Abstract:
A data write circuit is interposed between a CPU and memory, both of which operate based on the same number of bits (e.g., thirty-two bits). The CPU produces address data for designating a specific address in the memory, and access mode designation data for designating one of a byte access mode, half-word access mode, and word access mode. The data write circuit comprises a decoder for decoding the access mode designation data, a logic circuit for generating selection signals, and four selectors, each of which deals with 8-bit data consisting of eight prescribed bits of the original thirty-two bits. Each selector selects either first data read from the memory or second data output from the CPU. Therefore, each selector is capable of selecting the second data, which are substituted for the first data in the memory. Thus, it is possible to perform write operations in desired units in the memory.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to data write circuits that write data to semiconductor memories, in particular to data write circuits that are capable of writing data in prescribed units of bits, which differ from the units of bits originally employed in computers and digital signal processors (DSP), for example. 
     2. Description of the Related Art 
     Recently, small-size computers such as personal computers normally have central processing units (CPU) that operate in prescribed units of bits, such as thirty-two two bits. Hence, semiconductor memories installed in computers perform read/write operations in prescribed units of bits, such as thirty-two bits. Conventionally, however, there exist numerous software programs that perform read/write operations in units of bytes (eight bits). For this reason, conventional software programs require circuits that enable accessing memories in units of bytes under the control of 32-bit CPUs. 
     FIG. 11 is a block diagram showing a typical example of a circuit configuration having a byte-accessing capability under the control of a 32-bit CPU. Herein, reference numeral  1  designates a 32-bit CPU; reference numerals  2   a  to  2   d  designate static random-access memories (SRAMs) that perform read/write operations in units of bytes; reference numeral  3  designates a control circuit that realizes accessing in units of bytes. The CPU  1  provides 32-bit address data, which is designated by A( 31 : 00 ) wherein the number ‘31’ in the left side of the parentheses designates bit  31  within thirty-two bits, and the number ‘00’ in the right side of the parentheses designates bit  0  of thirty-two bits. Herein, the lowermost two bits, namely bit  0  and bit  1  of the 32-bit address data A( 31 : 00 ) are supplied to the control circuit  3 , and the other bits, namely address data A( 31 : 02 ) ranging from bit  2  to bit  31 , are commonly supplied to address terminals a(n: 0 ) of the SRAMs  2   a  to  2   d . The SRAM  2   a  has an input terminal i( 7 : 0 ) and an output terminal o( 7 : 0 ), both of which are arranged for the first byte ranging from bit  0  to bit  7  of the 32-bit data of the CPU  1 . Similarly, the SRAM  2   b  has an input terminal i( 15 : 8 ) and an output terminal o( 15 : 8 ), both of which are arranged for the second byte ranging from bit  8  to bit  15 ; the SRAM  2   c  has an input terminal i( 23  : 16 ) and an output terminal o( 23 : 16 ), both of which are arranged for the third byte ranging from bit  16  to bit  23 , and the SRAM  2   d  has an input terminal i( 31 : 24 ) and an output terminal o( 31 : 24 ), both of which are arranged for the fourth byte ranging from bit  24  to bit  31 . The aforementioned input terminals and output terminals of the SRAMs  2   a  to  2   d  are respectively connected with data terminals D( 31 : 00 ) for 32-bit data ranging from bit  0  to bit  31 , as follows: 
     SRAM  2   a : both the input terminal and output terminal are connected together with data terminals for the first byte ranging from bit  0  to bit  7  of the 32-bit data of the CPU  1 . 
     SRAM  2   b : both the input terminal and output terminal are connected together with data terminals for the second byte ranging from bit  8  to bit  15  of the 32-bit data of the CPU  1 . 
     SRAM  2   c : both the input terminal and output terminal are connected together with data terminals for the third byte ranging from bit  16  to bit  23  of the 32-bit data of the CPU  1 . 
     SRAM  2   d : both the input terminal and output terminal are connected together with data terminals for the fourth byte ranging from bit  24  to bit  31  of the 32-bit data of the CPU  1 . 
     In addition, the CPU  1  also provides to the control circuit  3  a signal VA that designates either a byte access mode or a word (32 bits) access mode. 
     In the aforementioned circuit configuration shown in FIG. 11, the CPU  1  provides a signal VA of ‘0’ to the control circuit  3  in order to perform write operations in units of words. In this case, the CPU  1  supplies the address data A( 31 : 02 ) to the SRAMs  2   a  to  2   d  respectively. Additionally, the CPU  1  supplies write data D( 31 : 00 ) consisting of bit  0  to bit  31  in such a way that bit  0  to bit  7  are supplied to the SRAM  2   a , bit  8  to bit  15  are supplied to the SRAM  2   b , bit  16  to bit  23  are supplied to the SRAM  2   c , and bit  24  to bit  31  are supplied to the SRAM  2   d . Incidentally, the lowermost two bits, namely bit  0  and bit  1 , of the address data A( 31 : 00 ) are irrelevant to the circuit operation. When the control circuit  3  detects the signal VA of ‘0’ output from the CPU  1 , it outputs write enable signals WENa to WENd to the SRAMs  2   a  to  2   d  respectively. Thus, all the write data D( 31 : 00 ) output from the CPU  1  are completely written to the SRAMs  2   a  to  2   d.    
     The CPU  1  provides a signal VA of ‘1’ to the control circuit  3  in order to perform write operations in units of bytes. In this case, the CPU supplies the address data A( 31 : 02 ) to the SRAMs  2   a  to  2   d  respectively, while it supplies address data A( 1 ) and A( 0 ) to the control circuit  3 . Additionally, the CPU  1  commonly supplies write data D to all the SRAMs  2   a  to  2   d . Then, the CPU  1  supplies a write enable signal WEN to one of the SRAMs  2   a  to  2   d , which is designated by the address data A( 0 ) and A( 1 ). In the case where both the address data A( 0 ) and A( 1 ) are set to ‘0’, for example, the CPU  1  supplies a write enable signal WENa to the SRAM  2   a . Thus, the write data D output from the CPU  1  are written to only the SRAM  2   a.    
     As described above, the data write circuit having byte-accessing capability conventionally requires multiple memories in order to perform read/write operations in units of bytes. Therefore, the data write circuit performs control for each of the memories. This results in unwanted complexity in the configuration of the control circuit. In addition, the overall size of the memory chip(s) increases because of increases in the wiring regions between memories. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a data write circuit having word/byte-accessing capabilities, which is realized by a simple configuration of a control circuit with a noticeable reduction of the size of a memory chip. 
     A data write circuit of this invention is interposed between the CPU and memory, both of which operate based on the same number of bits (e.g., thirty-two bits). The CPU produces address data for designating a specific address in the memory, and access mode designation data for designating one of the byte access mode, half-word access mode, and word access mode. The data write circuit comprises a decoder for decoding the access mode designation data, a logic circuit for generating selection signals, and four selectors, each of which deal with 8-bit data consisting of eight prescribed bits of the original thirty-two bits. Each selector selects either first data read from the memory or second data output from the CPU. 
     When the access mode designation data designates the byte access mode, one of the selectors is forced to select the second data, which are substituted for the first data in the memory. That is, it is possible to replace a specific byte in the memory with new data provided from the CPU. For example, only the low-order eight bits consisting of bit  0  to bit  7  are replaced with the corresponding data output from the CPU in the memory. When the access mode designation data designates the half-access mode, two of the selectors are forced to select the second data, which are substituted for the first data in the memory. That is, it is possible to replace a specific half-word in the memory by new data provided from the CPU. When the access mode designation data designates the word access mode, all the selectors are forced to select the second data, which are substituted for the first data in the memory. Thus, it is possible to entirely replace a word in the memory by new data provided from the CPU. Incidentally, it is possible to use a register instead of the memory. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, aspects, and embodiments of the present invention will be described in more detail with reference to the following drawings, in which: 
     FIG. 1 is a circuit diagram showing an example of computer circuitry using a data write circuit in accordance with the preferred embodiment of the invention; 
     FIG. 2A is a time chart showing a system clock signal CPU-CK for use in a CPU shown in FIG. 1; 
     FIG. 2B is a time chart showing a chip select signal CSN output from the CPU; 
     FIG. 2C is a time chart showing a write enable signal WEN output from the CPU; 
     FIG. 2D is a time chart showing access mode designation data DS having a value ‘0’ output from the CPU; 
     FIG. 2E is a time chart showing address data A( 31 : 0 ) output from the CPU; 
     FIG. 2F is a time chart showing output data O( 31 : 0 ) of the CPU; 
     FIG. 2G is a time chart showing a memory clock signal M-CK for use in a memory shown in FIG. 1; 
     FIG. 2H is a time chart showing a signal CSN 1  output from the data write circuit based on the chip select signal CSN; 
     FIG. 2I is a time chart showing a signal WEN 1  output from the data write circuit based on the write enable signal WEN; 
     FIG. 2J is a time chart showing address data A 1 (n: 0 ), for use in the memory, that is output from the data write circuit based on address data A( 31 : 2 ) of the CPU; 
     FIG. 2K is a time chart showing input data  11 ( 31 : 0 ) of the memory; 
     FIG. 2L is a time chart showing output data  01 ( 31 : 0 ) of the memory; 
     FIG. 3A is a time chart showing a system clock signal CPU-CK; 
     FIG. 3B is a time chart showing a chip select signal CSN; 
     FIG. 3C is a time chart showing a write enable signal WEN; 
     FIG. 3D is a time chart showing access mode designation data DS having a value ‘1’; 
     FIG. 3E is a time chart showing address data A( 31 : 0 ) of the CPU; 
     FIG. 3F is a time chart showing output data O( 31 : 0 ) of the CPU; 
     FIG. 3G is a time chart showing a memory clock signal M-CK; 
     FIG. 3H is a time chart showing a signal CSN 1  output from the data write circuit based on the chip select signal CSN; 
     FIG. 3I is a time chart showing a signal WEN 1  output from the data write circuit based on the write enable signal WEN; 
     FIG. 3J is a time chart showing address data A 1 (n: 0 ) of the memory; 
     FIG. 3K is a time chart showing input data I 1 ( 31 : 0 ) of the memory; 
     FIG. 3L is a time chart showing output data  01 ( 31 : 0 ) of the memory; 
     FIG. 4A is a time chart showing a system clock signal CPU-CK; 
     FIG. 4B is a time chart showing a chip select signal CSN; 
     FIG. 4C is a time chart showing a write enable signal WEN; 
     FIG. 4D is a time chart showing access mode designation data DS having a value ‘2’; 
     FIG. 4E is a time chart showing address data A( 31 : 0 ) of the CPU; 
     FIG. 4F is a time chart showing output data O( 31 : 0 ) of the CPU; 
     FIG. 4G is a time chart showing a memory clock signal M-CK; 
     FIG. 4H is a time chart showing a signal CSN 1  output from the data write circuit based on the chip select signal CSN; 
     FIG. 4I is a time chart showing a signal WEN 1  output from the data write circuit based on the write enable signal WEN; 
     FIG. 4J is a time chart showing address data A 1 (n: 0 ) of the memory; 
     FIG. 4K is a time chart showing input data I 1 ( 31 : 0 ) of the memory; 
     FIG. 4L is a time chart showing output data O 1 ( 31 : 0 ) of the memory; 
     FIG. 5A is a time chart showing a system clock signal CPU-CK; 
     FIG. 5B is a time chart showing a chip select signal CSN; 
     FIG. 5C is a time chart showing a write enable signal WEN; 
     FIG. 5D is a time chart showing access mode designation data DS having a value ‘0’; 
     FIG. 5E is a time chart showing address data A( 31 : 0 ) of the CPU; 
     FIG. 5F is a time chart showing input data I( 31 : 0 ) of the CPU; 
     FIG. 5G is a time chart showing a memory clock signal M-CK; 
     FIG. 5H is a time chart showing a signal CSN 1  output from the data write circuit based on the chip select signal CSN; 
     FIG. 5I is a time chart showing a signal WEN 1  output from the data write circuit based on the write enable signal WEN; 
     FIG. 5J is a time chart showing address data A 1 (n: 0 ) of the memory; 
     FIG. 5K is a time chart showing input data I 1 ( 31 : 0 ) of the memory; 
     FIG. 5L is a time chart showing output data O 1 ( 31 : 0 ) of the memory; 
     FIG. 6A is a time chart showing a system clock signal CPU-CK; 
     FIG. 6B is a time chart showing a chip select signal CSN; 
     FIG. 6C is a time chart showing a write enable signal WEN; 
     FIG. 6D is a time chart showing access mode designation data DS having a value ‘0’; 
     FIG. 6E is a time chart showing address data A( 31 : 0 ) of the CPU; 
     FIG. 6F is a time chart showing output data O( 31 : 0 ) of the CPU; 
     FIG. 6G is a time chart showing a memory clock signal M-CK; 
     FIG. 6H is a time chart showing a signal CSN 1  output from the data write circuit based on the chip select signal CSN; 
     FIG. 6I is a time chart showing a signal WEN 1  output from the data write circuit based on the write enable signal WEN; 
     FIG. 6J is a time chart showing address data A 1 (n: 0 ) of the memory; 
     FIG. 6K is a time chart showing input data I 1 ( 31 : 0 ) of the memory; 
     FIG. 6L is a time chart showing output data O 1 ( 31 : 0 ) of the memory; 
     FIG. 7A is a time chart showing a system clock signal CPU-CK; 
     FIG. 7B is a time chart showing a chip select signal CSN; 
     FIG:  7 C is a time chart showing a write enable signal WEN; 
     FIG. 7D is a time chart showing access mode designation data DS having a value ‘1’; 
     FIG. 7E is a time chart showing address data A( 31 : 0 ) of the CPU; 
     FIG. 7F is a time chart showing output data O( 31 : 0 ) of the CPU; 
     FIG. 7G is a time chart showing a memory clock signal M-CK; 
     FIG. 7H is a time chart showing a signal CSN 1  output from the data write circuit based on the chip select signal CSN; 
     FIG. 7I is a time chart showing a signal WEN 1  output from the data write circuit based on the write enable signal WEN; 
     FIG. 7J is a time chart showing address data A 1 (n: 0 ) of the memory; 
     FIG. 7K is a time chart showing input data I 1 ( 31 : 0 ) of the memory; 
     FIG. 7L is a time chart showing output data O 1 ( 31 : 0 ) of the memory; 
     FIG. 8A is a time chart showing a system clock signal CPU-CK; 
     FIG. 8B is a time chart showing a chip select signal CSN; 
     FIG. 8C is a time chart showing a write enable signal WEN; 
     FIG. 8D is a time chart showing access mode designation data DS having a value ‘2’; 
     FIG. 8E is a time chart showing address data A( 31 : 0 ) of the CPU; 
     FIG. 8F is a time chart showing output data O( 31 : 0 ) of the CPU; 
     FIG. 8G is a time chart showing a memory clock signal M-CK; 
     FIG. 8H is a time chart showing a signal CSN 1  output from the data write circuit based on the chip select signal CSN; 
     FIG. 8I is a time chart showing a signal WEN 1  output from the data write circuit based on the write enable signal WEN; 
     FIG. 8J is a time chart showing address data A 1 (n: 0 ) of the memory; 
     FIG. 8K is a time chart showing input data I 1 ( 31 : 0 ) of the memory; 
     FIG. 8L is a time chart showing output data O 1 ( 31 : 0 ) of the memory; 
     FIG. 9A is a time chart showing a system clock signal CPU-CK; 
     FIG. 9B is a time chart showing a chip select signal CSN; 
     FIG. 9C is a time chart showing a write enable signal WEN; 
     FIG. 9D is a time chart showing access mode designation data DS having a value ‘0’; 
     FIG. 9E is a time chart showing address data A( 31 : 0 ) of the CPU; 
     FIG. 9F is a time chart showing output data O( 31 : 0 ) of the CPU; 
     FIG. 9G is a time chart showing a memory clock signal M-CK; 
     FIG. 9H is a time chart showing a signal CSN 1  output from the data write circuit based on the chip select signal CSN; 
     FIG. 9I is a time chart showing a signal WEN 1  output from the data write circuit based on the write enable signal WEN; 
     FIG. 9J is a time chart showing address data A 1 (n:O) of the memory; 
     FIG. 9K is a time chart showing input data I 1 ( 31 : 0 ) of the memory; 
     FIG. 9L is a time chart showing output data O 1 ( 31 : 0 ) of the memory; 
     FIG. 10 is a block diagram showing an example of a decoder using the data write circuit for use in an AV amplifier; and 
     FIG. 11 is a block diagram showing a typical example of a data write circuit having byte-accessing capability using four SRAMs interconnected together with a CPU and a control circuit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     This invention will be described in further detail by way of examples with reference to the accompanying drawings. 
     FIG. 1 is a circuit diagram showing an example of computer circuitry using a data write circuit in accordance with the preferred embodiment of the invention. Herein, reference numeral  11  designates a 32-bit CPU;  12  designates a data write circuit; and  13  designates a memory that is accessed in units of words (32 bits). This computer circuitry has various types of memory accessing capabilities, wherein the memory  13  is accessed in units of bytes (8 bits), in units of half-words (16 bits), and in units of words (32 bits). The data write circuit  12  is not necessarily designed for the memory  13 , which can be replaced by a 32-bit register. Details of the data write circuit  12  that deal with the 32-bit register will be described later. 
     In the data write circuit  12  shown in FIG. 1, reference numeral  16  designates a NAND gate that is supplied with a chip select signal CSN from the CPU  11 ;  17  designates a decoder that decodes address data A( 31 : 2 ) output from the CPU  11 ;  18  designates an OR gate that is supplied with a write enable signal WEN from the CPU  11 ;  19  designates a D flip-flop (or D-type flip-flop: DF) whose clock terminal receives a memory clock signal M-CK (see FIG.  2 G). The memory clock signal M-CK has a frequency that is two times the frequency of a system clock signal CPU-CK (see FIG. 2A) of the CPU  11 . 
     In addition, reference numeral  21  designates a decoder that decodes access mode designation data DS output from the CPU  11 . That is, the decoder  21  decodes the access mode designation data DS to provide three outputs, namely ‘0’ designating a byte (8-bit) transfer mode, ‘1’ designating a half-word (16-bit) transfer mode, and ‘2’ designating a word (32-bit) transfer mode. Reference numerals  23  to  26  designate three-input-type AND gates, each of which performs an AND operation based on address data A( 1 ) and A( 0 ) (namely, bit  1  and bit  0  of the 32-bit address data) output from the CPU  11 , and the value of the output terminal ‘0’ of the decoder  21 . Specifically, the AND gates  23 - 26  differ from each other in the arrangement of negated inputs (or inverters), which are represented by small circle marks in FIG.  1 . That is, the third input of the AND gate  24  receiving the address data A( 0 ) is a negated input; the second input of the AND gate  25  receiving the address data A( 1 ) is a negated input; and the second and third inputs of the AND gate  26  receiving the address data A( 1 ) and A( 0 ) respectively are negated inputs. Reference numerals  27  and  28  designate two-input-type AND gates, each of which performs an AND operation based on the address data A( 1 ) and the value of the output terminal ‘1’ of the decoder  21 . Herein, the second input of the AND gate  28  receiving the address data A( 1 ) is a negated input. Reference numerals  29  to  32  designate three-input-type OR gates. The first inputs of the OR gates  29  and  30  receive the output of the AND gate  27 , while the first inputs of the OR gates  31  and  32  receive the output of the AND gate  28 . The second inputs of the OR gates  29  to  32  receive the outputs of the AND gates  23  to  26  respectively. All the third inputs of the OR gates  29  to  32  receive the value of the output terminal ‘2’ of the decoder  21 . Reference numerals  35  to  38  designate 8-bit selectors, each of which has two inputs (namely, ‘0’ and ‘1’) for receiving 8-bit data. In response to the outputs of the OR gates  29  to  32 , the selectors  35  to  38  each select one of the two inputs (namely, ‘0’ or ‘1’) thereof, so that the selected input is output therefrom. 
     Next, the overall operation of the computer circuitry shown in FIG. 1 will be described in detail with reference to the time charts shown in FIGS. 2A-2L through FIGS. 9A-9L. All addresses and data are expressed in hexadecimal notation, wherein in the time charts, the number following ‘0x’ is represented in hexadecimal notation. 
     1. Memory Byte Write 
     FIGS. 2A to  2 L are time charts that are used to explain the overall operation of the computer circuitry of FIG. 1 for performing write operations on the memory  13  in units of bytes. As shown in FIG. 2E, the address data A( 31 : 00 ) of the CPU  11  designate address 40000 in the duration between times t 1  and t 3 , and then they designate address 40001 in the duration after time t 3 . That is, these charts are used to explain write operations of the CPU  11  that write data ‘1’ at address 40000 and then write data ‘2’ at address 40001. Specifically, in order to write data ‘1’ to the memory  13 , the CPU  11  provides output data O( 31 : 00 ) consisting of four series of write data ‘01010101’, which is shown in FIG.  2 F. In order to write data ‘2’ to the memory  13 , the CPU  11  provides output data O( 31 : 00 ) consisting of four series of write data ‘02020202’. FIG. 2A shows a system clock signal CPU-CK for the CPU  11 . FIG. 2G shows a memory clock signal M-CK that is synchronized with the system clock signal CPU-CK and whose frequency is two times the frequency of the system clock signal CPU-CK. 
     In order to enable the aforementioned write operations in which the CPU  11  writes data ‘1’ at address 40000 and then writes data ‘2’ at address 40001, the CPU  11  outputs a chip select signal CSN (see FIG.  2 B), a write enable signal WEN (see FIG.  2 C), and access mode designation data DS (see FIG. 2D) as well as the aforementioned address data A( 31 : 00 ) and output data O( 31 : 00 ) at time t 1 . The chip select signal CSN is supplied to the NAND gate  16  shown in FIG. 1, wherein it is transmitted through the NAND gate  16  that is opened in response to the output of the decoder  17 , so that the NAND gate  16  outputs a chip select signal CSN 1  (see FIG. 2H) to the memory  13 . 
     The OR gate  18  and the D flip-flop (DF)  19  convert the write enable signal WEN to a signal WEN 1  (see FIG. 21) that is synchronized with the memory clock signal M-CK. This signal WEN 1  is supplied to the memory  13 . The access mode designation data DS having a value ‘0’ is supplied to the decoder  21 , which in turn provides ‘1’ at the output terminal ‘0’ thereof Within the address data A( 31 : 00 ), high-order address data A( 31 : 2 ) are supplied to the address terminal of the memory  13  as address data A 1 (n: 0 ), which is shown in FIG.  2 J. Specifically, the address data A 1 (n: 0 ) designate address 10000 in the duration between times t 1  and t 3 . Within the address data A( 31 : 00 ), the remaining low-order address data A( 1 ) and A( 0 ), both of which are presently set to ‘0’, are supplied to the AND gates  23  to  28 . 
     The time t 1  corresponds to a leading edge of a pulse of the system clock signal CPU-CK as well as a leading edge of a pulse of the memory clock signal M-CK. At time t 2  that corresponds to a leading edge of the next pulse of the memory clock signal M-CK, the memory  13  reads the contents of the address data A 1 (n: 0 ) to output data O 1 ( 31 : 0 ) (see FIG. 2L) from the output terminal thereof Specifically, the memory  13  outputs data ‘0’ as the output data O 1 ( 31 : 0 ) in the duration between times t 2  and t 3 . The output data O 1 ( 31 : 0 ) of the memory  13  are supplied to the data input terminal of the CPU  11  as data I( 31 : 0 ). They are also supplied to the input terminals ‘0’ of the selectors  35  to  38  respectively. 
     When the AND gates  23  to  26  are supplied with the value (i.e., ‘1’) of the output terminal ‘0’ of the decoder  21 , and the address data A 1 ( 1 ) and A( 0 ) (both ‘0’), only the AND gate  26  outputs ‘1’, which is supplied to the selector  38  via the OR gate  32 . Thus, the selector  38  selects the input terminal ‘1’ thereof, so that it selectively outputs 8-bit data consisting of bit  0  to bit  7  (namely, data ‘01’, see FIG. 2F) of the output data O( 31 : 00 ) of the CPU  11 . That is, the selector  38  selectively outputs the 8-bit data to the data input terminal of the memory  13 . All the remaining AND gates  23 - 25 ,  27 , and  28  output ‘0’, and all the remaining OR gates  29 - 31  output ‘0’. The output data O 1 ( 31 : 0 ) are divided into four elements, namely O 1 ( 31 : 24 ), O 1 ( 23 : 16 ), O 1 ( 15 : 8 ), and O 1 ( 7 : 0 ), which are respectively supplied to the input terminals ‘0’ of the selectors  35  to  38 . In the aforementioned condition, only the selector  38  selects the input terminal ‘1’ thereof to provide the data ‘01’ to the data input terminal of the memory  13 , while the other selectors  35 ,  36 , and  37  select the input terminals ‘0’ thereof to provide the output data O 1 ( 31 : 24 ), O 1 ( 23 : 16 ), and O 1 ( 15 , 8 ) directly back to the data input terminal of the memory  13 . 
     In summary, when both the address data A( 1 ) and A( 0 ) are set to ‘0’, only the 8-bit data consisting of bit  0  to bit  7  of the output data O 1 ( 31 : 0 ) of the memory  13  are replaced by the 8-bit data consisting of bit  0  to bit  7  of the output data O( 311 : 0 ) of the CPU  11 , which are supplied to the data in put terminal of the memory  13  as a part of the input data I 1 ( 31 : 0 )(see FIG.  2 K). At time t 2 , when the write enable signal WEN  1  (see FIG. 21) starts decreasing, the aforementioned input data I 1 ( 31 : 0 ) are written to the memory  13  at a specific address (i.e., address 10000) designated by the address data A 1 (n: 0 ). 
     At time t 3 , the address data A( 31 : 0 ) of the CPU  11  designate address 40000, the data write circuit  12  supplies to the memory  13  the address data A((n: 0 ) that designate the foregoing address 10000 again. At time t 4 , the written contents of the memory  13  at address 10000 are read and supplied to the selectors  35  to  38  respectively. At this time, the address data A( 1 ) and A( 0 ) are respectively set to ‘0’ and ‘1’, while the decoder  21  outputs ‘1’ at the output terminal ‘0’ thereof. Therefore, the AND gate  25  outputs ‘1’, which is supplied to the selector  37  via the OR gate  31 . Thus, the selector  37  selects the input terminal ‘1’ thereof to output 8-bit data consisting of bit  8  to bit  15  of the output data ( 31 : 0 ) of the CPU  11 . As shown in FIG. 2F, the selector  37  selectively outputs data ‘02’, which is supplied to the data input terminal of the memory  13 . The other selector selectors  35 ,  36  and  38  select the input terminals ‘0’ thereof to provide the output data O 1 ( 31 : 24 ), O 1 ( 23  : 16 ), and O 1 ( 7 : 0 ) back to the data input terminal of the memory  13 . At time t 4  when the signal WEN 1  starts decreasing, the aforementioned output data of the selectors  35  to  38  are written into the memory  13  at address 10000. 
     In the duration between times t 1  and t 3  in which the address data A( 31 : 0 ) of the CPU  11  designate address 40000, the 8-bit data O( 7 : 0 ) consisting of bit  0  to bit  7  of the output data O( 31 : 0 ) of the CPU  11  are written to a part of the address 10000 ranging from bit  0  to bit  7 . In the duration after time t 3  in which the address data A( 31 : 0 ) of the CPU  11  designate address 40001, the 8-bit data O( 15 : 8 ) consisting of bit  8  to bit  15  of the output data O( 31 : 0 ) of the CPU  11  are written to a part of the address 10000 ranging from bit  8  to bit  15 . 
     2. Memory Half-Word Write 
     FIGS. 3A to  3 L are time charts that are used to explain the overall operation of the computer circuitry for performing write operations on the memory  13  in units of half-words (i.e., 16 bits). As shown in FIG. 3E, the address data A( 31 : 0 ) of the CPU  11  designate address 40000 in the duration between times t 1  and t 3 , and then designate address 40002 in the duration after time t 3 . Hence, the CPU  11  writes data ‘1’ to the memory  13  at address 40000, and then it writes data ‘2’ to the memory  13  at address 40002. In order to write the data ‘1’ into the memory  13 , the CPU  11  provides two series of data ‘00010001’ as the output data O( 31 : 0 ) (see FIG.  3 F). In order to write the data ‘2’ into the memory  13 , the CPU  11  provides two series of data ‘00020002’ as the output data O( 31 : 0 ). In this case, the CPU  11  outputs data ‘1’ as the access mode designation data DS. 
     When the CPU  11  supplies the decoder  21  with the access mode designation data DS having a value ‘1’, the decoder  21  outputs ‘1’ at the output terminal ‘1’ thereof, which is supplied to the AND gates  27  and  28  respectively. Both the address data A( 1 ) and A( 0 ) are set to ‘0’ when the address data A( 31 : 0 ) designate address 40000. At this time, the AND gate  28  outputs data ‘1’, which is delivered to the selectors  37  and  38  via the OR gates  31  and  32 . Thus, the selectors  37  and  38  select the input terminals ‘1’ thereof to output the data O( 15 : 8 ) and O( 7 : 0 ) of the output data O( 31 : 0 ) of the CPU  11 , while the other selectors  35  and  36  select the input terminals ‘2’ thereof to output the data O 1 ( 31 : 24 ) and O 1 ( 23 : 16 ) of the output data O 1 ( 31 : 0 ) of the memory  13 . That is, the low-order sixteen bits of the output data O 1 ( 31 : 0 ) of the memory  13  are replaced with the low-order sixteen bits of the output data O( 31 : 0 ) of the CPU  11 . In other words, the low-order sixteen bits of the output data O( 31 : 0 ) of the CPU  11  are newly written to the memory  13 , while the high-order sixteen bits of the output data O 1 ( 31 : 0 ) are directly retained in the memory  13 . In the next duration when the address data A( 31 : 0 ) designate address 40002, the address data A( 1 ) and A( 0 ) are set to ‘1’ and ‘0’ respectively. At this time, the AND gate  27  outputs data ‘1’, which is delivered to the selectors  35  and  36  via the OR gates  29  and  30 . Thus, the selectors  35  and  36  select the input terminals ‘1’ thereof to output the data O( 31 : 24 ) and O( 23 : 16 ) of the output data O( 31 : 0 ) of the CPU  11 , while the other selectors  37  and  38  select the input terminals ‘0’ thereof to output the data O 1 ( 15 : 8 ) and O 1 ( 7 : 0 ) of the output data O 1 ( 31 : 0 ) of the memory  13 . That is, the high-order sixteen bits of the output data O 1  ( 31 : 0 ) of the memory  13  are replaced with the high order sixteen bits of the output data O( 31 : 0 ) of the CPU  11 . Hence, the high-order sixteen bits of the output data O( 31 : 0 ) of the CPU  11  are newly written to the memory  13 , while the low-order sixteen bits of the output data O 1 ( 31 : 0 ) are directly retained in the memory  13 . 
     3. Memory Word Write 
     FIGS. 4A to  4 L are time charts that are used to explain the overall operation of the computer circuitry for performing write operations on the memory  13  in units of words (i.e., 32 bits). As shown in FIG. 4E, the address data A( 31 : 0 ) of the CPU  11  designate address 40000 in the duration between times t 1  and t 3 , and then designate address 40004 in the duration after time t 3 . In this case, the CPU  11  writes data ‘1’ into the memory  13  at address 40000, and then it writes data ‘2’ into the memory  13  at address 40004. In response to the address data A( 31 : 0 ) of the CPU  11  shown in FIG. 4E, the address data A 1 (n: 0 ) of the memory  13  sequentially designate address 10000 and address 10001 in synchronization with address 40000 and address 40004 respectively. In addition, the CPU  11  sequentially produces write data ‘00000001’ and ‘00000002’. Further, the CPU  11  provides the access mode designation data DS having a value ‘2’. 
     When the CPU  11  supplies the decoder  21  with the access mode designation data DS having a value ‘2’, the decoder  21  outputs ‘1’ at the output terminal ‘2 ’ thereof, which is supplied to each of the OR gates  29  to  32 . All the OR gates  29  to  32  output the same data ‘1’ to each of the selectors  35  to  38 . Therefore, all the selectors  35  to  38  select the input terminals ‘1’ thereof to output the data O( 31 : 24 ), O( 23 : 16 ), O( 15 : 8 ), and O( 7 : 0 ) of the output data O( 31 : 0 ) of the CPU  11 . That is, all bits of the output data O( 31 : 0 ) of the CPU  11  are supplied to the memory  13  via the selectors  35  to  38 , wherein they are written to the address designated by the address data A 1 (n: 0 ). 
     4. Memory Read FIGS. 5A to  5 L are time charts that are used to explain the overall operation of the computer circuitry for performing read operations on the memory  13 . Herein, the memory read mode is designed to enable read operations in units of words because even though the CPU  11  reads data from the memory  13  in units of words, it can process data in units of bytes therein. Hence, there is no need to provide different types of read operations that are performed in units of bytes and in units of half-words. 
     At time t 1 , the address data A( 31 : 0 ) of the CPU  11  (see FIG. 5E) designate address 40000, so that the high-order thirty bits (namely, data ‘10000’) are supplied to the address terminal of the memory  13  as the address data A 1 (n: 0 ) (see FIG.  5 J). At time t 2 , the CPU  11  starts reading data ‘201’ (see FIG. 5L) from the memory  13 , so that the read data are supplied to the data input terminal of the CPU  11  as the input data I( 31 : 0 ) (see FIG.  5 F). In the CPU  11 , 8-bit data consisting of bit  0  to bit  7  of the input data I( 31 : 0 ) are subjected to data processing. At time t 3 , the address data A( 31 : 0 ) of the CPU  11  designate address 40001, whereas the address data A 1 (n: 0 ) of the memory  13  still designate address 10000 (see FIG.  5 J). At time t 4 , the same data ‘201’ are read from the memory  13  and are then supplied to the data input terminal of the CPU  11 , which is shown in FIGS. 5F and 5L. In the CPU  11 , 8-bit data consisting of bit  8  to bit  15  of the input data I( 31 : 0 ) are subjected to data processing. 
     5. Register Byte Write 
     FIGS. 6A to  6 L are time charts for explaining the write operations that are performed in units of bytes in the computer circuitry of FIG. 1, which is modified to use a (32×M-bit) register ‘ 13   a  ’ instead of the memory  13 . Similar to the aforementioned memory byte write mode, the CPU  11  provides the access mode designation data DS having a value ‘0’, which is supplied to the decoder  21 . Hence, the decoder  21  outputs ‘1’ at the output terminal ‘0’ thereof, which is supplied to each of the AND gates  23  to  26 . As a result, only one selector is designated by the low order address data A( 1 ) and A( 0 ) of the address data A( 31 : 0 ) of the CPU  11  and is selected from among the selectors  35  to  38 , so that the selected selector outputs a prescribed 8-bit part of the output data O( 31 : 0 ) of the CPU  11  to the register  13   a . Thus, the output data O 1 ( 31 : 0 ) of the register  13   a , a part of which is replaced with the prescribed 8-bit part of the output data O( 31 : 0 ) of the CPU  11 , are written into the register  13   a.    
     Specifically, at time t 1 , the address data A( 31 : 0 ) of the CPU  11  (see FIG. 6E) designate address 40000, so that the register  13   a  is supplied with the address data A 1 (n:O) designating address 10000. Hence, data ‘0’ is read from the register  13   a  and is supplied to each of the selectors  35  to  38 , for example. Both the low-order address data A( 1 ) and A( 0 ) are set to ‘0’ when the address data A( 31 : 0 ) designate address 40000 in the duration between times t 1  and t 3 . Therefore, only the AND gate  26  outputs data ‘1’, which is supplied to the selector  38  via the OR gate  32 . Thus, the selector  38  selects the input terminal ‘1’ thereof to output the data O( 7 : 0 ) consisting of bit  0  to bit  7  of the output data O( 31 : 0 ) of the CPU  11 . That is, the output data O 1 ( 31 : 0 ) of the register  13   a , whose low-order 8-bit portion consisting of bit  0  to bit  7  is replaced with the low-order 8-bit data O( 7 : 0 ) of the CPU  11 , are supplied to the data input terminal of the register  13   a  as the input data I 1 ( 31 : 0 ). At time t 3  when the signal WEN 1  starts increasing, the input data I 1 ( 31 : 0 ) are written into the register  13   a , and then they are output from the data output terminal of the register  13   a  (see FIG.  6 L). 
     6. Register Half-Word Write 
     FIGS. 7A to  7 L are time charts for explaining the write operations that are performed in units of half-words in the computer circuitry of FIG. 1 that use the register  13   a  instead of the memory  13 . In this case, the CPU  11  outputs the access mode designation data DS having a value ‘1’ to the decoder  21 . Details of the register half-word write mode are similar to the foregoing memory half-word write mode, which was described in conjunction with FIGS. 3A to  3 L. As a result, the output data O 1 ( 31 : 0 ) of the register  13   a , whose high-order 16-bit portion or low-order 16-bit portion is replaced with the corresponding portion of the output data O( 31 : 0 ) of the CPU  11 , are written into the register  13   a.    
     7. Register Word Write 
     FIGS. 8A to  8 L are time charts for explaining the write operations that are performed in units of words in the computer circuitry of FIG. 1 that uses the register  13   a  instead of the memory  13 . In this case, the CPU  11  outputs the access mode designation data DS having a value ‘2’ to the decoder  21 . Details of the register word write mode are similar to the foregoing memory word write mode, which was described in conjunction with FIGS. 4A to  4 L. As a result, the output data O( 31 : 0 ) of the CPU  11  are entirely supplied to the data input terminal and are written into the register  13   a.    
     8. Register Read 
     FIGS. 9A to  9 L are time charts for explaining the read operations that are performed in the computer circuitry of FIG. 1 that uses the register  13   a  instead of the memory  13 . Details of the register read mode are similar to the foregoing memory read mode, which was described in conjunction with FIGS. 5A to  5 L. 
     Next, an example of the application of the data write circuit will be described with reference to FIG. 10, which is a block diagram showing the configuration of a decoder for use in an AV amplifier. In FIG. 10, a CPU  40  accesses a memory bank  43  or a register bank  44  via an internal memory interface  41  and a memory management unit (MMU)  42 . Thus, the CPU  40  performs read/write operations on the memory bank  43  or the register bank  44 . In addition, a DSP  45  accesses the memory bank  43  or the register bank  44 . The aforementioned data write circuit of this invention can be installed in the internal memory interface  41  to enable write operations in units of bytes or in units of half-words with respect to the memory bank  43  or the register bank  44 . 
     As described heretofore, the data write circuit of this invention performs write operations in prescribed units of bits between the CPU and memory (or register), each of which operates based on the same number of bits (e.g., thirty-two bits). Specifically, the data write circuit of this invention requires a single memory (or register) to perform write operations in prescribed units of bits (e.g., bytes, half-words), the number of which is reduced compared with the original number of bits (e.g., words), without using multiple memories. As a result, this invention contributes to a noticeable reduction in the overall area of the memory chip, which may be reduced by approximately 20% compared with the conventional circuitry. 
     As this invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, the present embodiment is therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalents of such metes and bounds are therefore intended to be embraced by the claims.