Abstract:
A digital signal processor (DSP) is disclosed which is capable of starting a new arithmetic operation even when results of completed arithmetic operations cannot be written into accumulators because they already contain results. The DSP includes an arithmetic operation unit, a bus, at least one accumulator and a bypass device. The bus transfers data to be processed by the arithmetic operation unit in addition to the results of arithmetic processing performed by the arithmetic operation unit. The at least one accumulator holds the results of arithmetic processing and delivers the results to the bus. The bypass device may contain a delay element that delays delivery of the results to the bus for the same length of time that it would take for the results to be delivered to the bus via the at least one accumulator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. application Ser. No. 08/990,100 filed Dec. 12, 1997, now U.S. Pat. No. 6,205,459. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a digital signal processor (DSP) for carrying out signal processing under the control of a control processor and a digital signal processing system including the control processor and the DSP. 
     2. Prior Art 
     FIG. 1 shows the arrangement of a conventional DSP that carries out signal processing under the control of a control processor. In the figure, symbols C and D designate external registers which are storage means for storing input data to be processed by the DSP, and non-final and final results of the processing. 
     A multiplication/addition block  100  is connected to the external registers C and D via buses  13  and  14 . The multiplication/addition block  100  is comprised of a multiplication/addition unit  10  for executing arithmetic (or arithmetic-logic) operations on input data supplied via the buses  13  and  14 , and two accumulators ACC 0  and ACC 1  for storing results of the arithmetic operations by the multiplication/addition unit  10 . 
     The multiplication/addition unit  10  is comprised of two internal registers A and B, a multiplier  11 , and an ALU (arithmetic-logic unit)  12 . The internal registers A and B are for temporarily storing input data to be used in the arithmetic (or arithmetic-logic) operations by the ALU  12 . Input data used for a multiplication operation is supplied to the multiplier  11  necessarily via the bus  13  or  14  through the internal register A or B, while input data for an arithmetic (or arithmetic-logic) operation other than a multiplication is supplied to the ALU  13  via the bus  13  or  14  through the internal register A or B. 
     The ALU  12  carries out arithmetic (or arithmetic-logic) operations on input data supplied from the internal registers A, B, the multiplier  11  and/or the accumulators ACC 0 , ACC 1 . The accumulators ACC 0 , ACC 1  are for storing results of the arithmetic operations by the ALU  12 . The data written into the accumulators ACC 0 , ACC 1  are delivered to the bus  14  or again input to the ALU  12 . 
     In FIG. 1, the buses and other signal lines are shown with numerals, such as 24 and 48, which indicate the bit widths of these signal lines. As shown in the figure, the buses  13 ,  14  and the output signal lines from the internal registers A, B each have a bit width of 24 bits, while a signal line from the multiplier  11 , which outputs results of multiplication of 24-bit data by 24-bit data, has a bit width of 48. A signal line from the ALU  12 , which occasionally accumulates output data from the multiplier  11 , i.e. a result of a multiplication operation thereof, which has a bit width of 48 bits, has a bit width of 56 with an overhead of 8 bits added to the bit width of the output data from the multiplier  11 . 
     The component elements and parts of the DSP described above are controlled by a program stored in advance in memory means, by so-called pipeline control. That is, assuming, for instance, that the DSP carries out convolution of time-series sample data with a predetermined sequence of filter coefficients, this convolution operation is carried out in the following manner: 
     First, at a certain clock timing, the multiplier  11  multiplies a first set of sample data and a coefficient stored in the internal registers A and B, respectively, and delivers a result of the multiplication (first multiplication) to the ALU  12 . At the same time, a second set of sample data and a coefficient are written into the internal registers A and B, respectively. 
     Then, at the next clock timing, the result of the first multiplication is written from the ALU  12  e.g. into the accumulator ACC 0 , and at the same time a result of multiplication of the second set of the sample data and the coefficient (second multiplication) is supplied from the multiplier  12  to the ALU  12  and further a third set of sample data and a coefficient are written into the internal registers A and B. 
     Then, at the next clock timing, the result of the first multiplication delivered from the accumulator ACC 0  and the result of the second multiplication delivered from the multiplier  11  are added together (i.e. accumulated) by the ALU  12 , and a result of this addition is written into the accumulator ACC 0 . At the same time, a result of multiplication of the third set of the sample data and the coefficient (third multiplication) is delivered from the multiplier  11  to the ALU  12 , and further a fourth set of sample data and a coefficient are written into the internal registers A and B. 
     Hereafter, multiplication of sample data and a coefficient, and accumulation of a result of the multiplication are repeatedly carried out in the same manner. Then, when multiplication operations of all sets of sample data and coefficients and accumulation of all results of the multiplication operations are completed, a result of this convolution operation, which is obtained at this time point as contents of the accumulator ACC 0 , is delivered to the bus  14 , from which it is supplied to an external device. 
     Thus, arithmetic operations constituting a convolution operation, such as a multiplication operation and an addition operation, are carried out in parallel by respective devices, which enable the arithmetic operations to be executed efficiently. 
     Although the operation of the DSP is described above by referring to an example of the convolution operation, there is a case where a further multiplication operation is carried out on output data from the multiplication/addition unit  10 , depending on kinds of arithmetic processing to be carried out. In this case, the output data from the multiplication/addition unit  10  is delivered via the accumulator ACC 0  or ACC 1  to the bus  14 , from which it is written into the internal register A or B of the multiplication/addition unit  10 . 
     In the conventional DSP, when the ALU  12  has carried out an arithmetic operation, the ALU  12  cannot start the next arithmetic operation before a result of the arithmetic operation is stored in the accumulator ACC 0  or ACC 1 . Therefore, if the ACLU  12  has completed an arithmetic operation before results of its preceding arithmetic operations written into the accumulators ACC 0  and ACC 1  are transferred to another device, the ACLU  12  cannot write the result of the new arithmetic operation into any of the accumulators ACC 0  and ACC 1 , so that the ALU  12  has to wait starting the next arithmetic operation until the accumulator ACC 0  or ACC 1  becomes available. 
     Further, some kinds of arithmetic processing require lots of arithmetic operations to be executed within a predetermined time period. When such a kind of arithmetic processing is executed by the DSP, if the start of the next arithmetic operation is delayed due to unavailability of the accumulators ACC 0  and ACC 1 , there can be a case where all the required arithmetic operations cannot be completed within the predetermined time period. Conventionally, in such a case, data stored in one accumulator ACC 0  or ACC 1  is transferred to one of the external registers C and D, and a result of an arithmetic operation by the ALU  12  is stored in the one accumulator ACC 0  or ACC 1  which is made available by the transfer of the data therefrom, thereby enabling the ALU  12  to start the next arithmetic operation. In general, however, an accumulator of this kind has a bit accuracy corresponding to the bit width of 56 bits which is higher than a bit accuracy required of data processed by the DSP, which corresponds to the bit width of 24 bits, and hence if the contents of the accumulator are once written into an external register, this degrades the bit accuracy of the data, and in the worst case the data itself can be lost. 
     Further, there can be a case where the accumulator ACC 0  or ACC 1  suffers from an overflow during processing by the DSP. If the contents of this accumulator are delivered to the bus  14 , the data can be lost by operation of an overflow-protect circuit if it is arranged in the path of delivery of the contents of the accumulator to the bus. 
     In the conventional DSP, it takes different time periods to execute arithmetic operations of identical contents, depending on whether the arithmetic operations are executed by using internal registers or by using external registers. FIG. 2 shows an example of two instructions for execution of arithmetic operations of identical contents by using internal registers and by using external registers, respectively. 
     First, an instruction shown in a left-hand column of FIG. 2, i.e. “reg_a=acc 0 +=reg_a*reg_b”, which means “Multiply data stored in the internal register A by data stored in the internal register B, store the sum of the resulting product and data stored in the accumulator ACC 0 , in the accumulator ACC 0 , and store this data stored in the accumulator ACC 0  in the internal register A”, is carried out by sequentially executing the following three steps: 
     Step 1: The multiplier  11  multiplies the data stored in the internal register A by the data stored in the internal register B; 
     Step 2: The ALU  12  adds together the data stored in the accumulator ACC 0  and the product from the multiplier  11 , and the resulting sum is stored in the accumulator ACC 0 ; and 
     Step 3: The data stored in the accumulator ACC 0  is stored in the internal register A. 
     Each of the above steps is carried out over one clock, and therefore it takes a total of three clocks to carry out the above instruction. 
     In contrast, an instruction shown in a left-hand column of FIG. 2, i.e. “reg_a=acc 0 +=reg_c*reg_d”, which means “Multiply data stored in the external register C by data stored in the external register D, store the sum of the product and data stored in the accumulator ACC 0 , in the accumulator ACC 0 , and store this data stored in the accumulator ACC 0  in the internal register A”, is identical in contents of arithmetic operations to the above-mentioned instruction, but it takes a total of four clocks to carry out the instruction, since it is required to sequentially execute the following four steps: 
     Step 1: Data stored in the external register C is transferred to the internal register A, and data stored in the external register D to the internal register B; 
     Step 2: The multiplier  11  multiplies the data stored in the internal register A by the data stored the internal register B; 
     Step 3: The ALU  12  adds together the data stored in the accumulator ACC 0  and the product from the multiplier  11 , and the resulting sum is stored in the accumulator ACC 0 ; and 
     Step 4: The data stored in the accumulator ACC 0  is stored in the internal register A. 
     That is, the arithmetic operations carried out by using the external resisters require transfer of data from the external registers to the internal registers and hence it takes one clock longer to complete the arithmetic operations than when arithmetic operations of the identical contents are carried out by using the internal registers. Although in the above example, multiplications are carried out, even in a case where additions are carried out, similarly there arise a difference in processing time equivalent to one clock between when data in the external internal registers are added together and when data in the internal registers are added together. 
     As described above, according to the conventional DSP, even if arithmetic operations of identical contents are carried out, it takes different time periods to carry out the arithmetic operations, depending on whether internal registers or external internal registers are used. This makes it necessary to carry out timing adjustment, such as changing timing of start of execution of each arithmetic operation (changing a bus request line number) depending on whether the arithmetic operation uses internal registers or external internal registers, rendering timing designing of the program even more difficult to carry out. For example, if an instruction for arithmetic operations using the external registers C and C is first carried out, and then another instruction for arithmetic operations using the internal registers A and B is carried out one clock later, there arises an inconvenience that results of multiplications carried out according to these instructions are delivered to the bus  14  at the same timing. In addition to such an inconvenience related to the timing of outputting results of arithmetic operations carried out according to instructions, there arises, depending on how a program is written, an inconvenience that data which is prepared by executing a preceding instruction and stored in an accumulator is overwritten by data prepared by executing another instruction subsequent to the preceding instruction if this data is written into the accumulator before the data prepared by the preceding instruction is delivered from the accumulator to a proper destination. Therefore, the programmer has to be very careful that instructions of a program are each carried out at such a suitable timing as will not cause the above-mentioned inconveniences, by always confirming a time period required to execute each instruction. This requires a great deal of labor of the programmer. 
     FIG. 3 shows the arrangement of a digital signal processing system comprised of a DSP of a kind described above and a control processor. More specifically, the digital signal processing system is comprised of a DSP  1 , a RISC (reduced instruction set computer)-CPU (central processing unit)  2 , and a RAM (random access memory), all built in a single chip. 
     The RAM  3  is a dual port RAM which can be accessed both by the DSP  1  and the RISC-CPU  2 . The DSP  1  is provided with an MMU (memory management unit)  15  for controlling writing of data into the RAM  3  and reading of data therefrom. Further, the RISC-CPU  2  is also provided with an MMU similar to the MMU  5 . The RAM  3  is used not only as a work RAM used both by the DSP  1  and the RISC-CPU  2  but also as means for passing data between the DSP  1  and the RISC-CPU  2 . 
     The DSP  1  is identical in construction with that described above with reference to FIG. 1 except that the external registers C and D are connected to the MMU  15 , and therefore detailed description thereof is omitted. 
     When data is transferred between the DSP  1  and the RISC-CPU  2 , the external registers C and D are used by the DSP  1  as means for storing data to be transferred therefrom to the RISC-CPU  2  and vice versa. More specifically, data to be transferred from the DSP  1  to the RISC-CPU  2  is stored in advance of the transfer in the register C or D, and then the MMU  15  stores the data stored in the register C or D at desired addresses within the RAM  3 . The data stored in the RAM  3  is subsequently read by the MMU of the RISC-CPU  2 . On the other hand, data to be transferred from the RISC-CPU  2  to the DSP  1  is first stored in the RAM  3 , and the data stored in the RAM  3  is read by the MMU  15  of the DSP  1  and stored in the register C or D, for use in arithmetic operations or the like. 
     FIG. 4 shows another type of digital signal processing system, which is distinguished from the FIG.  3  digital signal processing system in that the external registers C and D of the DSP  1  are connected to the RISC-CPU  2  and a RAM  5  via a bus  4 . According to this arrangement of the system, the RISC-CPU  2  directly writes data in the external registers C, D, whereby the data is transferred from the RISC-CPU  2  to the DSP  1 , while on the other hand the RISC-CPU  2  reads data stored in the registers C and D, whereby the data is transferred from the DSP  1  to the RISC-CPU  2 . The RAM  5  is a single port RAM which is connected to the bus  4  and used by the DSP  1  and the RISC-CPU  2  as a work RAM. 
     Now, to carry out a multiplication/addition operation by the conventional digital signal processing system shown in FIG. 3, two pieces of data are required for each multiplication/addition operation. However, the DSP  1  can only read out one piece of data from the RAM  3  per one reading operation, so that two reading operations are required to obtain data required by one multiplication/addition operation. This has been an obstacle to continuous high-speed execution of multiplication/addition operations. 
     One typical use of the digital signal processing system is a filtering operation in which externally-supplied time-series sample data are convolved with a predetermined sequence of coefficients. In most cases, one of two pieces of data for a multiplication/addition operation is data which incessantly varies, while the other is data of coefficients which are fixed in value. However, in the FIG. 3 system, the MMU  15  of the DSP  1  uniformly controls reading of two kinds of data which are thus different in characteristics, which imposes burden much burden on the MMU  15  and forms a bottle neck to efficient supply of data and coefficients to the arithmetic operations of multiplication/addition. In view of the efficiency of the processing, it is desirable that data which is incessantly updated and data of coefficients having fixed values should be efficiently read under different types of reading control suitable for their respective different characteristics. However, insofar as the common RAM is used for managing the storing of such data and coefficients, it is very difficult to carry out such different types of reading control. 
     On the other hand, in the FIG. 4 digital signal processing system, data required by the DSP for multiplication/addition operation are written into the external registers C and D by the RISC-CPU  2 . However, these external registers C, D are connected to the RISC-CPU  2  via the common bus  4 , so that the data has to be written into these registers through two reading operations separately carried out. Therefore, the DSP  1  has to wait for the RISC-CPU  2  to carry out writing of data two times before it starts the arithmetic operations of multiplication/addition. 
     Further, when the RISC-CPU  2  and the DSP  1  are interfaced by way of the registers C and D as in the FIG. 4 system, the RISC-CPU  2  and the DSP  1  are required to operate such that close relationship is maintained therebetween, which complicates control to be executed by each of these devices. 
     SUMMARY OF THE INVENTION 
     It is a first object of the invention to provide a DSP (digital signal processor) which is improved in accuracy, processing speed, facility of programming, etc., namely, improved in performance as a whole, as well as a digital signal processing system incorporating the digital signal processor. 
     It is a second object of the invention to provide a DSP which is capable of starting a new arithmetic operation even when results of completed arithmetic operations cannot be written into accumulators which already store data. 
     It is a third object of the invention to provide a DSP which does not require different time periods to execute arithmetic operations depending on whether the arithmetic operations are each to be carried out by using internal registers or by using external registers, and hence facilitates programming therefor. 
     It is a fourth object of the invention to provide a digital signal processing system which is capable of efficiently supplying data required in carrying out arithmetic operations from a control processor thereof to a DSP thereof. 
     To attain the first and second objects, according to a first aspect of the invention, there is provided a digital signal processor comprising an arithmetic operation unit, a bus that transfers data to be processed by the arithmetic operation unit and results of arithmetic processing by the arithmetic operation unit, at least one accumulator that holds the results of the arithmetic processing by the arithmetic operation unit and delivers the results to the bus, and a bypass device that delivers the results by bypassing the at least one accumulator. 
     Preferably, the bypass device comprises a delay element that delays delivery of the results of the arithmetic processing by the arithmetic operation unit to the bus, the delay element delivering the results to the bus at the same timing that the results would be delivered by way of the at least one accumulator. 
     More preferably, a digital signal processor includes at least one external register connected to the bus, for storing the data to be processed by the arithmetic operation unit, the arithmetic operation unit having at least one internal register connected to the at least one external register via the bus, the data to be processed by the arithmetic operation unit being transferred from the at least one external register to the at least one internal register by way of the bus. 
     Further preferably, the digital signal processor carries out a filtering operation, the at least one external register comprising a plurality of external registers, the at least one internal register comprising a plurality of internal registers, one of the external registers and one of the internal registers each storing time-series data, while another one of the external registers and another one of the internal registers each storing filter coefficients. 
     To attain the first and third objects, according to a second aspect of the invention, there is provided a digital signal processor including an arithmetic operation unit, at least one internal register connected to the arithmetic operation unit, and at least one external register, the arithmetic operation unit carrying out arithmetic processing by receiving data stored in the at least one internal register to carry out an arithmetic operation using the at least one internal register, and receiving data stored in the at least one external register to carry out an arithmetic operation using the at least one external register. 
     The digital signal processor according to the second aspect of the invention is characterized by comprising a path commonly used for transfer of the data stored in the at least one internal register for the arithmetic operation using the at least one internal register and the data stored in the at least one external register for the arithmetic operation using the at least one external register, and a variable delay device interposed in the path, wherein based on changeover of the delay time of the variable delay device, the arithmetic operation unit carries out the arithmetic processing in a predetermined constant time period irrespective of whether the arithmetic processing is the arithmetic operation using the at least one internal register or the arithmetic operation using the at least one external register. 
     Preferably, the digital signal processor includes a bypass device that delivers the data stored in the at least one external register to the arithmetic operation unit by bypassing the at least one internal register when the arithmetic operation using the at least one external register is carried out by the arithmetic operation unit. 
     Preferably, the arithmetic operation unit includes a multiplication device having an output, and an addition device connected to the output of the multiplication device, the multiplication device carrying out a multiplication operation only for the arithmetic operation using the at least one internal register. 
     Preferably, the digital signal processor includes at least one accumulator that holds results of the arithmetic processing by the arithmetic operation unit, and a bus connected to the at least one external register, the at least one internal register, and the at least one accumulator, for transfer of the data stored in the at least one internal register and the at least one external register for the arithmetic processing by the arithmetic operation unit and the results of the arithmetic processing by the arithmetic operation unit. 
     More preferably, the digital signal processor carries out a filtering operation, the at least one external register comprising a plurality of external registers, the at least one internal register comprising a plurality of internal registers, one of the external registers and one of the internal registers each storing time-series data, while another one of the external registers and another one of the internal registers each storing filter coefficients. 
     To attain the first and fourth objects, according to a third aspect of the invention, there is provided a digital signal processing system comprising a control processor, a signal processor that carries out signal processing under control of the control processor, a plurality of dual port RAM&#39;s that pass data between the control processor and the signal processor, the dual port RAM&#39;s each having two of ports, and a bus to which the control processor and the dual port RAM&#39;s are connected, the control processor having a first memory management unit connected to one of the ports of each of the dual port RAM&#39;s via the bus, for transfer of data to and from a selected one of the dual port RAM&#39;s, the signal processor having a plurality of second memory management units each connected to another one of the ports of the each of dual port RAM&#39;s, for transfer of data to and from the each of the dual port RAM&#39;s independently of each other. 
     Preferably, the digital signal processing system carries out a filtering operation, the signal processor having an arithmetic operation unit, a bus that transfers data to be processed by the arithmetic operation unit and 
     results of arithmetic processing by the arithmetic operation unit, a plurality of external registers having the second memory management devices, a plurality of internal registers connected to the external registers via the bus, one of the external registers and one of the internal registers each storing time-series data, while another one of the external registers and another one of the internal registers each storing filter coefficients. 
    
    
     The above and other objects, features and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing the arrangement of a conventional DSP; 
     FIG. 2 shows a table which is useful in explaining operations of the DSP  2  according to respective instructions for carrying out arithmetic operations which are identical in contents but different in respect of the use of internal or external registers to carry out the arithmetic operations; 
     FIG. 3 is a block diagram showing the arrangement of a conventional digital signal processing system; 
     FIG. 4 is a block diagram showing the arrangement of another conventional digital signal processing system; 
     FIG. 5 is a block diagram showing the arrangement of a DSP according to a first embodiment of the invention; 
     FIG. 6 is a block diagram showing the arrangement of a DSP according to a second embodiment of the invention; 
     FIG. 7 shows a table similar to that shown in FIG. 2, which is useful in explaining operations of the FIG. 6 DSP; 
     FIG. 8 is a block diagram showing the arrangement of a DSP according to a third embodiment of the invention; 
     FIG. 9 is a block showing the arrangement of a digital signal processing system according to a fourth embodiment of the invention; and 
     FIG. 10 is a block showing the arrangement of a digital signal processing system according to a fifth embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Now, the invention will be described in detail with reference to drawings showing embodiments thereof. 
     In figures showing the embodiments of the invention, elements and parts corresponding to those of the prior art described hereinbefore are designated by identical reference numerals, detailed description of which is omitted. 
     A first embodiment of the invention will now be described, which is capable of more efficiently carrying out arithmetic operations to thereby enhance the performance of the DSP. 
     Referring first to FIG. 5, there is shown the arrangement of a 24-bit fixed-point DSP. This embodiment is distinguished from the conventional DSP described hereinbefore with reference to FIG. 1 in which results of arithmetic operations by the multiplication/addition unit  10  within the multiplication/addition block  100  are delivered to the bus  14  necessarily via the accumulator ACC 0  or ACC 1 , in that a multiplication/addition block  100 A is provided with a delay element  20  as an additional element, which functions as means for bypassing the accumulators ACC 0  and ACC 1 , via which results of arithmetic operations by the multiplication/addition unit  10  can be directly delivered to the bus  14 . The delay element  20  has a delay time thereof set such that a time period over which data is input to the internal register A or B and processed and the processed data is delivered via the accumulator ACC 0  or ACC 1  to the bus  14  is equal to a time period over which data is input to the internal register A or B and processed and the processed data is delivered via the delay element  20  to the bus  14 . 
     According to this arrangement of the DSP, even when data which has been not yet transferred is still stored in the accumulator ACC 0  or ACC 1  and hence new results of arithmetic operations cannot be written therein, the results of the arithmetic operations can be delivered to the bus  14  via the delay element  20 . Therefore, it is possible for the multiplication/addition unit  10  to start a new arithmetic operation with data of results of preceding arithmetic operations being stored in the accumulators ACC 0  and ACC 1 . 
     That is, in actuality, all the results of arithmetic operations output from the multiplication/addition unit  10  need not be written into the accumulators ACC 0  and ACC 1 , but what is required to be written thereinto is only data which is required to be returned to the ALU  12  for an addition (accumulation) operation of the data and a new result of a multiplication operation by the multiplier  11 . According to the present embodiment, data which are not to be accumulated, i.e. data to be delivered to an external device, data to be stored in the external register C or D, or data to be again input to the internal register A or B, can be directly delivered to the bus  14  via the delay element  20  as means for bypassing the accumulators ACC 0  and ACC 1 . Therefore, the ALU  12  of the multiplication/addition unit  10  can start a new arithmetic operation even when the accumulators ACC 0  and ACC 1  are unavailable at the time a preceding arithmetic operation has been completed, which enable arithmetic operations to be efficiently carried out. 
     Further, according to the present embodiment, it is not required to transfer data stored in the accumulator ACC 0  (or ACC 1 ) to the external register C (or D) before the ALU  12  starts a new arithmetic operation, thereby preventing data stored in the accumulator from becoming degraded in bit accuracy or being lost. 
     Moreover, according to the present embodiment, a result of an arithmetic operation by the multiplication/addition unit  10  is delivered via the delay element  20  to the bus  14  at the same timing that the same result would be delivered via the accumulator ACC 0  (or ACC 1 ) to the bus  14 , so that an instruction can be issued which instructs transfer of an output from the multiplication/addition unit  10  via the delay element  20  to a memory device or the internal register A or B at the same timing that data which would be obtained by the same arithmetic operation and then stored in the accumulator ACC 0  (ACC 1 ) is delivered to the memory or the internal register A or B. That is, a program adapted to the DSP according to the present embodiment can be composed so as to employ the timing of issuing instructions which is the same as one by the conventional DSP. In a case where this advantage over the prior art is not needed, a three-state buffer may be employed instead of the delay element  20 , whereby output data from the multiplication addition unit  10  may be directly delivered to the bus  14 . 
     Next, a second embodiment of the invention will be described, which is capable of facilitating programming to thereby enhance the performance of the DSP. 
     FIG. 6 shows the arrangement of a DSP according to the second embodiment. In this embodiment, data paths from the internal registers A and B to the ALU  12  are commonly provided for data to be processed by using the internal registers and data to be processed by using the external registers. This embodiment is distinguished from the conventional DSP described hereinabove with reference to FIG. 1 in that as shown in FIG. 6, variable delay means comprised of a delay element  21  and a selector  22  and variable delay means comprised of a delay element  23  and a selector  24  are interposed in the respective data paths. The delay elements  21  and  23  of the variable delay means effect a delay of one clock. The selector  22  (or  24 ) of the variable delay means selects between output data from the internal register A (or B) and output data from the delay element  21  (or  23 ), and delivers the selected output data to the ALU  12 . 
     Which data is to be selected by each of the selectors  22  and  24  of the variable delay means can be determined depending on the kind of an instruction to be executed by the DSP, and the selection is carried out according to a control signal generated by an instruction decoder, not shown. More specifically, when an arithmetic operation is carried out by using the external registers, control signals are supplied to the selectors  22  and  24  for selecting output data from the internal register A and B, while when an arithmetic operation is carried out by using the internal registers, control signals are supplied to the selectors  22  and  24  for selecting output data from the delay elements  21  and  23 . 
     Further, according to the present embodiment, instructions for multiplication operations using the external registers are excluded from each set of instructions, that is, instructions for multiplication operations are necessarily carried out by using the internal registers. This restriction is intended to execute all instructions over an equal time period. 
     FIG. 7 shows an example of instructions useful in describing operations of the DSP according to the present embodiment. Now, the operation of the DSP will be described with reference to FIG.  7 . 
     First, an instruction in a left-hand column of FIG. 7 is the same instruction that was described hereinabove with reference to FIG. 2, which instructs a multiplication operation to be carried out by using the internal registers A and B (see the instruction in the left-hand column in FIG.  2 ). The operation of the DSP according to this instruction is not different from the above described operation of the conventional DSP, and it takes three clocks to complete the arithmetic operations. 
     Next, an instruction in a central column of FIG. 7, i.e. “reg_a=reg_a+reg_b”, means “Add together data stored in the internal register A and data stored in the internal register, and store a result of the addition in the internal register A”. This instruction instructs arithmetic operations to be carried out by using the internal registers, and therefore output data from the delay elements  21  and  23  are selected by the respective selectors  22  and  24  to carry out the arithmetic operations. More specifically, this instruction is carried out by sequentially executing the following steps: 
     Step 1: The delay elements  21  and  23  deliver the output data from the internal registers A and B, respectively, after a delay of one clock; 
     Step 2: The output data from the delay elements  21  and  23  delivered respectively from the selectors  22  and  24  are added together by the ALU  12 , and a result of the addition is stored in the accumulator ACC 0 ; and 
     Step 3: The data stored in the accumulator ACC 0  is stored in the internal register A. 
     Thus, it also takes three clocks to carry out this instruction. 
     Next, an instruction in a right-hand column of FIG. 7, i.e. “reg_a=acc 0 =reg_c+reg_d”, means “Add together data stored in the external register C and data stored in the external register D, and store a result of the addition in the internal register A”. This instruction instructs the arithmetic operations to be carried out by using the external registers, and therefore output data from the internal registers A and B are selected by the respective selectors  22  and  24  to carry out the arithmetic operations. More specifically, this instruction is carried out by sequentially executing the following steps: 
     Step 1: Data stored in the external register C is transferred to the internal register A, and data stored in the external register D is transferred to the internal register B; 
     Step 2: The output data from the internal registers A and B delivered respectively through the selectors  22  and  24  are added together by the ALU  12 , and a result of the addition is stored in the accumulator ACC 0 ; and 
     Step 3: The data stored in the accumulator ACC 0  is stored in the internal register A. 
     Thus, it also takes three clocks to carry out this instruction. 
     As described above, according to the present embodiment, there is no difference in the number of clocks required to carry out arithmetic operations irrespective of whether the arithmetic operations are carried out by using the internal registers or by using the external registers, or irrespective of whether the arithmetic operations are multiplication operations or addition operations. This saves labor required of the programmer in determining the timing of start of execution of each instruction of a program he is preparing. 
     FIG. 8 shows the arrangement of a DSP according to a third embodiment of the invention. This embodiment is distinguished from the second embodiment described above in that delay elements  25  and  27  as well as selectors  26  and  28  are additionally provided. The delay element  25  transfers data delivered to the bus  13  to the selector  26  after a delay of one clock, while the delay element  27  transfers data delivered to on the bus  14  to the selector  28  after a delay of one clock. 
     The selector  26  selects either output data from the internal register A or output data from the delay element  25  and supplies the same to the multiplier  11 , the delay element  21 , and the selector  22 , and the selector  28  selects either output data from the internal register B or output data from the delay element  27  and supplies the same to the multiplier  11 , the delay element  23 , and the selector  24 . Which data should be selected by the selectors  26  and  28  is determined depending on the kind of an instruction to be executed, i.e. when arithmetic operations are to be carried out by using the external registers, output data from the delay elements  25  and  27  are selected, while when arithmetic operations are to be carried out by using the internal registers A and B, output data from the internal registers A and B are selected. 
     According to the present embodiment, when an instruction for arithmetic operations using the external registers is carried out, output data from the external registers C and D are supplied not via the internal registers A and B but via the delay elements  25  and  27  to the ALU  12 . As a result, the present embodiment is advantageous in that the arithmetic operations using the external registers can be carried out without updating the contents of the internal registers A and B. 
     A fourth embodiment of the invention will now be described, which is capable of efficiently passing data between a control processor and a DSP controlled by the control processor, to thereby enhance the whole performance of a digital signal processing system including the control processor and the DSP. 
     FIG. 9 shows the arrangement of a digital signal processing system according to the fourth embodiment. In the figure, component elements and parts corresponding to those appearing in FIG. 4 are designated by identical reference numerals, and detailed description thereof is omitted. 
     The digital signal processing system according to the present embodiment has three RAM&#39;s  31  to  33 . The RAM  33  is used by the RISC-CPU  2  as a work RAM. The RAM&#39;s  31  and  32  are dual port RAM&#39;s and play the role of passing data between the RISC-CPU 2  and the DSP  1 . 
     One port of each of the RAM&#39;s  31  and  32  as well as the RAM  33  are connected via a common bus  20  to the RISC-CPU  2 . These RAM&#39;s  31  to  33  are physically formed by respective three separate portions, but when viewed from the RISC-CPU  2 , they constitute one RAM having contiguous addresses. In the illustrated example, the RAM  33  corresponds to addresses 0000 to 7FFF, the RAM  31  to addresses 8000 to 8FFF, and the RAM  32  to addresses 9000 to 9FFF. The RISC-CPU  2  passes data between these RAM&#39;s  31  to  33  via the common bus  20 . 
     In a DSP  1   a  of the digital signal processing system, external registers C′ and D′ correspond to the external registers C and D employed in the conventional digital processing system described hereinabove with reference to FIGS. 3 and 4. However, the registers C′ and D′ of the present embodiment are each provided an MMU (memory management unit). The MMU of the external register C′ is connected to another port of the RAM  31  which corresponds to the addresses 8000 to 8FFF, while the MMU of the external register D′ is connected to another port of the RAM  32  which corresponds to the addresses 9000 to 9FFF. These MMU&#39;s control writing and reading of data by the RAM&#39;s  31  and  32 , independently of each other. More specifically, when viewed from the RISC-CPU  2 , the RAM&#39;s  31  and  32  form a single RAM together with the RAM  33 , but when viewed from the DSP  1   a , the RAM&#39;s  31  and  32  are two RAM&#39;s independent of each other. 
     Now, the operation of the digital signal processing system according to the present embodiment will be described. For instance, in the case where a filtering operation is carried out by the digital signal processing system, before the filtering operation is started, the RISC-CPU  2  stores in advance a sequence of coefficients required by the DSP  1   a  for its arithmetic operations in one of the RAM&#39;s  31  and  32  (in the RAM  32 , for example). 
     Then, when the DSP  1   a  starts the filtering operation, the RISC-CPU  2  sequentially stores time-series data on which arithmetic operations are to be carried out in the other RAM  31 . 
     On the DSP  1   a  side, the MMU of the external register C′ sequentially reads a predetermined number of pieces of time-series data corresponding to a predetermined past time period from the RAM  31  whenever a predetermined sampling time period elapses, and the read data are temporarily stored in the external register C′, and then transferred to the internal register A. On the other hand, in parallel with the above operation, the MMU of the register D′ sequentially reads the predetermined sequence of coefficients from the RAM  32 , and the read data are transferred by way of the external register D′ to the internal register B. Thus, the multiplication/addition operations of data and coefficients transferred to the internal registers A and B are sequentially carried out to thereby perform the filtering operation. 
     The reading of data from the RAM  31  by the MMU of the external register C′ and the reading of coefficients from the RAM  32  are carried out in parallel with each other and independently of each other. Therefore, the DSP  1   a  can read the data and coefficients at the same time, which enables the multiplication/addition operations to be efficiently carried out. 
     A fifth embodiment of the invention will now be described, which incorporates essential elements of the first, second, and fourth embodiments to thereby enhance the whole performance of a digital signal processing system including the control processor and the DSP. 
     More specifically, as shown in FIG. 10, this embodiment is distinguished from the fourth embodiment described above in that it additionally incorporates a bypass path comprised of the delay element  20  of the first embodiment shown in FIG. 5, and the delay elements  21 ,  23  and the selectors  22 ,  24  of the second embodiment shown in FIG.  6 . 
     According to this embodiment, all the advantageous effects obtained by the first, second, and fourth embodiments described above can be obtained. 
     The embodiments described heretofore are illustrated only bay way of example, and not by way of limitation, and can be modified as desired within the scope of the invention.