Abstract:
A system for extending standard processors using either undefined op-codes or sparse address spaces to maintain the use of legacy processor tools and reduce the complexity of the design process. The disclosure describes a method and apparatus for adding circuitry to processing units that allows partitioning of the design into a fixed processing unit derivative and a configurable subsystem. The legacy processor unit language tools work with the fixed processing unit derivative while the logic design tools work well with the configurable subsystem. In one embodiment, the configurable subsystem is implemented with easily available programmable Logic Devices (PLD&#39;s and FPGA&#39;s).

Description:
CLAIMING BENEFIT OF EARLIER FILING DATE AND CROSS-REFERENCES TO OTHER APPLICATIONS 
     This application claims priority to U.S. provisional application entitled “A Microprocessor Interface To A Configurable Subsystem”, filed on Nov. 19, 1998, Ser. No. 60/109,235. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is generally in the field of microprocessor design and more specifically in the field of microcontroller design and extensions thereto. 
     2. Description of the Related Art 
     Certain microprocessors or micro-controllers have been sold in quantities of hundreds of millions. The processors have been available for years, in some cases for decades (8048 since 1976, 8051 since 1980). As a result, tens or hundreds of thousands of programmers have learned the architecture and written programs for the architecture. In addition, sophisticated tools have been designed to facilitate designing with these architectures. 
     Recently, configurable array technologies have evolved to the point that complete processors can be designed and implemented in a single configurable array, such as an Altera Flex 10K FPGA (Field Programmable Gate Array). In theory, designers can now tailor such popular processor architectures as they wish, by adding features or changing memory size. However, in practice, the task of designing a processor or modifying its architecture is a tremendous task, and generally requires giving up the tools that have been developed for the original (unmodified) architecture. The modified processor is no longer supported by the legacy processor tools and creating a modified processor is complex and time consuming. Moreover, the logic tools needed to design with programmable logic are incompatible with the legacy tools that have evolved to support current processor system architectures such as the one shown in FIG.  1 . 
     What is needed are ways to modify a processing system so that the “legacy” tools developed for the unmodified architecture can still be used and so that a configurable subsystem can attach to the processing system to extend the functions of the architecture without involving alterations to the processing system. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to a method and apparatus that satisfies the above needs. A system in accordance with the present invention includes an central bus for carrying address, data and control signals relating to the address and data on the central bus and an I/O port, connected to the central bus, where the I/O port forms an extended bus having address and data signals and control signals, including a select signal, relating to the address and data signals on the extended bus. The system further includes a functional unit connected to the central bus and having an inexhaustively decoded space, where the use of an unassigned location in the space causes the activation of the select signal on the extended bus. Data is transferable between the central bus and the extended bus when the select signal is activated on the extended bus. 
     In one version of the present invention, the functional unit includes an instruction processing unit and a register set residing in the inexhaustively decoded space. Executing an instruction that references an unassigned location in the space causes the activation of the select signal. 
     In another version of the present invention, the functional unit includes an instruction processing unit for executing instructions residing in the inexhaustively decoded space and the instruction processing unit executes an instruction at an unassigned location in the inexhaustively decoded space to cause the activation of the select signal. 
     A method in accordance with the present invention includes the steps of: forming an extended bus from an I/O port connected to an central bus of a processing system, where the extended bus includes address data and control signals, including a select signal, relating to the address and data signals on the extended bus; executing an instruction in a functional unit connected to the central bus of the processing system and having an inexhaustively decoded space, where the instruction uses an unassigned location in the inexhaustively decoded space to cause the activation of the select signal on the extended bus; and transferring data between the central bus and the extended bus in response to executing the instruction causing the activation of the select signal. 
     An advantage of the present invention is that use of legacy tools for the unmodified processing system architecture is preserved because the instruction set activating the select signal is a standard instruction or an additional instruction. In either case, the instruction set and architecture of the unmodified processing system are not altered and existing tools still work with the modified system. 
     Another advantage is that configurable subsystem is kept separate from the processing system so that the designer has the flexibility to implement extended functions for the processing system independently of the processing system. This advantage is brought about by the extended bus to which the configurable subsystem attaches. The extended bus is designed to easily interface to most, if not all, of the available programmable logic arrays on the market, thereby delivering a great deal of flexibility in designing the configurable subsystems using the logic array vendors&#39; tools. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
     FIG. 1 shows a basic processing system which can be extended in accordance with the present invention; 
     FIG. 2 shows a simplified block diagram of a particular processing system, such as an 8051, for use in the present invention; 
     FIG. 3 shows an extension to the basic 8051 architecture capable of supporting both computational and input/output architectural extensions; 
     FIG. 4 shows the signals of an extended bus, called the XFSR-bus, for the typical processing system; 
     FIG. 5 shows a write operation in which data is transferred from a location in the processing system to configurable subsystem circuitry attached to the extended bus; 
     FIG. 6 shows a read operation in which data is transferred from configurable subsystem circuitry attached to the extended bus to a location in the processing system; 
     FIG. 7 shows a sequential interpretation of a group of addresses assigned to the configurable subsystem. 
     FIG. 8, shows an arbitrary processing system capable of being extended in accordance with the present invention; 
     FIG. 9 shows a multi-channel configurable subsystem; 
     FIG. 10 shows a more expanded view of a prototyping system for verifying the combined processing system and configurable subsystem; and 
     FIG. 11 shows several alternative implementations for the configurable subsystem in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a basic processing system which can be extended in accordance with the present invention. The basic processing system  20  of FIG. 1 includes one or more functional units such as a processing unit  100 , a data storage unit  102 , a register set  104  and I/O port  106  or combinations thereof, all connected to a central bus  108 . The processing unit  100  fetches and executes instructions from a program store unit  110  and typically includes a timing and control unit, an Arithmetic Logic Unit (ALU) and a program counter for addressing, over bus  112 , instructions from the program storage unit  110 . The data storage unit  102  is used to store data used by a program and the register set  104  contains one or more special purpose registers needed by the processing unit for executing instructions. Lastly, I/O port  106  allow the processing system to send and receive data to and from external devices over bus  114 . The various units of the processing system communicate with each other over the central bus  108 , which carries address, data and control information. 
     FIG. 2 shows a simplified block diagram of a particular processing system  120 , such as an 8051, for use in the present invention. In FIG. 2, the processing system includes a program storage unit  122 , typically implemented by a Read-Only Memory for storing the program and a special function register set (SFR)  124 , which includes an accumulator  126 , a timer  128 , an interrupt control register  130 , a serial port control register  132 , and an instruction pointer register  134 . It is important to note that, for the 8051, the various registers of the special function register set are assigned to an address in a register address space that is larger than the number of implemented registers. This is illustrated by the hatched area  136  and means that some addresses in the register address space are not assigned to a register of the register set, thus forming an inexhaustively decoded address space. The processing unit includes an ALU  140 , which is connected to the accumulator  126  in the register set  124  (and possibly other registers) and the central bus  142 , and a program counter  144 , which is connected to the instruction pointer register  134  of the special function register set  124  and the program ROM  122 . The processing unit further includes an instruction decoder and timing and control unit (not shown) for carrying out the execution of instructions in the processing unit. A register set address decoder  146 , connected to the central bus  142 , decodes addresses on the central bus  142  to select a register in the register set  124 . The I/O unit is not shown but is connected to the central bus and is discussed below. 
     FIG. 3 shows an extension to the basic 8051 architecture capable of supporting both computational  152  and input/output architectural extensions  154 . In particular, the address decoder  146  for the special function register set  124  is modified to provide a select line  150  on which a select signal, CS/ is activated when one of the unassigned addresses  136  is present on the address decoder input. The unassigned address  136  is caused to be present on the input to the address decoder when the processing unit executes an instruction that references the unassigned address  136 . In FIG. 3, the select line  150  is used to enable either a computation extension  152  to the processing system or an input/output extension  154 . 
     The computation extension  152  is a function, F(x)  158 , performed on data, x, received via an interface  156  from the central bus and whose results y=F(x)  158 , are returned via the interface  156  to the central bus. 
     An I/O architectural extension  154  is a channel, filter or translator function  160  in which data is passed to or from the central bus through an interface  156  from or to external circuitry. 
     FIG. 4 shows the signals of an extended bus  170 , called the XSFR-bus, for the typical processing system. The extended bus  170  includes one or more data paths  172  and  174  for transferring data to and from the central bus  142  to the extended bus  170  and a set of control signals which relate to the data transfers between the central bus  142  and extended bus  170 . In particular, for an embodiment of the present invention in which an 8051 or 8051-derivative is used, the control signals include, CLK  176 , ALE  178 , WR/  180 , RD/  182 , RDY  184  and CS/  186  (where a / after a signal name indicates the signal is active when it is in a logic low state). In this embodiment, the CLK  176  signal is a signal derived from a clock operating the processing system; ALE  178  is a signal which signals the presence of address information on the extended bus; RD/  182  captures data from the extended bus; WR/  180  indicates the presence of output data on the extended bus; RDY  184  indicates that the processing unit should stall while maintaining certain control signals active on the extended bus; and CS/  186  indicates that an unassigned address  136  is referenced in an instruction being executed by the processing unit. Devices attached to the extended bus are enabled when the CS/ signal  186  is active. FIG. 4 also shows a portion of the processing unit, the instruction decoder and timing and control unit  188 , which is responsible for decoding instructions and issuing timing and control signals to carryout the execution of instructions in the processing unit. Signals such as ALE  178 , RD/  182 , WR/  180  and CLK  176  for the extended bus  170  are provided by the timing and control unit  188  which is modified to force an external bus cycle (ALE and RD/ or WR/ ) when an instruction references an unassigned address  136  in the register set address space. 
     Operation of the system shown in FIG. 4 in one version of the invention is shown in FIGS. 5 and 6. 
     FIG. 5 shows a write operation in which data is transferred from the central bus to configurable subsystem circuitry attached to the extended bus. In this process, the program counter accesses the next instruction from the program store unit. For a move instruction as the next instruction, the processing unit is instructed to move data from a source location to a target location. During the execution of the move instruction, the processing unit references an unassigned address  136  in FIG. 4 in the special function register space as the target location. In this case, the timing and control unit  188  of the processing unit accesses the data from the source, places the data onto the central bus  142 , and latches the data into a temporary latch, not shown. The processing system then places the unassigned register address  136  of the target onto the central bus, and the timing and control unit generates an ALE (address latch enable) signal  178  on the extended bus, as well as any necessary signals to enable the register set address decoder. The address decoder, in response, activates the special select signal, CS/  186 , to enable circuitry attached to the extended bus. This circuitry uses the falling edge  190  of the ALE signal  178  to capture the unassigned address  136  for decoding as shown in FIG.  5 . After the address is captured, the timing and control unit  188  then causes the data from the source from the temporary latch to be placed onto the central bus, and a WR/ signal  180  to be activated. Because the register set address decoder recognizes the unassigned address  136 , it does not enable any register in the register set. The WR/ signal  180 , combined with the captured address and the CS/ signal  186 , causes the desired data  194 , appearing on the central bus, to be transferred to the extended bus interface  172  and into the configurable subsystem at a location based on the unassigned address. Typically, the configurable subsystem captures the data  194  on the trailing edge  192  of the WR/ signal  180 . 
     FIG. 6 shows the transfer of data from configurable subsystem circuitry attached to the extended bus to a location in the processing system. In this case, the processing unit executes a move instruction in which the source location is an unassigned register set address and the target location is a location, such as a register, within the processing system. The move instruction is fetched from the program store unit and interpreted by the instruction decoder. The instruction decoder and the timing and control unit then cause the source address (an unassigned address) to be placed onto the extended bus  170  and the ALE signal  178  to be activated. The register set address decoder recognizes the unassigned address  136  and produces the CS/ signal  186  to enable the configurable subsystem to capture the address from the extended bus  170  on the trailing edge  190  of the ALE signal  178 . The register set address decoder causes the registers in the register set to ignore this phase of the move operation. The configurable subsystem then decodes the latched, unassigned address and places its data  198  onto the extended bus  170  in response to the decoded address, the CS/ signal  186 , and the RD/ signal  182 . The trailing edge of the RD/ signal  196  is used to capture the data appearing on the extended bus  170  into a temporary latch (not shown) for transfer to the target register as described above. 
     In one embodiment of the present invention, the select signal, CS/  186 , is timed to occur before the trailing edge of the ALE signal  178 . This serves to qualify the ALE signal  178  so that the circuitry attached to the extended bus  170  need only activate when it receives an ALE  178  when CS/  186  is active. 
     In another embodiment of the present invention, the select signal, CS/  186 , is allowed to arrive later than the trailing edge of the ALE signal  178  but before either the RD/  182  or WR/  180  signal. This serves to qualify the RD/  182  or WR/  180  signals so that the circuitry attached to the extended bus  170  need only activate when it receives a RD/  182  or WR/  180  signal with CS/  186  active. In this case, the circuitry must latch every address on the trailing edge of ALE  178  because CS/  186  arrives too late to qualify ALE  178 . 
     In some embodiments of the present invention, more than one unassigned address in the register set address space is decoded. In particular, the select line is activated when a group of addresses is decoded. One convenient group is a set of four addresses starting on a quad-byte boundary. For example, in an 8051 or 8051-derivative processing system addresses  200 ,  201 ,  202  and  203  ( 0 C 8 H- 0 CAH) are unassigned in the 8051. Another similar group includes the addresses  204 ,  205 ,  206  and  207  ( 0 CCH- 0 CFH) which are also unassigned in the 8051. Another convenient group of addresses is a set of eight addresses starting on a octbyte boundary. 
     In one version of the invention, the register set address is modified to distinguish between these two contiguous groups of addresses and to provide a separate select line for each group. In this case, the address decoder activates a signal, CS 1 /, on a first select line when a reference to an unassigned address falls within the addresses in the first group, and the address decoder activates a signal, CS 2 /, on a second select line when a reference to an unassigned address falls within the addresses in the second group. The existence of two select lines means that two configurable subsystems are attachable to the extended bus. In other versions of the invention, a greater or lesser number of configurable subsystems are attachable to the extended bus. 
     Each of the configurable subsystems in the quadbyte boundary case has four addresses to each of which a separate function is assigned. In this case, one assignment of addresses is to designate the “00” address to a control function  210 , a “01” address to a data out (from the processing system) function  214 , “02” to a data in (to the processing system) function  212 , and “03” to a status function  216 . This configuration is shown in FIG.  7 . 
     Furthermore, once a function assignment is made for these addresses, in one version of the invention, the configurable subsystem treats the addresses as randomly accessible. In a different version of the invention, the configurable subsystem treats an access to an unassigned address as part of a sequential process. 
     As show in FIG. 7, prior to any access to the unassigned space, the configurable subsystem is in the IDLE state  218 . The receipt of data written to the “00” address, i.e., control data, causes the configurable subsystem to move to a WAIT state  220 , in which the subsystem waits for data from the processing system. When data is written to the “01” address to the configurable subsystem, the subsystem moves to a PROCESS state  222  to process the new data and put the result in a data out register. A read at the “02” address by the processing system to obtain the result data causes the subsystem to move to the NEXT state  224  in which status information from the “03” address is obtained. 
     For a configurable subsystem to treat accesses to the unassigned addresses as sequential process, a finite state machine is designed into the configurable subsystem circuitry. This state machine, in some cases, operates from the CLK signal  176 , in FIG. 4, provided from the processing system. This arrangement allows the state machine to cycle through a number of internal states between the states mentioned above, increasing the flexibility of the state machine. Furthermore, in some cases, the state machine has a RDY line  184 , in FIG. 4 connected to the processing system. As described above, the RDY line, when active, stalls the processing unit of the processing system. This arrangement permits the processing speed of the configurable circuitry to be variable and prevents the processing system from having to repeatedly poll the status address of the configurable subsystem to determine the state of the subsystem. 
     In some processing systems, there is no suitable set of registers occupying a sparsely populated address space. FIG. 8 shows an arbitrary processing system having a timing and control unit  230 , a number of functional units, A  232 , B  234 , C  236  and one or more busses  238  to which the functional units are connected. In this arbitrary processing system, data can be transferred from unit A  232  to unit B  234  by enabling the output of unit A  232  onto the bus  238  and causing B  236  to capture the data from the bus  238  into its local storage circuits. The actual timing and control of such transfers can be multi-staged (pipelined) and complex without departing from the present invention. It is desired to modify this arbitrary processing system to have an extended bus  244  to which a configurable subsystem  240  is added in accordance with the present invention. 
     In FIG. 8, the modification consists of adding another functional unit, D  242  to the bus and forming, from that functional unit, an extended bus  244  with the appropriate timing and control signals as required. The new unit is provided with I/O pins which make up the extended bus for this arbitrary processing system. 
     Beyond adding the functional unit, D, the instruction set of the processing unit in the arbitrary processing system is modified. This modification is an the addition of an instruction which, when executed, operates to transfer data to or from the added functional unit. The added functional unit D  242  has the responsibility of transferring data between the processing system bus to which it is connected and the extended bus  244  to which the configurable subsystem  240  is connected. Preferably, the added instruction uses an unassigned op code from an inexhaustively decoded instruction space. 
     The modified processing system operates as follows. First, the new instruction is fetched by the processing unit. Second, the processing unit decodes the new instruction and, according to the instruction, fetches data from the next location in the program store. Third, the just-fetched data is interpreted by the processing unit. If the data specifies a write to the extended bus, the next data item in the program store or data in another known location is fetched and written to the added functional unit, D. This unit then places the data on the extended bus for the configurable subsystem along with suitable timing signals that enable the subsystem to capture the data. If the data specifies a read from the extended bus, the next data item in the program store specifies or an implied location, such as the accumulator, becomes the destination location of the data to be received from the extended bus. The functional unit D  242  is triggered by the execution of the new instruction to operate suitable timing signals on the extended bus to capture the data from the configurable subsystem and the functional unit D notifies the processing unit when data is available from the unit. The processing unit then transfers the data to the specified location in the processing system. Timing signals, in one case, are similar to the timing signals discussed above for an 8051 or 8051-derivative. 
     Since only one bit of the data item fetched after the new instruction is needed to specify a read or a write to the extended bus, the remaining bits in the data item are free to be assigned to certain functions. One function for the remaining bits is to denote an address within the added functional unit D. Another function is to denote the number of bytes to be transferred from the processing system to the extended bus and visa-versa. 
     As described above, an instruction is added to the instruction set of the processing system by assigning one of the unassigned (undefined) op codes to the added instruction. Furthermore, the compiler or assembler tool for the processor must also be altered to reflect the presence of the added instruction. One common technique is to use the “define-byte” feature of a compiler or assembler as shown below. 
     
       
         
               
               
             
           
               
                   
                   
               
             
             
               
                   
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     Here the “CPU code” consists of standard instructions to be executed by the processing unit of the processing system. The “db” is an assembler “directive” that tells the assembler to simply define the next byte as specified. For example, the line “db undefined_opcode_value” will result in the value of the undefined opcode being inserted in-line with the preceding CPU code. When the CPU executes this portion of the program, the undefined opcode will be picked up and an attempt will be made to execute the code. The unaltered processing unit either ignores the additional instruction or traps it as an exception. However, a suitable alteration to the instruction decoder and timing and control unit of the processing unit causes the altered processing system to recognize and execute the new instruction. Therefore, the tools and ability to interpret the undefined opcode as the extended bus-code to interface to our extended bus are present. In most cases, the undefined opcode can be placed in a table with all of the defined opcodes, and the same branching mechanism used for interpreting the undefined code as for the defined opcodes. 
     Prototyping 
     A processing system having an extended bus for connecting to a configurable subsystem is described above. The altered processing system and the configurable subsystem must be tested prior to placing the combined system into production. This calls for a technique for prototyping the combined system to verify its correctness and that it meets design requirements. For example, referring to FIG. 9, one such combined system has a modified 8051 or 8051-derivative processing system  250  with three configurable subsystems attached  252 ,  254 ,  256 . Each configurable subsystem,  252 ,  254 ,  256  is connected to channel  258 ,  260 ,  262  on the extended bus, where a channel is defined as a group of contiguous addresses for communicating between the configurable subsystem and the altered processing system. Any number of channels is possible depending on the number of contiguous address blocks available from the register set address space. Each of the configurable subsystems  252 ,  254 ,  256  in FIG. 9 is either a standard cell or an Field Programmable Gate or Logic Array (FPGA or FPLA). It is preferred that standard cells be implemented on the same silicon substrate  264  as the modified processing system and that the FPGA be implemented as a separate devices from the modified processing system. 
     FIG. 10 shows a more expanded view of a prototyping system for verifying the combined processing system and configurable subsystem. The processing system  268  includes an extended processing system  270  having an extended bus  272  and a configurable subsystem  274  connected to the extended bus  272  along with an external boot ROM  276  for downloading programs to the processing system program store unit  278  and a prototyping area (“wire-wrap” area)  280  for adding circuitry or wiring needed during the prototyping. The configurable subsystem  274  also has an interface  282  for in-circuit program support. 
     Various tools, such as Verilog TM and FPGA tools  284 , are available to store programs in the external boot ROM  276  for the processing system  270  and to configure the FPGA  274 . Processing system support tools, such as the 8051 programming tools  286 , are available to configure the processing system&#39;s program store  278 . 
     The prototyping system  268  of FIG. 10, is set up and operates as follows. The processing system  270 , e.g., an 8051-derivative, is modified using existing Verilog or VHDL tools  284  so that the 8051 Special Function Register address decoder recognizes a selected set of unassigned addresses  136  in the Special Function Register address space in accordance with one embodiment of the present invention. A reference to those addresses activates the CS/ signal  290  on the extended bus  272 . The timing and control unit  292  of the processing unit in the 8051-derivative  270  is modified to receive a RDY signal  294  from the extended bus  272  and to force an external 8051 bus cycle to occur when the unassigned address  136  is referenced. 
     A printed wiring assembly (PWA)  268  is designed to support the modified 8051  270  with the extended bus  272  and a programmable logic device (PLD)  274 . The PLD may be supported by a specific socket or by a “wire-wrap area”  280  on the printed wiring assembly. During development of the modified 8051  270 , an FPGA is used to implement the 8051 itself. This FPGA is denoted by FPGA-8051. However, production versions utilize ASIC (Application Specific Integrated Circuit) implementations of the modified 8051 instead of FPGA versions. The FPGA-8051 must be configured either by downloading code from the Verilog/VHDL tools  284  or via the pre-loaded boot ROM  276 . Also, the program store and the data storage of the FPGA-8051 either must be downloaded from the 8051 tool set  286  or from the boot ROM  276 . 
     To test the system, the 8051 program writes known patterns to the addresses of the extended bus  136 . As shown in FIG. 5, the data appears on the 8051 central bus, and the Special Function Register address decoder generates the CS/ signal on the extended bus. The read RD/ and write signals WR/ operate as described in reference to FIG.  5  and FIG. 6, and the central bus contains data appropriate to the program in the program store of the processing system. During this period the RDY signal  294  is forced inactive and the timing and control circuit modifications are tested to assure that the extended bus logic  296  is functional. After all extended bus control timing is verified, the extended bus  272  is ready to be used by an external PLD. 
     Next, the logic design Verilog/VHDL tools  284  are used to define an extended bus interface  298  for a standard PLD device (including PAL, EPLD, FPGA, etc.) and the in-circuit programming support interface  282  is used to download the PLD code. At this point the modified 8051 is capable of driving the configurable subsystem  274 , via the extended bus  272 . 
     After successfully testing and verifying the above subsystem, the modified 8051  270  is converted to an ASIC. The ASIC version includes an 8051 with an extended bus  272  and extended bus support. The ASIC is built in a standard package using 3 or 5 volt supplies and is used as the basis of configurable logic subsystem design. The PWA would typically be redesigned for the ASIC-8051 and would then serve as a universal prototype for configurable subsystems based on the 8051 microprocessor. 
     To summarize the prototyping process, the following steps are followed. First, the 8051 program is developed and simulated using legacy 8051 simulators. Second, FPGA code is developed and simulated using Verilog or VHDL simulators. Next, the prototyping system is powered up. Following this, the 8051 program is downloaded with the 8051 held in reset. Next, the FPGA code is downloaded and the 8051 is then released from reset. The modified 8051 then exercises and tests and debugs the PLD device during which the extended bus pins are monitored using standard tools such as a logic analyzer. 
     FIG. 11 shows several alternative implementations for the configurable subsystem in accordance with the present invention. Configuration A  300  represents an unmodified processing system, such as the 8051, in which the processing system implements the desired extended functionality in software. This is the simplest way to achieve the desired extended functionality but does not work for many cases because of the slow speed of the software in implementing the extended functionality. 
     Configuration B  302  combines a modified processing system  304  with a field programmable configurable subsystem  306  which is connected to the modified processing system via an extended bus  308  as described above. This configuration is best for rapid development but be somewhat more costly depending on the size of the FPGA. 
     Configuration C  310  combines a modified processing system  304  with a pre-programmed PGA 312, such as is available from American Micro Systems, Inc. of Pocatello, Id. and which is connected to the modified processing system via an extended bus  308 . This configuration is best suited for medium volume production and minimizes the parts cost. 
     Finally, Configuration D  314  integrates the modified processing system with its extended bus and the configurable subsystem  316  onto the same silicon substrate  318 . This configuration is best for high volume production and low cost. 
     The processing system in each of the above configurations is either an 8051 or 8051-derivative with an extended bus accessible via the unassigned addresses of the 8051 register set or an arbitrary processing system having an extended bus and a new instruction added to the unassigned instruction code space to reference the extended bus. 
     Thus, having the extended bus permits tradeoffs among the various configurations and gives flexibility to the designer. 
     Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.