory locations to be accessed from any port without blocking. The full N-port design is the fastest of the multi-port architectures, but is also the largest. 
   In accordance with the second architecture, a single-port memory with contention resolution is employed that functions as a single-server, multi-queue system. The single-port memory with contention resolution is the slowest of the multi-port architectures, but is also the smallest. 
   In accordance with the third architecture, a plurality of independent memory banks with contention resolution are employed. So long as each processor seeks data in a different memory bank, there is no contention. In contrast, when two processors seek data in the same memory bank, there is contention and one of them has to wait. An advantage of the third architecture is that its speed and size are a function of the number of memory banks used, and, therefore, its space-time parameters can be tailored for the application. For example, when the third architecture has a large number of memory banks, its speed and size approach that of the full N-port design, but when the third architecture has only 2 memory banks, it&#39;s speed and size approach that of the single-port memory. 
     FIG. 2  depicts a graph of the space-time parameters for three multi-port architectures in the prior art. 
   Although the three principal architectures provide a variety of space-time parameters, there are special-purpose applications that need a multi-port architecture with better space-time parameters than are exhibited by architectures in the prior art. 
   SUMMARY OF THE INVENTION 
   The present invention is an N-port memory architecture that is faster than a traditional N-bank memory bank architecture and smaller than a full N-port design. This is accomplished by recognizing that there are special-purpose applications where the traditional N-bank memory bank architecture can be enhanced to provide almost the same speed as the full N-port design. One of these applications has to do with the storage of multi-dimensional arrays. 
   The illustrative embodiment is an memory bank architecture that has been enhanced in two ways. First, the architecture has been modified to store multi-dimensional arrays so that: (1) N contiguous elements in a row can be accessed without blocking, (2) N contiguous elements in a column can be accessed without blocking, (3) some N-element multi-dimensional sub-arrays can be accessed without blocking, and (4) all N/2-element multi-dimensional sub-arrays can be accessed without blocking. This is advantageous in system designs that handle multi-dimensional arrays, such as video decoding systems, etc. 
   Second, the architecture has been modified so that the above can happen and that any element can be read from, and written to, on any data port. This is particularly advantageous for loading and unloading data into the vector registers of a single-instruction, multiple-data processor, such as that used for video decoding. 
   The illustrative embodiment comprises: (i) P memory locations identified by addresses 0 through P−1, wherein P is a positive integer greater than 1, and (ii) N independent memory banks that are each uniquely identified by a different memory bank, wherein N is a positive integer and 1&lt;N≦P, and (iii) logic for decoding each of the addresses into a memory bank; wherein addresses p+(c−1) decode into different memory banks for all p and all c, wherein 0≦p+(c−1)&lt;P, wherein p is a positive integer and pε{0, . . . , P−1}, wherein c is a positive integer and cε{1, . . . , C}, and wherein C is a positive integer and C≦N; and wherein addresses p+N(r−1) decode into different memory banks for all p and all r, wherein 0≦p+N(r−1)&lt;P, wherein r is a positive integer and rε{1, . . . , R}, and wherein R is a positive integer and R≦N. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  depicts a block diagram of a multi-processor and a multi-port memory. 
       FIG. 2  depicts a graph of the space-time complexity for three multi-port architectures in the prior art. 
       FIG. 3  depicts a block diagram of an N-port memory in accordance with the illustrative embodiment of the present invention in which N=8. 
       FIG. 4  depicts a block diagram of the logical structure of memory  301 , which is of a linear memory with P memory locations identified by addresses 0 through P−1, wherein P is a positive integer greater than 1. 
       FIG. 5  depicts a block diagram of the salient components of memory  301 , which comprises storage  501 , N=8×N=8 data switch  502 , and N=8×N=8 address switch and decoder  503 , interconnected as shown. 
       FIG. 6  depicts a block diagram of the salient components of storage  501 , which comprises N=8 independent memory banks  501 - 1  through  501 - 8 . 
       FIG. 7   a  depicts a mapping of the elements in a multi-dimensional array to memory banks. 
       FIG. 7   b  depicts how N contiguous elements of a the first column are all stored in different memory banks, and, therefore, can be read without contention. 
       FIG. 7   c  depicts how N contiguous elements in the third row are all stored in different memory banks, and, therefore, can be read without contention. 
       FIG. 7   d  depicts how a subarray of N/2 contiguous elements—elements ( 3 , 2 ), ( 4 , 2 ), ( 3 , 3 ), and ( 4 , 3 )—are all stored in different memory banks, and, therefore, can be read without contention. 
       FIG. 7   e  depicts how some, but not all, subarrays of N contiguous elements are stored in different memory banks, and, therefore, can be read without contention. 
       FIG. 8  depicts a mapping of multi-dimensional array elements to logical addresses. 
       FIG. 9  depicts a mapping of logical addresses to memory banks. 
       FIG. 10  depicts a block diagram of the salient components of address switch and decoder  502 , which comprises N=8×N=8 address switch  1001  and address decoder  1002 . 
   

   DETAILED DESCRIPTION 
     FIG. 3  depicts a block diagram of an N-port memory in accordance with the illustrative embodiment of the present invention in which N=8. It will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention for any value in which N is a positive integer. 
   Memory  301  comprises N=8 data ports and N=8 address ports. A word can be read from or written to memory  301  on a data port independently of whether a word is read from or written to memory  301  on another port. In other words, any combination of N=8 words can be read from and written into memory  301  in one cycle. For example, a word can be written into memory  301  on data ports  1 ,  6 , and  8 , while words are read from memory  301  on data ports  2 ,  3 ,  4 ,  5 , and  7 . In all cases, the data on port n, wherein nε{1, 2, . . . , N}, is associated with the address on address port n. 
     FIG. 4  depicts a block diagram of the logical structure of memory  301 , which is a linear memory with P memory locations identified by addresses 0 through P−1, and wherein P is a positive integer greater than 1. In accordance with the illustrative embodiment, P=16,384=0×3FFF=2^14, but it will be clear to those skilled in the art how to make and use alternative embodiments of the present invention for any value of P. So although memory  301  has multiple ports, the reading of an address on one address port yields the same data as on another port because they both refer to the same logical memory location. 
     FIG. 5  depicts a block diagram of the salient components of memory  301 , which comprises storage  501 , N=8×N=8 data switch  502 , and N=8×N=8 address switch and decoder  503 , interconnected as shown. 
   Storage  501  comprises P memory locations, N address ports,  510 - 1  through  510 - 8 , and N data ports,  513 - 1  through  513 - 8 . In accordance with the illustrative embodiment, each logical memory location corresponds to only one of the address ports  510 - 1  through  510 - 8  and one of the data ports  513 - 1  through  513 - 8 . 
   The constraint that each logical memory location in memory  501  corresponds to only one of the address ports  510 - 1  through  510 - 8  means that a logical address on one of address ports  511 - 1  through  511 - 8  must be routed to the correct one of address ports  510 - 1  through  510 - 8 . This is the function performed by address switch and decoder  503 . In other words, address switch and decoder  503  must:
         i. decode each logical address on each of address ports  511 - 1  through  511 - 8 ,   ii. generate a physical memory address in storage  501  that corresponds to that logical address, and   iii. route the physical address to the appropriate one of address ports  510 - 1  through  510 - 8 .
 
In accordance with the illustrative embodiment, address switch and decoder  503  comprises an N×N non-blocking crossbar switch, but it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention in which another structure provides the requisite functionality.
       

   The shuffling of addresses between address ports  511 - 1  through  511 - 8  and address ports  510 - 1  through  510 - 8 , without more, destroys the isomorphic relationship in which the data on port  512 -n is associated with the address on address port  511 -n. To preserve this relationship, data switch  502  performs the inverse shuffle of address switch and decoder  503 . For example, if logical address 0×0000 is presented on address port  511 - 3  during a read operation, the data in logical address 0×0000 should appear on data port  512 - 3 . But within memory  301 , address switch and decoder  503  might route the corresponding physical address to address port  510 - 1  which would cause the desired word to emerge on data port  513 - 1 . To ensure that the word emerges on data port  512 - 3 , data switch  513  routes the word from data port  513 - 1  to  512 - 3 . 
   In accordance with the illustrative embodiment, data switch  502  is an N×N non-blocking crossbar switch, but it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention in which another structure provides the requisite functionality. 
   There is another advantage to the combination of address switch and decoder  503  and data switch  502  and that is that it enables the word at any logical address to be read from, or written to, any of data ports  512 - 1  through  512 - 8 . This is particularly advantageous when, for example, memory  301  is used to load and unload the vector registers in a single-instruction, multiple-data processor. 
     FIG. 6  depicts a block diagram of the salient components of storage  501 , which comprises N=8 independent memory banks  601 - 1  through  601 - 8 . Each memory bank is a single-port memory that comprises P/N=2^13=2048 words. Because storage  501  comprises independent memory banks only one word from each memory bank can be read or written to in a single cycle. 
   Although the worst-case contention situation cannot be eliminated the average-case can be by distributing words that are often accessed together across different memory banks. There are special-purpose applications where group of words are often accessed together and one of those applications involves the storage of multi-dimensional arrays, such as those commonly manipulated in video coding and coding (e.g., H.264, MPEG, etc.). For example, in video decoding, the elements in a row, a column, and a contiguous block tend to be accessed far more frequently together than random elements in the array. 
   In accordance with the illustrative embodiment, each element of a J×K two-dimensional array, wherein J and K are both positive integers greater than 1, is assigned to one of the memory banks so that three conditions are satisfied:
         i. the coordinates for N contiguous elements in a row of the two-dimensional array decode into different memory banks; and   ii. the coordinates for N contiguous elements in a column of the two-dimensional array decode into different memory banks; and   iii. the coordinates for the elements in an L by M two-dimensional subarray of the two-dimensional array decode into different memory banks, wherein L and M are both positive integers, 1≦L≦J, 1≦M≦K, and 2≦L*M≦N/2.
 
It will be clear to those skilled in the art, after reading this disclosure, how to generate any of the many suitable mappings between array coordinates and memory banks—and one illustrative mapping is depicted in  FIG. 7   a.  
       

     FIG. 7   b  depicts how N contiguous elements of a the first column are all stored in different memory banks, and, therefore, can be read without contention. The reader can verify that the same is true for all columns. 
     FIG. 7   c  depicts how N contiguous elements in the third row are all stored in different memory banks, and, therefore, can be read without contention. The reader can verify that the same is true for all columns. 
     FIG. 7   d  depicts how a subarray of N/2 contiguous elements—elements ( 3 , 2 ), ( 4 , 2 ), ( 3 , 3 ), and ( 4 , 3 )—are all stored in different memory banks, and, therefore, can be read without contention. The reader can verify that the same is true for all subarrays of N/2 contiguous elements. 
     FIG. 7   e  depicts how some, but not all, subarrays of N contiguous elements are stored in different memory banks, and, therefore, can be read without contention. The reader can verify that the same is true for some, but not all, subarrays of N contiguous elements. 
   One corollary of the above constraints is that, in accordance with the Pigeon Hole Principal, at least two coordinates for any N+1 elements decode into the same memory bank. 
   In accordance with the illustrative embodiment, each element of a J×K two-dimensional array is assigned a logical address in, for example, row-column order as depicted in  FIG. 8 . It will be clear to those skilled in the art how to assign the elements to logical addresses in accordance with a different, but suitable, scheme. 
   In addition, address switch and decoder  503  comprises logic for decoding each of the addresses into:
         i. a memory bank, and   ii. a unique physical address into that memory bank so that the following three conditions are satisfied:   i. addresses p+(c−1) decode into different memory banks for all p and all c, wherein 0≦p+(c−1)&lt;P, wherein p is a positive integer and pε{0, . . . , P−1}, wherein c is a positive integer and cε{1, . . . , C}, and wherein C is a positive integer and C≦N; and   ii. addresses p+N(r−1) decode into different memory banks for all p and all r, wherein 0≦p+N(r−1)&lt;P, wherein r is a positive integer and rε{1, . . . , R}, and wherein R is a positive integer and R≦N; and   iii. addresses p+(c−1)+N(r−1) decode into different memory banks for all p, all c, and all r, wherein 0≦p+(c−1)+N(r−1)&lt;P, and wherein 1≦C*R≦N/2.
 
The result will be a mapping of logical addresses to memory banks, such as that depicted in  FIG. 9 .
       

   Here too, because there are only N memory banks, the Pigeon Hole Principal holds—at least two addresses in every set of N+1 addresses decode into the same memory bank. 
     FIG. 10  depicts a block diagram of the salient components of address switch and decoder  502 , which comprises N=8×N=8 address switch  1001  and address decoder  1002 . 
   Address switch  1001  is combinational logic that receives a P-bit logical address on each of address ports  511 - 1  through  511 - 8  and that outputs a (log 2 P-log 2 N)-bit physical address on each of address ports  510 - 1  through  510 - 8 . Address switch  1001  shuffles the addresses under the control of address decoder  1002  using a non-blocking cross-bar switch, but performs the logical address to physical memory address translation on its own so that each P-bit logical address assigned to a single memory bank generates a unique (log 2 P-log 2 N)-bit physical address. It will be clear to those skilled in the art how to accomplish this. 
   It is to be understood that the above-described embodiments are merely illustrative of the present invention and that many variations of the above-described embodiments can be devised by those skilled in the art without departing from the scope of the invention. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.