Patent Publication Number: US-7587557-B2

Title: Data sharing apparatus and processor for sharing data between processors of different endianness

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to a data sharing apparatus containing a memory and two processors of different endianness, as well as such processors for the apparatus, and particularly to data sharing between processors. 
     (2) Description of the Related Art 
     In the case where the basic word length of a processor is 2 bytes (16 bits) or more, there are two formats for byte ordering in storing data of 2 bytes or more into a memory, the so-called “big-endian” and “little-endian”. This is because the basic word length has 2 bytes or more, as opposed to the byte units in which addresses are assigned in a memory. 
     First, the big-endian format shall be explained. 
     Big-endian is a format for the ordering of a byte sequence (byte order) during the storage of data of 2 bytes or more, into a memory having byte unit addresses. It refers to the format that stores data in an ascending order of memory addresses, beginning with the byte at the big end (in other words, the Most Significant Bit (MSB) side. In this case, “B” should stand for byte, rather than bit). However, endian refers to byte sequence ordering and not to bit sequence ordering, so the order of bits within a byte does not change. 
       FIG. 1  is an explanatory diagram of the big-endian format. For example, in storing a 4 byte data ‘ 89 ABCDEF’ (hexadecimal, hereinafter, the same for all within ‘ ’ marks) into the memory areas addressed  100  to  103  in the big-endian format, the data is stored as ‘ 89 ’, ‘AB’, ‘CD’, ‘EF’, sequentially, in the addresses  100  to  103 . 
       FIG. 2  shows the same example as  FIG. 1 , only that it illustrates a memory image (memory data) having 32 bits as the basic word length. The memory address is in byte units, and the lower address (addressed byte position according to the least significant 2 bits of the address) within the basic word length is, from the left, “0”, “1”, “2”, “3”, in ascending order. The most significant byte ‘ 89 ’ is stored in the address “0”. 
     In this manner, in the big-endian format, data is stored into memory, in an ascending order of addresses, from the byte on the MSB-side. 
       FIG. 3  illustrates a typical structure for the connection of a big-endian-type CPU  2  and a memory  3   b . A CPU is also simply referred to as a processor. 
     As shown in the same diagram, the input/output terminals D[31:24], D[23:16], D[15:8], D[7:0] of the CPU  2  are connected, respectively, to the lower addresses “ 0 ”, “ 1 ”, “ 2 ”, “ 3 ” of the memory  3   b , via a data bus. As a result, byte data is stored, in an ascending order of memory addresses, starting from the MSB. Furthermore, in the memory  3   b , a memory image of the case where a structure sample  4  is defined in a program for the CPU  2 , is shown. A LONG INTEGER w is word data in the basic word length (32-bit) composed of the 4 bytes, w 3 , w 2 , w 1 , w 0 , in sequence from the MSB. The SHORT INTEGER x is half-word data, composed of x1, and x0, in sequence from the MSB. The same is true for a SHORT INTEGER y. CHAR a, b, c, and d, each represent byte data. 
     Next, the little-endian format shall be explained. 
     Little-endian refers to the format which stores data in an ascending order of memory addresses, from the byte on the little end (in other words, Least Significant Bit (LSB) side) first. 
       FIG. 4  is an explanatory diagram of the little-endian format. It shows the same data as in  FIG. 1 , where the 32-bit data ‘ 89 ABCDEF’ is stored, sequentially, in the addresses  100  to  103 , as ‘EF’, ‘CD’, ‘AB’, and ‘ 89 ’. 
       FIG. 5  illustrates a memory image of the same example in  FIG. 4 , with 32 bits as the basic word length. Compared to  FIG. 2 , the lower address sequence is inverted, with “0”, “1”, “2”, “3”, starting from the right. 
       FIG. 6  illustrates a typical structure for the connection of a little-endian-type CPU  1  and a memory  3   a . As shown in the diagram, the input/output terminals D[7:0], D[15:8], D[23:16], D[31:24] of the CPU  1  are connected, respectively, to the lower address “ 0 ”, “ 1 ”, “ 2 ”, “ 3 ” of the memory  3   a , via a data bus. As a result, byte data is stored, in an ascending order of memory addresses, starting from the byte on the LSB-side. Furthermore, in the memory  3   a , a memory image of the case where a structure sample  4 , the same as in  FIG. 3 , is defined in a program for the CPU  1 , is shown. Compared to the memory image shown in  FIG. 3 , the byte sequence is in reverse. 
     As such, big-endian and little-endian are formats for byte sequence ordering, in storing data of 2 bytes or more into a memory, each having an opposite ordering sequence from the other. 
     Due to such incompatibilities, a structure that converts differences in endianness is necessary in order to preserve the identity of shared data in a system where a big-endian CPU and little-endian CPU co-reside. 
     Patent Document 1 (See Japanese Laid-open Patent Application No. 06-69978) discloses a packet communication apparatus including an endian conversion unit for converting packet data from a big-endian processor into the little-endian format, during packet communication between a big-endian CPU and a little-endian CPU. 
     In addition, Patent Document 2 (See Japanese Laid-open Patent Application No. 2000-3304) discloses a data alignment apparatus that performs a memory access by absorbing differences in data size, alignment, and endianness, through the conversion of data to be stored, based on the access address and access size. 
     However, in the technology disclosed in patent documents 1 and 2, it is necessary to add specialized hardware to processors equipped with an endian conversion unit. Moreover, since delays arise due to endian conversion, the technology poses a barrier to high speed memory access, even carrying with it the problem of hindering the speed of other processors, in the case where data is shared. 
     Furthermore, in general, image processing is carried out by big-endian processors, with communication being carried out by little-endian processors. With the inclusion of image processing functions in mobile phones in recent years, little-endian processors and big-endian processors have come to be connected to the same bus. This then creates a need for real-time sharing of data. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a processor and memory sharing apparatus that realizes the sharing of memory data through an extremely simple structure, in the case where a big-endian processor and a little-endian processor are connected to a bus. 
     In order to achieve the above-mentioned objective, the processor in the present invention is a processor connected to a memory via a data bus, comprising an address conversion unit operable to convert at least one lower bit of an address so as to indicate a reversed position of data in the data bus, and output the converted address to the memory, when the processor performs a memory access for the data with a smaller width than the data bus, wherein the processor is connected to the data bus in a way that data is transmitted to and from the memory in a byte order based on an endianness which is opposite to an endianness of said processor. 
     According to this structure, since the first processor and the second processor access the memory in the same byte order for data with the same width as the data bus, the sharing of data with the same width as the data bus through an extremely simple structure is realized as an effect of the present invention. In addition, through the address conversion unit, it becomes possible for the processor to access the memory for data with a smaller width than the data bus. Furthermore, since the processor can be realized through simple hardware such as the address conversion unit, it does not become an impediment to memory access speed. 
     Here, the processor (first processor) may be structured to execute a program that defines structure data which includes data that is smaller than a basic word length, said structure data being shared between the first processor and another processor (second processor) of a different endianness via the memory, said data being defined in an order within the basic word length, and said order being in reverse to an order in a definition of said structure data in a program to be executed by the second processor. 
     According to this structure, data can be shared between the first processor and the second processor via the memory, even for data with a smaller width than the data bus. In the case where the width of the data bus is 32 bits (4 bytes), for example, with the provision of the address conversion unit, the address “ 0 ” in terms of the first processor, and the address “ 3 ” in terms of the second processor, both indicate the same area in the memory (likewise, the addresses “ 1 ”, “ 2 ” and “ 3 ” in terms of the first processor, are the addresses “ 2 ”, “ 1 ” and “ 0 ” in terms of the second processor). Furthermore, in the case where the structure data of the first program is defined as being the 4 bytes “B 0 ”, “B 1 ”, “B 2 ” and “B 3 ”, the structure data of the second program is defined as being the 4 bytes “B 3 ”, “B 2 ”, “B 1 ” and “B 0 ”. For the byte data “B 0 ” that is defined as “B 0 ”, the first processor accesses the address “ 0 ”, and the second processor accesses the address “ 3 ”. In this manner, sharing of data is possible, not only for data access of the same width as the data bus, but also for the case of data access of a smaller width. 
     Furthermore, the processor may be structured to include a cache memory connected to the data bus in a byte order based on the endianness of the processor, wherein the processor may be structured to read data from the memory through the cache memory in data units of the same width as the data bus. 
     According to this structure, since data with the same width as the data bus is read from the memory through the cache memory, caching data for a processor of different endianness through an extremely simple structure is possible as an effect of the present invention. In addition, address conversion is not necessary for cache access, and the processor can access data through the same access operation, regardless of whether the particular data is located in the shared memory or the cache memory. 
     Furthermore, the data sharing apparatus in the present invention is a data sharing apparatus comprising a first processor, a second processor, and a memory, said first and second processors being of different endianness, wherein both the first processor and the second processor are connected to the memory via a data bus, in a byte order based on the endianness of the first processor. 
     According to this structure, since the first processor and the second processor access the memory in the same byte order for data with the same width as the data bus, sharing of such data, through an extremely simple structure is realized as an effect of the present invention. 
     Here, the data sharing apparatus may be structured to further include an address conversion unit operable to convert at least one lower bit of an address so as to indicate a reversed position of data in the data bus, and output the converted address to the memory, when the second processor performs a memory access for the data with a smaller width than the data bus. 
     According to this structure, the second processor is able to access the memory for data with a smaller width than the data bus through the address conversion unit. 
     Here, the memory stores structure data to be accessed by a first processor and a second processor, the first processor executes a first program that defines the structure data, and the second processor executes a second program that defines structure data which includes data that is smaller than the basic word length, said data being defined in an order within the basic word length, and said order being in reverse to an order in the first program. 
     According to this structure, sharing of data between the first processor and the second processor via the memory is possible, not only for data with the same width as the data bus, but also for data with a smaller width than the data bus. In addition, uncacheable data (data which cannot be stored in the cache memory) can be shared between processors of different endianness. 
     Here, the data sharing apparatus may be structured to further include a transfer unit operable to control data transfer by direct memory access (DMA), wherein, in the case where a source and a destination require data of different endianness and data with a smaller width than the data bus is to be transferred, the transfer unit reverses an order of said data within a basic word length, for the source and the destination. 
     According to this structure, DMA transfer between a source and a destination of different endianness is possible, with the data order required by each, being provided during the DMA transfer. In addition, the processor is able to use the data at the destination (transferred data), even through half-word and word unit access. 
     Here, the transfer unit may be structured to include a conversion unit operable to convert at least one lower bit of an address of either the source or the destination so as to indicate a reversed position of the data in the data bus, and output the converted address to the memory, in the case where a source and a destination require data of different endianness and data with a smaller width than the data bus is to be transferred. 
     According to this structure, since the conversion unit only converts lower bits of an address, it can be realized through an extremely simple structure. 
     Here, the data sharing apparatus may be structured to further include a cache memory connected to the data bus in a byte order based on the endianness of the second processor. 
     According to this structure, since data with the same width as the data bus is read from the memory through the cache memory, data, for the first processor which has a different endianness, can be cached through an extremely simple structure as an effect of the present invention. 
     Furthermore, the data sharing method in the present invention is structured in the same way as the above-mentioned data sharing apparatus. 
     As explained above, according to the data processing apparatus in the present invention, (A) by connecting the big-endian-type second processor to the shared memory, in the same byte order as the little-endian-type first processor (See  FIG. 7 , bus connection points  42  and  43 ), data can be shared by the little-endian-type first processor and the big-endian-type second processor through word unit access of the shared memory. In addition, it is possible for the cache memory of the big-endian processor to cache from the shared memory in word units. 
     In addition, although the addition of (B) address conversion to (A) does not go so far as to enable the second processor to share data by half-word and byte unit access, it does enable access of the little-endian memory. 
     Furthermore, in addition to (A) and (B), (C) by defining variables which are shorter than the basic word length, in reversed orders within the basic word length, in structure definition within application programs for the first processor and the second processor, data can be shared by the little-endian-type first processor and the big-endian-type second processor, not only through access of the shared memory in word units, but also through access in half-word and byte units 
     Also, in addition to (A) and (B), since during (D) DMA transfer, the same address conversion in (B) is used, data of different endianness is performed of DMA transfer, and data sharing for the source and destination through half-word unit and byte unit access becomes possible. 
     As further information about technical background to this application, Japanese Patent Application No. 2003-075198 filed on Mar. 19, 2003 is incorporated herein by reference. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings that illustrate a specific embodiment of the invention. 
       In the Drawings: 
         FIG. 1  is an explanatory diagram of the big-endian format. 
         FIG. 2  is an explanatory diagram of the big-endian format. 
         FIG. 3  illustrates a typical structure for the connection of a big-endian CPU and a memory. 
         FIG. 4  is an explanatory diagram of the little-endian format. 
         FIG. 5  is an explanatory diagram of the little-endian format. 
         FIG. 6  illustrates a typical structure for the connection of a little-endian CPU and a memory. 
         FIG. 7  is a block diagram showing the structure of the data processing apparatus in the first embodiment. 
         FIG. 8  is a diagram showing the input/output logic of the address conversion by the address conversion unit  21 . 
         FIG. 9  is an explanatory diagram of the case where the first processor  10  and the second processor  20  access the shared memory  30  in word units. 
         FIG. 10A  is an explanatory diagram of the case where the first processor  10  and the second processor  20  access half-word data in the shared memory  30 , using the same address. 
         FIG. 10B  is an explanatory diagram of the case where the first processor  10  and the second processor  20  access the same half-word data in the shared memory  30 , using different addresses. 
         FIG. 11A  is an explanatory diagram of the case where the first processor  10  and the second processor  20  access byte data in the shared memory  30 , using the same address. 
         FIG. 11B  is an explanatory diagram of the case where the first processor  10  and the second processor  20  access the same byte data in the shared memory  30 , using different addresses. 
         FIG. 12  is a diagram showing memory images for the shared memory  30  and cache memory  23 . 
         FIG. 13  is an explanatory diagram of data sharing in half-word units and byte units, based on the definition of a structure. 
         FIG. 14  is a diagram showing an example of another structure definition. 
         FIG. 15  is a diagram showing the process of reversing the order of a structure definition within a source program. 
         FIG. 16  is a block diagram showing the structure of the data processing apparatus in the second embodiment of the present invention. 
         FIG. 17  is a block diagram showing the detailed structure of the DMAC  32 . 
         FIG. 18  is an explanatory diagram showing the input/output logic of the address conversion by the address conversion unit  104 . 
         FIG. 19A  is an example of a time chart illustrating a DMA transfer by the DMAC  32 . 
         FIG. 19B  is a diagram showing memory images for the source memory and the destination memory. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     First Embodiment 
       FIG. 7  is a block diagram showing the structure of a data processing apparatus in the present embodiment. As in the diagram, the data processing apparatus includes a first processor  10 , a second processor  20 , a shared memory  30 , an address conversion unit  21 , and a cache memory  23 , that are connected to each other via a data bus (D bus, in the diagram). 
     This data processing apparatus achieves the sharing of data in a memory in an extremely simple structure, through the following three points: (A) the big-endian-type second processor  20  is connected to the shared memory  30  with the same byte order as the little-endian-type first processor  10 , (B) the address conversion of the second processor  20 , and (C) the respective methods in which structures of data in a program are defined by the first processor  10  and the second processor  20 , where (A) and (B) are hardware structures, and (C) is a software structure. 
     (A. Bus Connection) 
     In  FIG. 7 , “ 0 ”, “ 1 ”, “ 2 ”, “ 3 ” within the shared memory  30  and the cache memory  23  are byte positions to be addressed according to the lower address (here, the lowest two bits of an address). 
     The first processor  10  is a little-endian processor, and is connected to the shared memory  30  via the bus connection points  41  and  42 . This connection is the same as a typical connection for little-endian processors (See  FIG. 6 ), where the first processor  10  is connected to the shared memory  30  in the little-endian byte order. In other words, as the little end D[7:0] of the data input/output terminal is connected to the address “ 0 ” of the shared memory  30 , the first processor  10  stores a byte sequence in an ascending order of memory addresses, starting from the little end. 
     The second processor  20  is a big-endian processor, and is connected to the shared memory  30  via the bus connection points  43  and  42 . This connection relates to the above-mentioned (A). That is, this connection method is not the typical big-endian processor connection method (See  FIG. 3 ), but rather the same one as a typical connection for a little-endian processor (See  FIG. 6 ). In this connection, the second processor  20  is connected to the ascending order of addresses in the memory  30 , not from the big end, but instead, from the little end byte D[7:0] input/output terminal. To put it in another way, the byte order of the second processor  20  has been switched, from big-endian to little-endian, by way of the bus connection points  43  and  42 . 
     By this connection, the sharing of data through the access of 32-bit data (hereinafter, word) in the shared memory  30  by the first processor  10  and second processor  20  becomes possible. In addition, it also becomes possible to cache (read-in) data into the cache memory  23 , in 32-bit units, from the memory  30 . However, data sharing through access of 16-bit data (half-word), and 8-bit data (byte) cannot be achieved by this connection alone. This can be achieved, by combining this connection with the previously mentioned (B) and (C). 
     In the shared memory  30 , the byte positions addressed “ 3 ”, “ 2 ”, “ 1 ”, “ 0 ”, indicated by the lower address (the least significant 2 bits of the address (A 1 , A 0 )) correspond to the data input/output terminals D[31:24], D[23:16], D[15:8], and D[7:0] of the first processor  10 . 
     (B. Address Conversion) 
     The address conversion unit  21  converts the lower address of the second processor  20  for outputting to the shared memory  30 . In relation to the above-mentioned (B), this address conversion converts the lower address of data that is smaller than the 32-bit basic word length, to indicate a reversed position within the basic word length, for the concerned data. 
       FIG. 8  is a table showing the input/output logic of the address conversion by the address conversion unit  21 . The inputs for the address conversion unit  21  are “access size” and “address”, both of which are inputted from the second processor  20 . Access size indicates any from among, a word access, a half-word access, or a byte access. The “address” is the 2 bits from the lowest position on the address bus, in other words, A 1  and A 0 . 
     The output for the address conversion unit  21  is the “converted address”. With regards to the “converted address”, as shown in the same table, the address conversion unit  21  outputs A 1  and A 0 , as they are (in other words, without conversion), in the case of a word access. In the case of a half-word access, only A 1  is inverted, A 0  being outputted as it is. For a byte access, both A 1  and A 0  are inverted, then outputted. Here, it is assumed that for a word access and a half-word access, the second processor  20  does not access misaligned data. 
       FIG. 9  is an explanatory diagram of the case of a word access by the second processor  20 . In the case where the second processor  20  accesses in word units, it is possible to share the data in the shared memory  30 , with the first processor  10 . In this case, the address conversion unit  21  does not convert the addresses. 
       FIG. 10A  is an explanatory diagram of the case of a half-word access by the second processor  20 . In a half-word access, the address conversion unit  21  converts the lower address “ 0 ” (A 1 , A 0 =0, 0) to the lower address “ 2 ” (1, 0), and the lower address “ 2 ” (A 1 , A 0 =1, 0), to the lower address “ 0 ”. In the same diagram, when the second processor  20  executes an instruction (movhw#89ABh,mem(0)) to store the half-word ‘ 89 AB’ into the address “ 0 ”, ‘ 89 AB’ is stored in the addresses “ 2 ” and “ 3 ”, as shown in a memory image (memory data)  4   b . In contrast, when the first processor  10  executes the same instruction, ‘ 89 AB’ is stored in the addresses “ 0 ” and “ 1 ”, as shown in the memory image  4   a.    
     In this manner, in a half-word access, the lower address “ 0 ” in the shared memory  30  in terms of the first processor  10  is the lower address “ 2 ” in terms of the second processor  20 . Likewise, the address “ 2 ” in terms of the first processor  10  is the address “ 0 ” in terms of the second processor  20 . 
       FIG. 11A  is an explanatory diagram of the case of a byte access by the second processor  20 . In a byte access, for example, the lower address “ 0 ” (A 1 , A 0 =0, 0) is converted to the address “ 3 ” (1, 1) by the address conversion unit  21 . In the same diagram, when the second processor  20  executes an instruction (movb#89h,mem(0)) to store the byte ‘ 89 ’ into the address “ 0 ”, ‘ 89 ’ is stored into the address “ 3 ”, as shown in a memory image  5   b . In contrast, when the first processor  10  executes the same instruction, ‘ 89 ’ is stored in the address “ 0 ”, as shown in a memory image  5   a.    
     As such, in a byte access by the second processor  20 , the lower address “ 0 ” is converted to the address “ 3 ”, the address “ 1 ” to “ 2 ”, the address “ 2 ” to “ 1 ”, and the address “ 3 ” to “ 0 ” by the address conversion unit  21 . In other words, in a byte access, the lower addresses “ 0 ”, “ 1 ”, “ 2 ”, “ 3 ” in the shared memory  30 , in terms of the first processor  10  are, in terms of the second processor  20 , the addresses “ 3 ”, “ 2 ”, “ 1 ”, “ 0 ” in the shared memory  30 . 
     Incidentally, even with the addition of the address conversion in (B) to (A), the sharing of data via the shared memory  30 , between the second processor  20  and the first processor  10 , is still not achieved up to half-word and byte units. There is significance in the point that, together with the switching of byte orders in (A), the address conversion in (B) makes half-word and byte access possible for the second processor  20 . 
     In other words, although without the inclusion of the address conversion unit  21 , the second processor  20  can access data from the shared memory  30  in word units, it will no longer be able access in half-word and byte units. For instance, in the example shown in  FIG. 10A , the addresses “ 0 ” and “ 1 ” in the shared memory  30  are a half-word on the lower-side (right-side of the diagram) of the word, as shown in the memory image  4   a . However, in outputting to the addresses “ 0 ” and “ 1 ”, the second processor  20  outputs ‘ 89 AB’ to the half-word on the high-side of the word. In this condition, assuming that the address “ 0 ” is designated in the shared memory  30  without address conversion, invalid information in the bus will be written into the lower-side half-word (addresses “ 0 ”, “ 1 ”) within the shared memory  30 , where ‘ 89 AB’ is not inputted. 
     In this way, the significance of the address conversion in (B) lies in the resolution of half-word and byte access discrepancies arising from the byte data switching in (A). In addition, since the address conversion unit  21  performs on a simple combinational circuit, delays in lower address output brought about by conversion are negligible. 
     Moreover, with regard to half-words and bytes, since the addresses in terms of the first processor  10 , mismatch with the addresses in terms of the second processor  20 , as in  FIG. 10A  and  FIG. 11A , data sharing in half-word and byte units is not considered possible. For this mismatch, for example, the first processor  10  and second processor  20  can directly designate the mismatching addresses for shared half-word and byte data, as shown in  FIG. 10B  and  FIG. 11B , thus enabling data sharing, not only in word units, but also in half-word and byte units as well. This point shall be achieved in the present embodiment through the previously mentioned (C). 
     The cache memory  23  is connected to the second processor  20 , via the connection point  44 , in a typical big-endian processor and memory connection method (See  FIG. 3 ). In  FIG. 7 , the cache memory  23  is provided outside the second processor  20 . However, it can be included within the second processor  20 . The cache memory  23  can cache data from the shared memory  30 , in word units. This is done through the previously mentioned byte order switching connection in (A). 
       FIG. 12  is a diagram showing memory images of the shared memory  30  and cache memory  23 . Apart from having different lower address orders, the cache memory  23  is able to hold the same memory image as the shared memory  30  by caching data from the shared memory  30  in word units. However, the cache memory  23  is not able to cache from the shared memory  30 , in half-word and byte units, with only the above-mentioned connection in (A). 
     (C. Structure Definition) 
       FIG. 13  is an explanatory diagram of the sharing of data in half-word and byte units, through (above-mentioned (C)) structure definition. A first program  10   a  to be executed by the first processor  10  is generated from a source program which defines a structure  10   b . In addition, a second program  20   a  to be executed by the second processor  20  is generated from a source program which defines the structure  20   b . The structure  20   b  defines the same variables as the structure  10   b . However, within the limits of the basic word length, it defines the order of a variable smaller than the basic word length, in reverse order. 
     For example, the 16-bit integer-type variables x and y are defined as “SHORT INTEGER y, x;” in the structure  10   b , and defined in reverse-order as “SHORT INTEGER x, y;” in the structure  20   b . Likewise, the character-type variables a, b, c and d are defined as “CHAR d, c, b, a;” in the structure  10   b , and defined in reverse-order, as “CHAR a, b, c, d;” in the structure  20   b . These structures are described by a programmer, or they may be described in reverse order by a compiler. 
     As such, by switching within the definitions in the structures, the mismatch of addresses in a half-word and byte access as shown in  FIG. 10A  and  FIG. 11A , can be allowed. In other words, as shown in  FIG. 10B  and  FIG. 11B , it is possible to point out the mismatching addresses, in the case where the first processor  10  and the second processor  20  execute a half-word or byte access for identical variables to be shared. 
     As shown in  FIG. 13 , the memory image of the shared memory  30  as defined in the structure  10   b , corresponds with the memory image as defined in the structure  20   b . However, the lower addresses of half-words and bytes in terms of the second processor  20  are in reverse order to those for the first processor  10  (mismatching). For example, the variable “a” is perceived by the first processor  10  as being stored in the lower address “ 3 ”, and perceived within the second processor  20  as being stored in the lower address “ 0 ”. In this manner, the variable “a” can be shared even in a byte access. 
     In addition, the variable “y” is perceived by the first processor  10  as being stored in the lower addresses “ 0 ” and “ 1 ”, and perceived within the second processor  20  as being stored in the lower address “ 2 ” and “ 3 ”. In this manner, the variable “y” can be shared even in a half-word access. 
     Moreover, as long as data is defined as the above-mentioned structure to be shared, the cache memory  23  can cache, not only in word units, but also in half-word and byte units. 
       FIG. 14  is a diagram showing an example of another structure definition. In a structure  10   c  for the first processor  10 , an 8-bit blank within the basic word length arises from the definition of the last unsigned 8-bit variable “CHAR b 2 ;”. For that reason, dummy byte data “CHAR bDummy 0 ;”, of the same bit length as the blank, is inserted into a structure  20   c . As a result, in the structure  20   c , a bit order, including the 8-bit blank, is defined in reverse order within the basic word length. 
       FIG. 15  is a diagram showing the reverse ordering of a definition of a structure within a program, as pre-processing prior to compiling. 
     In the diagram, a pre-processing unit  100  receives a first source program for the first processor  10 , and a second source program for the second processor  20 . It detects structures to be shared by both programs (S 11 ), and switches the order of variables, so that the order of variables within the basic word length for one source program is in reverse to that of the other source program (S 12 ). Here, dummy byte data, as shown in  FIG. 14 , or a dummy half-word may be inserted, as required. After which, both source programs are compiled in the respective compilers. 
     As a result, the need for a programmer to dwell on the ordering of structure data to be shared in programming is eliminated, and, as all that is required is to match the names of the variables to be shared, the burden on the programmer can be reduced. 
     As explained above, according to the data processing apparatus in the present embodiment, sharing of data between the little-endian-type first processor  10  and the big-endian-type second processor  20  by way of word unit access of the shared memory  30  is possible through (A) connecting the big-endian-type second processor  20  to the shared memory  30  in the same byte order as the little-endian-type first processor  10  (See  FIG. 7 , bus connection points  42 ,  43 ). Moreover, the cache memory of the big-endian processor can cache from the little endian shared memory, in word units. 
     Moreover, although the addition of the address conversion unit  21  (B) to (A) does not necessarily go so far as to enable the second processor  20  to share data through half-word and byte unit access, it does provide access to the little endian memory. For example, the shared memory  30 , which is connected to the first processor  10  in a little endian connection method, can be accessed by the big-endian-type second processor  20 , as a second processor  20  memory, through the simple structure in (A) and (B). 
     Furthermore, in addition to (A) and (B), (C) by defining, in the structure definition for the first processor  10  and the second processor  20 , a variable that is smaller than the basic word length in reverse order within the basic word length, data can be shared by the little-endian-type first processor  10  and the big-endian-type second processor  20 , not only by access of the shared memory  30  in word units, but also in half-word and byte units. 
     Note that although in the above-mentioned embodiment, the structure in the case where the first processor  10  is a little-endian, and the second processor  20  is a big-endian processor, is explained, the reverse case of endianness is also possible. In that case, the shared memory is connected to both processors as a big-endian memory, and the address conversion unit should be structured to convert the addresses of the little-endian processor. 
     Moreover, in the above-mentioned embodiment, the basic word length for the first processor  10  and the second processor  20  is assumed as 32 bits. However, a basic word length of another length such as 64 bits or 128 bits is possible. In this case, the address conversion unit  21  should be structured to convert the lower address which indicates the byte position within the basic word length. 
     Second Embodiment 
     In the above-mentioned embodiment, the issue of mismatches in half-word addresses and byte addresses arising in (A) and (B), when data in the shared memory  30  is taken from perspective of the first processor  10 , and when it is taken from the perspective of the second processor  20 , is resolved through the above-mentioned (C). In the present embodiment, a structure for resolving this mismatch by DMA transfer, without using (C), shall be explained. 
       FIG. 16  is a block diagram showing the structure of a data processing apparatus in the second embodiment of the present invention. The data processing apparatus in the diagram is different from that in  FIG. 7 , in having a Synchronous Dynamic Random Access Memory (SDRAM)  31 , a Direct Memory Access Controller (DMAC)  32 , an Input/Output device (I/O)  33 , and I/O  34 , added. Structural elements identical to those in  FIG. 7  are assigned the same numerals, and explanations will not be repeated. Explanations will focus on the points of difference. 
     The present data processing apparatus is included in Audio Visual (AV) devices such as a DVD recorder or a digital broadcast receiver. In this case, the second processor  20  performs image processing such as coding and decoding of “Moving Picture Experts Group” (MPEG) based, compressed moving picture data, and the first processor  10  controls the image processing apparatus, as a whole. The shared memory  30  is used in supplying and receiving various types of data between the first processor  10  and the second processor  20 . 
     The SDRAM  31  is used as a work memory by the first processor  10  and the second processor  20 . It holds stream data indicating MPEG compressed moving pictures, pictures being decoded, decoded reference pictures, and decoded audio data, and so on. 
     DMAC 32  performs address conversion to switch byte orders, in the case where, during a DMA transfer, the source data and destination data are data of different endianness, and the size of transfer data is smaller than a word. In addition, a DMA transfer is performed between memories, as well as between a memory and an I/O. Here, memory refers to the shared memory  30  and the SDRAM  31 , and I/O refers to the I/O  33 , I/O  34 , and so on. 
     The I/O  33  and the I/O  34  are 8-bit and 16-bit I/O devices, respectively, and are, for example, video output units for outputting decoded picture data as serial video data, or audio output units that output decoded audio data input as an audio signal, and so on. 
       FIG. 17  is a block diagram showing the detailed structure of DMAC  32 . DMAC  32  includes a first address counter  101 , a second address counter  102 , a terminal counter  103 , a data latch  105 , as well as a control unit  106 . 
     The first address counter  101  and the second address counter  102  hold source and destination addresses, respectively, and count-up or count-down during each transfer. Note that the opposite correspondence between both address counters and the source and destination is possible. 
     During burst transferring, the terminal counter  103  informs the control unit  106  when it is finished counting the number of data that need to be transferred, ending the DMA transfer. 
     In the case where there is a difference in endianness between the source data and the destination data, or between the I/O  33 ,  34  and the memory data, and data size is smaller than a word, the address conversion unit  104  converts the source/destination addresses outputted from the first address counter  101 , to reverse, within the basic word length, the order of the concerned data for the source and the destination. 
     Here, “difference in endianness” is used to refer to the access of source and destination data by processors of different endianness, or the use of source and destination data by processors and I/O devices of different endianness. An example is the case where source data is loaded/stored by the second processor  20  and destination data is loaded/stored by the first processor  10 . Another example is the case where source data is stored by the second processor  20  and the destination is an I/O requiring byte data, sequentially, starting from the LSB-side, and so on. 
     Furthermore, since the data loaded/stored by the second processor  20  is stored in the memory in the little-endian order, for DMA in word units, DMA transfer can be performed as is, without address conversion by the address conversion unit  104 . However, for DMA in half-word and byte units, which are smaller than a word, the addresses in terms of the first processor  10  and the addresses in terms of the second processor  20  mismatch, as shown in  FIG. 10A  and  FIG. 11A . As such, in the case where the source and destination require data of different endianness, the address conversion unit  104  converts addresses to correct the mismatch of addresses in the half-word and byte units. 
       FIG. 18  is an explanatory table showing the input/output logic of the address conversion by the address conversion unit  104 . The inputs for the address conversion unit  104  are “endian”, “access size”, and “address”. The “endian” is inputted from the control unit  106 , and indicates whether the source data and destination data are of the same endianness or not. To be more precise, the “endian” indicates whether the processor accessing the source data and the processor accessing the destination data are of the same endianness or not. Moreover, in the case where one of either the source or the destination is an I/O, and the byte order (or half-word order) of the transfer data required by such I/O is in the little-endian order, the “endian” shall be the “same” if the processor accessing the source and destination data is the first processor  10 , and “different” if it is the second processor  20 . The “access size” is inputted from the control unit  106 , and either refers to a word, half-word, or byte. The “address” is the least significant 2 bits of an address, inputted from the first address counter  101 . 
     As shown in the “converted address”, in the case where the endianness is the same, the address conversion unit  104  outputs the “address” as it is (without conversion), be it a word transfer, half-word transfer, or byte-transfer. In addition, even if the endianness is different, in the case of a word transfer, the “address” is outputted as it is (without conversion). In the case of different endianness in a half-word transfer, only A 1  is inverted for output. In the case of different endianness in a byte transfer, both the bits A 1  and A 0  are inverted for output. 
     In this manner, the input/output logic in the case of different endianness in the  FIG. 18  is the same as the input/output logic of the address conversion unit  21  shown in  FIG. 8 . However, what is important is that conversion is performed in order to correct such mismatches as the mismatching half-word and byte addresses for the second processor  20  and the first processor  10 , brought about by the address conversion unit  21 . 
     The data latch  105  temporary holds transfer data in memory to memory DMA transfer. 
     The control unit  106  internally contains a control register accessible to the first processor  10  or the second processor  20 . It controls the entirety of the DMAC  32 , according to the settings of the control register. The control register holds the start addresses of the source and the destination (in the case of an I/O, its address or as designated by an I/O select signal outputted from the control unit  106 ), the transfer data size, the endianness of the source and destination (either data accessed by the first processor  10  or data accessed by the second processor  20 ), and so on. 
       FIG. 19A  is an example of a time chart illustrating a DMA transfer by the DMAC  32 . In the diagram, the source memory is, for example, the SDRAM  31 , and the destination memory is the shared memory  30 . Here, an example is shown where a specified amount of data stored by the second processor  20 , in the SDRAM  31  addresses  100  and above, undergoes DMA transfer to the addresses  900  and above, in the shared memory  30 . In addition, “SA” indicates source addresses outputted from the first address counter  101  via the address conversion unit  104 . “SD” indicates half-word source data read out from a source memory. “DA” indicates destination addresses according to the second address counter  102 , and “DD” indicates half-word destination data to be written-into a destination memory. 
     In this case, the first address counter  101  increments addresses by two, with the address  100  as the start address, and the second address counter  102  increments by two, with the address  900  as its start address. Since it is a DMA transfer of half-words of different endianness, the address conversion unit  104  only inverts “A 1 ”. As a result, the source address SA is outputted to the address bus as  102 ,  100 ,  106 ,  104 , and so on, and the destination address data is outputted as  900 ,  902 ,  904 ,  906 , and so on. In so doing, the source data is transferred, starting from the half word on the little end. 
       FIG. 19B  is a diagram showing memory images of the source memory and destination memory for the DMA transfer in  FIG. 19A . As shown in  FIG. 19B , the half-word in the source memory lower address “ 0 ” is transferred to the destination memory lower address “ 2 ”, and the half-word in address “ 2 ” is transferred to the address “ 0 ”. 
     As such, after undergoing a DMA transfer, source data stored by the big-endian-type second processor  20 , may be accessed in the destination memory, in half word units by the first processor  10 . In this way, the DMAC  32  performs a DMA transfer, correcting the mismatches shown in  FIG. 10A  and  FIG. 11A . The data that was transferred can be accessed in half-word units by the first processor  10 , without any mismatch in address. In addition, assuming that the destination address  900  is the I/O  34 , supplying half-words, in sequence, from the LSB is possible by DMA transfer. Although DMA transfer of half-words is shown in  FIG. 19A  and  FIG. 19B , the same is true in the case of byte transfer. 
     Moreover, in the case of DMA transfer between a memory and an I/O, since the control unit  106  outputs a “chip select signal” directly to the I/O, data read out from the memory is transferred directly to the I/O (or a direction opposite to it) without passing the data latch  105 . In this case too, for example, data stored by the big-endian-type second processor  20  can be transferred to the I/O from the little end, in byte units or half-word units. The same is also true when the endianness is reversed. 
     As explained so far, according to the data processing apparatus in the present embodiment, since half-word address and byte address mismatches arising from the afore-mentioned (A) and (B) are corrected by the address conversion unit  104  during DMA transfer, data that has been transferred can be correctly read out by other processors through half-word or byte unit access. This is true even for data which is not defined as structure data in the afore-mentioned (C). In addition, half-word and byte address mismatches in DMA transfer between a memory and an I/O can likewise be corrected by the address conversion unit  104 . Whether an I/O needs data from the LSB-side, or data from the MSB-side, the transfer of the required data is possible through the DMAC  32 . 
     Furthermore, instead of converting the addresses outputted from the first address counter  101 , it is also possible to have the address conversion unit  104  convert addresses outputted from the second address counter  102 . 
     In addition, in the DMAC shown in  FIG. 17 , half-word or byte order is switched in destination data, through address conversion during half-word or byte transfer. However, it is also possible to have a structure in which source data is downloaded to the data latch  105  in word units, where it is performed of half-word or byte order switching, then written into the destination memory in word units. 
     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.