Abstract:
An application-specific single chip digital processor having flexible design expansion capability with minimal impact on the performance of a processor core. The processor core has an ALU and a register file (accumulators). The output of the ALU is connected to a multiplexer whose output is connected to the input of the register file. The output of the register file connects to one input of the ALU. A function unit, separate from the core, has an input connected to the output of the register file and an output connected to another input to the multiplexer. The core operates with a predefined instruction set. The function unit, which may be redesigned depending on the application, operates with a reserved (uncommitted) instruction set under control of the core.

Description:
This application is a continuation of application Ser. No. 08/415,098, filed on Mar. 31, 1995, abandoned, which is a continuation of application Ser. No. 07/651,067, filed on Feb. 5, 1991, abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to computer architecture in general, and more particularly, to digital processor architecture with expandable functionality for implementation on an integrated circuit. 
     2. Description of the Prior Art 
     Single chip digital processors, such as microcontrollers, microprocessors, or digital signal processors, are ubiquitous digital computers used in virtually every application where intelligent control, information processing, or real-time signal processing is important. The tasks a digital processor can efficiently fulfill are directly related to the internal organization, or architecture, of the processor. The more generalized the task(s), the more general the architecture. For example, the architecture of microprocessors, such as those used in personal computers, is more geared toward general information handling, compared to a microcontroller for a milling machine. Digital signal processors, or DSPs, are specialized digital computers. Unlike microprocessors, DSPs are adapted for efficient, often repetitive, processing of data. For example, a DSP may be used for filtering, detecting, and generating digitized analog signals in a modem. DSPs may also be used for processing signaling information from individual telephone lines and trunks in an electronic switching system. In either case, the task assigned to the DSP is specialized and repetitive--the faster the DSP can perform the assigned task, the greater the signal bandwidth that can be processed or the more telephone line signaling information that can be processed by one processor. 
     The architecture of a typical microprocessor has general purpose registers and arithmetic circuits arranged to process widely different tasks, such as text editing, spreadsheets, etc. As a consequence, the execution of those tasks are done not as quickly, or as efficiently, as a processor tailored for optimal performance of a particular task. A DSP, on the other hand, has specialized registers and arithmetic circuits for efficient processing of signals. For example, multiply-and-accumulate instructions are very common operations in signal processing algorithms and are usually implemented only in DSPs, not on microprocessors. 
     In certain applications, there is no commercially available microprocessor, microcontroller or DSP which can efficiently carry out a particular task in an efficient manner. An obvious solution is to design a new digital processor, or more likely a redesign of an old processor to add new functionality, which can efficiently carry out the desired task. However, this approach is expensive and time consuming. Further, adding new functionality to an existing processor may negatively impact the overall efficiency of the processor. For example, the addition of a new function to an existing microprocessor or DSP (i.e., new instructions and/or additions to the processor&#39;s hardware) may slow down the execution speed of the processor&#39;s original instructions. Generally, this is not considered a good solution since the overall efficiency of the processor, with the added functionality, may be lowered. 
     Typical techniques to add new functionality to an existing processor include register, peripheral, or memory mapping of new hardware for the processor to access, or adding a co-processor. However, each of these techniques require the existing processor to access the additional hardware as a separate entity, slowing down the exchange of information between the processor and the added hardware. At best, the added hardware (typically a co-processor on a separate chip from the processor) &#34;shadows&#34; the existing processor&#39;s registers. The co-processor then takes over execution from the processor when specific instructions are addressed to it. Because a co-processor operates independently of the processor, the processor usually waits for the co-processor to finish execution, otherwise the processor and co-processor will lose synchronization and data may be lost. To move data between the processor and a co-processor, the data is usually read into temporary storage and reread under control the processor. This slows down the execution of the main processor, negating some of the advantage in speed gained by utilizing the co-processor. 
     It is, therefore, one aspect of the invention to provide a new digital processor architecture which allows for the addition of new functionality without significant impact on the existing, or core, functions of the processor. 
     It is another aspect of the invention to provide a digital processor architecture with expandable functionality which is readily integratable into a single chip using application specific integrated circuit technology. 
     SUMMARY OF THE INVENTION 
     These and other aspects of the invention are generally possible in a digital processor implementing common functions and application-specific functions, disposed in an integrated circuit, having a processor core for implementing the common functions and responsive to a first set of instructions. The processor core has a register file and an ALU, the input of the register file being coupled to the output of the ALU, and the output of the register file being coupled to the input of the ALU. The digital processor is characterized by: a function unit, implementing the application-specific functions and responsive to a second set of instructions, having an output and having an input coupled to the output of the register file; and, a selecting means, disposed between the output of the ALU and the input of the register file, for switching the input of the register file between the output of the ALU and the output of the function unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The foregoing features of this invention, as well as the invention itself, may be more fully understood the following detailed description of the drawings, in which: 
     FIG. 1 is a simplified block diagram of an exemplary digital signal processor; and, 
     FIG. 2 is a simplified block diagram showing in more detail the DSP core and function unit of the exemplary DSP in FIG. 1. 
    
    
     DETAILED DESCRIPTION 
     The invention may be generally understood by referring to the illustrative embodiment shown in FIG. 2. Here, a processor core 2 in a digital processor (not numbered) has therein a data processing unit 10 and a control unit 17. The control unit 17 responds to a predefined set of instructions for controlling the operation of the data processing unit 10. In the data processing unit 10, one input of an arithmetic logic unit (ALU) 11 couples to the output of a register file (accumulators) 12 and the input to the register file 12 couples to the output of the ALU 11. Added to the core 2 is a function unit 7, responsive to the control unit 17, having an input coupled to the output of the register file 12. Further, the data processing unit 10 includes a selecting means (multiplexer) 13, disposed between the output of the ALU and the input of the register file 12, for selecting as input to the register file 12 the output of the function unit 7 or the ALU 11. A portion of the instruction set of the digital processor pertains to the function unit 7 and the remaining portion of the instruction set is fixed and pertains to the processor core 2. Alternatively, the core 2 responds to a first set of instructions, which are usually predetermined or fixed. The function unit 7 responds to a second set of instructions which are dependent on the operations of the function unit 7. 
     A more detailed explanation of the exemplary implementation of the above-described invention is given herein. In FIG. 1, an exemplary DSP 1, having a DSP core 2, is preferably integrated into a single IC. The DSP core 2 executes instructions from the internal RAM 3, internal ROM 4, or from the external memory (not shown) via interface 5. In addition, the DSP core 2 may communicate with external input/output (I/O) devices, such as a digital-to-analog converter (not shown) via the I/O interface 6. Adding extra functionality to the DSP 1 is a function unit 7 which communicates with the DSP core 2 via a function unit port on the DSP core 2. In addition, the function unit 7 may also communicate with the external I/O devices through the I/O interface 6 for expedient data transfers. 
     It is understood that the DSP core 2 remains substantially the same regardless of the functions to be implemented by the function unit 7. With current application-specific integrated circuit (ASIC) design techniques the DSP 1, except the function unit 7 (and the coding in the ROM 4, if necessary), may be easily replicated for each species of DSP 1 desired. The function unit 7 for each DSP 1 species may then be designed from a library of circuit functions to implement the special function in an efficient manner or it may be designed in a full custom manner for higher performance and smaller chip area. This reduces the cost and time to design application specific DSPs for individual customers while giving them the advantages of a standardized DSP core 2. Hence, the customer can take advantage of common software tools to develop a system utilizing the DSP 1 while tailoring the DSP 1 to meet the specific requirements of the customer&#39;s system. 
     The specific implementation of the DSP core 2 and an exemplary function unit 7 for the exemplary DSP 1 is shown in FIG. 2. The DSP core 2 has a data/arithmetic unit (DAU) 10 (part of which is shown here) which has an arithmetic logic unit (ALU) 11 and a set of accumulators, or registers, 12. To interface the function unit 7 with the DSP core 2, a multiplexer 13 is added between the output of the ALU 11 and the accumulators 12. When data is to be loaded into the accumulators 12 from the function unit 7, the multiplexer is configured to take data from the function unit 7 via bus 14. For DSP core 2 operations not involving the function unit 7, the multiplexer 13 is configured to take data from the ALU 11. Data to the function unit 7 comes from the output of the accumulators 12 via bus 15. In this way, the function unit 7 is substituted for the ALU 11 when function unit 7 operations are desired. Note that there is no substantial impact on the performance of the DSP core 2 by the function unit 7 except for a small propagation delay for data through the multiplexer 13. 
     The exemplary function unit 7 shown here efficiently performs barrel shifts and bit field extraction/insertion in logic 20 for telephone signaling applications. Operation of the logic 20 is beyond what is necessary for the understanding of the claimed invention and will not be described further, the actual function and operation of the function unit 7 is shown here to illustrate the advantages arising from the invention. Briefly, multiplexers 21, 22 select data for the logic 20 either from the DSP core 2 via bus 15, from the internal bus, or from alternate accumulators 23 and registers 24. Data output from the function unit 7 is passed back to the core 2 via bus 14, through multiplexer 13, and stored in accumulators 12. 
     A control unit 17 receives instructions from the memories (3, 4, or external memory, FIG. 1) to control the DAU 10 (and other circuits not shown in the DSP core 2) and, indirectly, the function unit 7. In the preferred embodiment, the instructions to control the core 2 are predefined or fixed; except for a few specific instructions discussed below, the control unit 17 interprets the instructions for the core 2 independently of the function unit 7. Hence, the core 2 is a &#34;fixed&#34; design with the flexibility designed into the function unit 7. 
     The function unit 7 is controlled by function unit control 25, which is in turn controlled by control unit 17 in the core 2. Control unit 17 receives instructions for the function unit 7 and sends commands to the function unit control 25 to implement those instructions. The subset of instructions reserved for the function unit 7 typically have a reserved number of bits thereof, commonly known a bit field, which is passed to the function unit control 25 via the command bus 18. 
     As stated above, the instructions for the core 2 are predefined except for a few specific instructions. For example, on branch instructions, the branch condition may be dependent on a flag from the function unit 7. Hence, in the exemplary function unit 7 shown here, flag bits, such as a bit value, sign, error flags, etc., are sent back to the control unit 17 for processing via flag bus 19. In addition, certain instructions for moving data between the accumulators 12 and the function unit 7, such as those requiring the multiplexer 13 to be reconfigured, may vary depending on the circuitry in the function unit 7. For example, in the exemplary embodiment disclosed herein, the core 2 communicates with the function unit 7 by placing one or two words on the bus 15 and reading back on bus 14 one or two words. Between the exchange of words on buses 14, 15, the core 2 may wait one or two clock cycles for the function unit 7 to process data. 
     Although the exemplary digital processor shown in FIGS. 1 and 2 is a digital signal processor, the invention described above may be used in conjunction with any digital processor, such as a microprocessor or controller, where the advantages of flexible expansion of the processor are desired. 
     Having described the preferred embodiment of this invention, it will now be apparent to one of skill in the art that other embodiments incorporating its concept may be used. It is felt, therefore, that this invention should not be limited to the disclosed embodiment, but rather should be limited only by the spirit and scope of the appended claims.