VARIABLE INSTRUCTION SET COMPUTER

Abstract: A source program is executed on microcontroller core (114) of a processing unit (100). The core (114) is capable of native instructions from a predetermined set of microcontroller specific instructions. In a pre-processing step, for the program statements of the source program a program-specific virtual machine is defined with a corresponding set of virtual machine instructions, such that the expression of the program statements in the sequence of instructions requires less storage space compared to using only native instructions. For the program-specific virtual machine an associated conversion means (132) is defined for converting the program-specific virtual machine instructions into the native instructions of the core (114). The source program statements are expressed in a sequence of instructions comprising instructions of the defined virtual machine. The sequence of instructions is stored in an instruction memory (120). The conversion means (114) is represented in the processing unit (100). During execution, instrutions are fetched from the instruction memory (120). The conversion means (114) is used to convert the fetched virtual machine instructions into native instructions for execution by the core (114).

Description: Variable instruction set computer The invention relates to a method for executing a source program on a processing unit comprising a predetermined microcontroller core for executing native instructions from a predetermined set of micro-controller specific instructions; the method comprising: a pre-processing step of expressing program statements of the source program in a sequence of instructions comprising virtual machine instructions: and storing the sequence of instructions in an instruction memory; and an execution step of fetching instructions from the instruction memory; using conversion means of the processing unit to convert virtual instructions fetched from the instruction memory into native instructions; and feeding the native instructions to the microcontroller core for execution The invention further relates to a processing unit for executing instructions of a virtual machine, the instructions being referred to as virtual machine instructions; the processing unit comprising: a predetermined microcontroller core for executing native instructions from a predetermined set of micro-controller specific instructions; the native instructions being different from the virtual machine instructions. an instruction memory for storing instructions including at least one of the virtual machine instructions; a converter comprising conversion means for converting a virtual machine instruction fetched from the instruction memory into at least one native instruction for execution by the microcontroller core. Increasingly, source programs are expressed in (compiled to) instructions of a virtual machine instead of native instructions of a microcontroller core on which the program is to be executed. A main reason for using a virtual machine is portability of programs between different machines (platforms). A program expressed in the virtual machine instructions of the virtual machine can be executed relatively easily on several concrete machines, using suitable interpreters operating on those machines. At this moment a driving force for using portable programs is Java, where Java programs are exchanged via Internet and can be executed on different native machines using processors with different instruction sets. Using a compiler, Java programs are expressed in Java byte codes (JBCs), which form the instructions of the Java Virtual Machine. The resulting code is usually referred to as a Java applet. Conventionally, programs expressed in virtual machine instructions are executed by means of software interpretation. The processor (CPU) executes a special interpreter program, where in a loop the processor fetches a virtual machine instruction. decodes it into a sequence of native instructions of the microcontroller core of the processor and executes each native instruction. This technique is slow and requires an additional interpreter program, which can be relatively large. To improve the execution speed, the so- called Just-In-Time (JIT) compilation technique is used. Just before starting execution of software module expressed in virtual machine instructions, the module is compiled to native code (i.e. , expressed in native machine instructions). In this way, the module needs to be stored twice in addition to the code for the compiler. The additional storage requirements of software interpretation are not desired for embedded systems. To avoid performance and storage overhead, it is preferred to use a hardware interpreter. In itself a hardware interpreter is known in the form of a Prolog pre-processor for Warren's abstract instruction set. In the paper "A Prolog pre-processor for Warren's abstract instruction set" by B. Knodler and W. Rosenstiel. Microprocessing and Microprogramming 18 (1986) pages 71-81 , a preprocessor is described for interpreting programs written in the Prolog programming language on a Motorola 68000 processor (MC68000). A compiler is used to translate the Prolog source program into instructions, which have been defined by Mr. Warren and which are generally used for executing Prolog programs. The set of Warren instructions forms a virtual machine designed for executing Prolog programs. The sequence of Warren instructions resulting from the compilation are loaded into RAM and executed by the MC68000 with the aid of the pre-processor. After power-on, the MC68000 first performs a booting procedure by executing native MC68000 instructions. At the end of the booting procedure, the MC68000 is ready to initiate the execution of a Prolog program. This is started by jumping to a predetermined address range. The pre-processor is a memory-mapped device, which is mapped to this range. When the pre-processor is addressed it reads a Warren instruction (of the translated Prolog program) from its own RAM, adaptively synthesizes a sequence of MC68000 instructions and constants and sends these directly to the CPU for execution. The MC68000 instructions for each Warren instruction are stored in ROM of the pre-processor. In general, the pre-processor translates one Warren instruction into a sequence of MC68000 instructions. The pre-processor contains its own RAM controller and ROM controller, which generate the addresses for the RAM and ROM of the pre-processor. The RAM controller manages the RAM instruction pointer. Each successive read operation of the MC68000 results in the pre-processor sending the next instruction (and optional constants) of the sequence to the CPU. If the sequence has been completed, a next read operation results in the first instruction of the sequence corresponding to the next Warren instruction of the program being send to the CPU. The known pre-processor supports one virtual machine (the Warren machine). It is an object of the invention to provide a method and processing unit of the kind set forth which is more flexible. It is a further object of the invention to provide a method and processing unit of the kind set forth wherein the program is represented in a more compact form. To meet the object of the invention, the method is characterised in that the method comprises the steps of defining for the program statements of the source program a program-specific virtual machine with a corresponding set of virtual machine instructions, such that the expression of the program statements in the sequence of instructions requires less storage space in the instruction memory compared to using only native instructions for expressing the program statements: defining for the program-specific virtual machine an associated conversion means for converting virtual machine instructions of the program-specific virtual machine into native instructions of the microcontroller core; and representing the associated conversion means in the processing unit. According to the invention, a program-specific virtual machine is defined for a program such that the program can be expressed in a form which is more compact than if the program was expressed in native instructions of the core. Also an associated conversion means is defined. The conversion means may for instance be implemented using a conversion table or microcode stored in ROM or dedicated logic, such as a PLD. The defined conversion means is represented in the processing unit. In this way, fast execution of the program is maintained, whereas at the same time a compact representation is achieved. This method is particularly suitable for use with embedded applications. In this case, the source program relates to all program statements initially represented in the embedded system (for instance, the program present in the system when the user purchases the system) . The program may be stored in a permanent memory, such as ROM, or stored in a re-programmable memory, such as EEPROM. For embedded applications it is highly desired that the code used to represent the embedded application program is compact and that the performance in executing the code is good. Typically, processing units used for embedded applications are based on a family concept, where for a specific application a processing unit is created from a given microcontroller core and I/O or storage components required for the application. It is desired to make no or at least no significant modifications to the core or the components in order to reduce costs. According to the invention, on the one hand virtual machine instructions are defined specifically for the program, giving full flexibility in achieving code compaction, whereas on the other hand an off-the-shelf core is used for executing the program. The required conversion from virtual machine instructions to native instructions is performed by a conversion means, which can be seen as a kind of pre-processor. The article "Optimizing an ANSI C Interpreter with Superoperators" , Todd A. Proebsting, POPL'95:322-332, 1/95 describes the use of superoperators as an optimisation technique for bytecoded interpreters. As a first step a compiler is used to express the program in virtual machine instructions. The virtual machine instruction are chosen such that the representation of the program using the virtual machine instructions is more compact. An example of such a virtual machine is a stack machine, which is known for its compact representation. Next, sequences of virtual machine instructions which occur frequently in the compiled code are identified and replaced by newly defined additional virtual machine instructions, where, for instance, one new instruction (a so-called superope- rator) replaces a sequence of four existing instructions. A superoperator can also be defined, by during the parsing phase of the compiler, identifying most frequently occurring terms in the expression tree which is built during an intermediate representation in the compiler. Only a limited number of superoperators are defined, e.g. , well less than 256. This allows a compact encoding of the superoperators (e.g. , using a single byte code). Part of the code range is then assigned for representing the original virtual machine instructions and part of the range is used for representing newly added virtual machine instructions representing the superoperators. By using only 20 superoperators already a significant code compaction can be achieved. The virtual machine instructions are interpreted using a software interpreter running on a MIPS R3000 or SPARC processor. No use is made of a hardware interpreter which in combination with a given embedded microcontroller core provides code compaction of a given embedded application program without a performance penalty . In an embodiment as defined in the dependent claim 2. a good level of code compaction can be achieved for different groups of program statements. Preferably, program statements relating substantially to a same subset of native instructions are grouped together and a virtual machine is defined for such a group. For instance, a different virtual machine may be defined for integer operations than for floating point operations. It is preferred to group program statements which relate to substantially the same subset of native instructions but use the instructions at a substantially differing frequency, allowing the definition of special virtual machine instructions for the most frequently occurring (sequences of) instructions. For a multi-tasking system, preferably program statements of one task are associated with one virtual machine. This simplifies development of the embedded program, since usually program statements for one task are produced under supervision of one engineer and in this approach the supervisory role can be extended to include the supervision over the production of compacted code and an associated conversion means Depending on the chosen implementation, selection of the conversion means associated with the virtual machine instruction to be executed could incur a delay of the execution. Instead of then selecting a conversion means for individual virtual machine instruction, the overhead may be reduced by switching the conversion means as part of the task switching. As long as then the same tasks executes uninterrupted, no overhead is involved in selecting the appropriate conversion means. It will be appreciated that the same virtual machine may also be used for several tasks which use a similar mix of native instructions in a similar frequency On the other hand, if the program for a task comprises more than one program module, where at least two of the program modules relate substantially to different subsets of native instructions or use instructions of a similar subset of instructions in substantially different frequencies, then it is preferred to use different virtual machines for those modules. Similarly, if an object-oriented programming technique is used, a virtual machine may be defined for each or a group of similar objects. In an embodiment as defined in the dependent claim 3, an instruction module and conversion data for converting the instructions of the module to native instructions is received. For instance, the processing unit may be supplied with an additional or replacement program module. The supply may be local, e.g. , by adding or replacing a ROM with the program module, or loading the program module from a portable storage medium, such as a floppy disk, into a programmable memory. The supply may also take place via a network, either a local area network or a wide area network, such as Internet. Increasingly, it is desired that embedded systems are 'open' to accept additional software or software which replaces part of the existing software at a stage after the initial programming of the system Where the initial software was optimally compressed using a dedicated virtual machine, using this machine for the newly received code will not always give good results or could even be impossible Even, if the supplier of the original software also supplies the new software, the nature of the software may be different, implying that a better compaction can be achieved by defining a separate virtual machine for the new software Increasingly, the received software will be of an entirely different nature than the original softw are As an example, the new software may be a Java applet expressed in Java bytecodes, whereas the original software was written in "C" and optimally compiled to a virtual machine matching the original program together with a corresponding conversion means It will be understood that the defined conversion means can not be used for converting the Java bytecodes since these codes are expressed using a different virtual machine To meet the object of the invention the processing unit is characterised in that the converter is operative to perform the conversion for a plurality of different virtual machines For instance, an embedded system may initially be supplied with an embedded program expressed using one virtual machine, which preferably is defined for the specific program At a later stage a software update is required, replacing substantially all or a part of the software or adding a software module For the entire application program which is then created it may be desired to use a new virtual machine which better reflects the then valid program (which may still contain parts of the old program as well) Particularly, since the required amount of software tends to grow over time a higher level of code compaction (reflected by a different virtual machine) may be required for the newly created application program In this way, different virtual machines are used over time, where at each moment in time only one virtual machine is used Alternatively, as described above, different virtual machines may be used at the same time for different parts of an embedded application program Different virtual machines may also be used for programs of a different origin, such an embedded program written in "C" and compacted using a program-specific virtual machine and an program, such as a Java applet, expressed using a different and usually predetermined virtual machine It will be appreciated that also multiple virtual machines may be present in the processing unit at the same time, where over time new virtual machines are added or existing virtual machines are replaced In an embodiment of the processing unit as defined in the dependent claim 5, the conversion means is of a re -programmable type This allows 'downloading' of a new conversion means into the processing unit whenever a new program is loaded. The reprogrammable conversion means may, for instance, be implemented using a re-pogrammable conversion table or microcode, e.g. , stored in (E)EPROM, or using dedicated re-progamma- ble logic, such as an E-PLD. In an embodiment of the processing unit as defined in the dependent claim 6, the converter comprises dedicated conversion means for each one of the virtual machines. In this embodiment, the processing unit is created with several dedicated conversion means. In principle it is possible to use one large virtual machine with one corresponding conversion means. By using several virtual machines more compact code can be achieved. For instance, assume that an embedded program is formed by two program modules of which one module is mainly involved with user interface aspects of the embedded system, requiring mainly native integer instruction, whereas the second module is mainly involved with signal processing, requiring mainlv floating point instructions. Assuming further that optimal virtual machines for each of the two modules comprise 256 virtual machine instructions each, the program modules are expressed using 8-bit codes One virtual machine covering both modules can be made bv combining the instruction of the two virtual machines, giving up to 512 virtual machine instructions (at least more than 256). Consequently, the code size will increase since now 9-bit codes are required. Such an increase can usually not be compensated by the fact that a combined conversion table may be smaller than two separate conversion table, due to a possible overlap in instructions In an embodiment of the processing unit as defined in the dependent claim 7, the converter distinguishes between conversion means for different virtual machines based on the location in the instruction memory where the virtual instruction to be executed is stored. For instance, if besides native instructions also virtual machine instructions of two different virtual machines are used, the memory may be subdivided into three regions; one region for each type of instruction. By determining in which region the address of the instruction to be executed lies, the converter can easily detect whether or not conversion is required (native instruction need not be converted) and which conversion means should be used for the conversion. In an embodiment of the processing unit as defined in the dependent claim 8, a separate indicator (e.g. , one or more bits) is used to differentiate between the native and the virtual machine and/or between different virtual machines. A separate indicator may be used for each instruction. Alternatively, one global indicator (e.g., a register) may be used which is set each time a change of machine occurs. In an embodiment of the processing unit as defined in the dependent claim 9, one virtual machine instruction is converted into one corresponding native instruction, which is coded more compactly. In this way, the conversion can be very simple. In an embodiment of the processing unit as defined in the dependent claim 10 one virtual machine instruction is converted into a predetermined sequence of a plurality of native instructions, giving a further level of compaction. A sequencer is used to control the feeding of the sequence of native instructions to the microcontroller core, for instance by inhibiting an increment of the instruction pointer (program counter) of the microcontroller while the sequence is being fed and enabling an increment when a sequence has been completed. Alternatively, the processing unit comprises an instruction fetcher for fetching an instruction from the instruction memory under control of its own instruction counter. Whenever a sequence has been completed, changing of the counter is enabled, and the instruction counter is set to a different value in response to a change of an instruction pointer (program counter) of the microcontroller core. While a sequence is being processed, a change of value of the instruction counter is inhibited. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments shown in the drawings. Figure 1 illustrates four possible architectural options of locating the converter in the processing unit; Figure 2 illustrates the process of defining the virtual machine and associated converter; Figure 3 illustrates the process for a program foremd of several coherent groups of program statements; and Figure 4 shows a block diagram of the converter; Figure 1 illustrates four possible architectural options of locating the converter in the processing unit 100. Three main components of the processing unit 100, are the microcontroller 110, the instruction memory 120 and a pre-processor 130. The preprocessor 130 comprises the converter 132. In all shown figures the microcontroller 110 comprises the instruction memory 120 and the pre-processor 130. As such, the processing unit 100 is not shown explicitly. By combining all main elements in the microcontroller 110, which preferably is a one-chip device, optimum performance can be achieved It will be appreciated that, if desired, the instruction memory 120 and/or the pre-processor 130 may be located outside the microcontroller 110, where the microcontroller bus 140 is extended outside the microcontroller 110 and, for instance, coupled to an external bus such as PCI The instruction memory 120 contains virtual machine instructions, such as instructions for a stack machine The instruction memory may also be used for storing data The invnetion is not limited to Harvard architecture, wherein data and instructions are separate The microcontroller 110 comprises a processor 112 with a predetermined microcontroller core 114, referred to as a native machine, for executing native instructions from a predetermined set of microcontroller specific instructions An example of a microcontroller suitable for executing embedded software is a RISC-type microcontroller, like the MIPS PR3001 range of microprocessors The processor may comprise an instruction cache 1 16 for storing native instructions before executing the instructions The native instructions of the microcontroller core 114 are different from the virtual machine instructions ot the virtual machine As such the microcontroller 1 10 is not able to directly execute virtual machine instructions stored in the instruction memory 120 In response to the processor 112 requesting an instruction, the pre-processor 130 issues the native instruction In order to be able to generate the native instruction the pre-processor 130 may fetch a virtual machine instruction from the instruction memory 120 using fetching means 134 The converter 132 of the pre- processor 130 is used for converting a virtual machine instruction, fetched from the instruction memory 120, into at least one native instruction In general, a virtual machine instruction is converted into a sequence of native instructions The pre-processor 130 further comprises a feeding means 136 for feeding native instructions of the sequence to the microcontroller core 114 for execution When executing a virtual machine program the microcontroller 110 in fact executes a native program generated by the pre-processor 130 Where normally an instruction pointer of the microcontroller 110 indicates the next instruction in the instruction memory 120 required by the microprocessor 110 to be executed next, now the instruction pointer indicates to the pre-processor 130 that a next native instruction is required (or a re-feedmg of a previous instruction) Consequently, the pre-processor 130 manages an independent virtual machine instruction pointer indicating the current (or next) virtual machine instruction in the instruction memory 120 The microcontroller does not require (explicit) knowledge of the virtual machine instruction or the virtual machine instruction pointer In Fig 1A the main components of the processing unit are interconnected via a general peripheral interconnect bus 140, such as the PI bus The pre-processor 130 is a peripheral on the bus The pre-processor 130 may act as a memory-mapped peripheral, where a predetermined range of addresses is allocated to the pre-processor In response to the processor 112 issuing on the bus 140 a request for an instruction with an address in this range, the pre-processor 130 issues the native instruction on the bus 140 When required, the pre-processor 130 fetches a virmal machine instruction from the instruction memory 120 via the bus 140 In Figs IB and 1C, the pre-processor 130 is located in between the procesor 112 and the instruction memory 120 If the pre-processor 130 needs to distinguish between native and virmal machine instructions, these configurations may delay execution of native instructions stored in the instruction memory 120 For reasons of clarity not all elements shown in Fig 1A are repeated in Fig IB 1C and ID In Fig ID, the pre-processor 130 is embedded in the processor 1 12 The pre-processor 130 is preferably located between an instruction cache 116 of the processor 1 12 and the core 1 14 This configuration allows for optimal performance but unlike the configurations of Figs 1A, IB and 1C, requires changes to the microcontroller 110 and, as such, the pre-processor 130 can not as an off-the-shelf design be used for different types of processors w ith the same type of core 114 In a pre-processing step, a program-specific virtual machine with a corresponding set of virtual machine instructions is defined for the program statements of a source program which needs to be executed by the processing unit 100 Furthermore, for the program-specific virmal machine an associated converter 132 is defined for converting virtual machine structions into native instructions of the microcontroller core The converter 132 provides for each of the virtual machine instructions one or, usually, a sequence of corresponding native instructions required for executing the virtual machine instruction of the native core 114 Figure 2 illustrates the process of defining the virtual machine and associated converter In step 205 the source program 200 is analysed Based on the analysis (e g , frequency of occurrence of operations), m step 210 a program-specific virtual machine 215 is defined In step 220 for the virtual machine an associated converter 225 is defined and in step 230 the source program is expressed m instructions of the virmal machine resulting in code 235 The virtual machine is defined such that the code requires less storage space in the instruction memory compared to using only native instructions for expressing the program statements Preferably, the program-specific virmal machine is defined by translating the program to virtual machine instructions of a predetermined virtual machine As a starting point a virtual machine, such as a stack machine, is chosen which is known to give a compact representation Due to the fact that a stack machine does not use explicit registers for storing operands, but instead operands are stored on the stack and operators always act on top element(s) of the stack, the number of operands which can be used is less restricted and management of the operands is simpler for a stack machine This makes it easier to build a compiler for a stack based machine than for a register based machine Furthermore, stack machine instructions tend to have a simpler structure than most register based machines However, it will be appreciated that also a suitable register based machine may be used To illustrate the invention, the stack machine of Annex A is used A sample program (which solves the eight queens problem) written in the "C" programming language is shown in Annex B The translation of the program to stack code of the \ irtual machine of Annex A is shown m Annex C By anah s g the code frequently occurring sequences of instructions are identified and additional virtual machine instructions are defined, each representing one of the identified sequences Annex D shows a list of suitable additional machine instructions Such an additional machine instruction can be seen as a "super virtual machine instruction" The program specific virtual machine is then formed by the basic instructions of Annex A in combination with the supeπnstructions of Annex D Annex E shows the expression of the program of Annex B into the virtual machine of Annex A and D Next, for the program- specific virtual machine an associated converter 132, as shown in Fig 1 , is defined for converting virtual machine instructions into native instructions of the microcontroller core The converter 132 provides for each of the virmal machine instructions (in the example, for the combination of the instructions in Annex A and D) one or usuall , a sequence of corresponding native instructions required for executing the virtual machine instruction on the native core 114 Using this approach, compaction of the code is achieved by using relatively few virmal machine instructions (in the example, 38 basic instructions and 40 super instructions) allowing a short representation (e g , 7 bits) for each non-parameteπsed instruction, compared to 16 or 32 bits usually used for representing native instructions The use of the supeπnstructions also results in requiring fewer instructions to represent the program In the example, expressing the program m the instruction of the basic virtyual machine requires 356 statements Using supeπnstructions reduces this to requiring 262 statements This compares favourably to the situation where the program is expressed in native instructions, where for a MIPS processor 608 statements are required Preferably, a program specific virtual machine is defined in the following way. The source program is converted to an intermediate level representation of so-called expression trees based on a predetermined virtual machine. An expression tree is constructed of nodes representing basic operations of the virtual machine. The expression tree represents the computation resulting from performing the basic operations of the nodes of the tree in a given sequence (e.g. , postfix order). The conversion of the program to expression trees may be performed using the parsing phase of a compiler, such as the Tight "C" compiler lcc' of Fraser and Hanson. Next the expression trees and coherent tree fragments (representing a sequence of operations) are identified. Each of the trees and fragments is a candidate to be represented by a superinstruction. Starting from the basic virmal machine instructions, the tree or tree fragment which gives most savings in code size is represented by a superinstruction. The superinstruction is added to the virmal machine. The program is expressed in the new set of virtual machine instructions to determine the saving. New superinstructions are added as long as still room is available for adding new instructions (e.g. , if the virtual machine instructions need to be stored as bytes, the number of instructions is limited to 256) or until no saving are achieved any more. Finally, the source program is expressed in instructions of the program-specific virtual machine and conversion data is generated. Once the program has been expressed in virtual machine instructions, the resulting code is stored in the instruction memory 120. For embedded systems, the code is usually stored in a ROM, which is mask-programmed, or a programmable ROM, such as a PROM or (E)EPROM. Similarly, the generated conversion data is represented in the converter 132 of the processing unit 100. The converter 132 may, for instance, be implemented using a conversion table or microcode stored in ROM. The converter 132 may also be implemented using logic, such as a PLD. For a re-programmable converter the same approach can be used based on the respective re-programmable techniques, such as (E)EP- ROM or E-PLD. Figure 3 illustrates a further embodiment according to the invention, wherein the source program 300, e.g. , programmed in "C" , is split in step 310 into several coherent groups of program statements, such as program modules, objects or task-specific code. Shown are modules 312 and 316. It will be appreciated that no explicit splitting may be required and the source program may already be available in a suitable modular organisation. For each of the groups 312 and 316, a program group specific virtual machine is defined (step 330) with a corresponding set of virtual machine instructions in a way similar to steps 205 and 210 of Fig. 2. Shown are the respective virtual machines 332 and 336. In step 340. the program statements of the groups 312 and 316 are expressed in instructions of the respective virtual machines 332 and 336, resulting m the respective code modules 342 and 346 The code modules are stored m the instruction memory 120 in step 350 In step 360 for the program group specific virtual machines 332 and 336 respective conversion means 362 and 366 are generated for converting virtual machine instructions into native instructions of the microcontroller core In step 370, the conversion means 362 and 366 are represented in the processing unit 100, for instance by programming a conversion table or logic in the processing unit 100 As a consequence, the converter 132 comprises a specific conversion means for each of the program groups To enable the processing unit 100 to select the group- specific conversion means associated with a fetched virmal instruction, in step 390 selection data is stored in the processing unit During execution a virtual machine instruction is fetched from the instruction memory 120 The selection data is then used to locate the conversion means associated w ith the virtual machine to which the virtual machine instruction belongs Figure 4 shows a block diagram of the converter 132 wherein the converter 132 comprises several conversion means (shown are 400, 410 and 420) Each of the conversion means is used for converting virmal machine instructions of a specific v irtual machine It will be appreciated that the converter 132 mav comprise fully autonomous conversion means, each capable of performing the full conversion Alternatively, the converter 132 may use some shared logic under control of separate conversion data (such as a table) for each of the virtual machines The processing unit 100 comprises the selection data 430 The selection data may be represented in any suitable way, e g , stored in the instruction memory 120 or, if the conversion means is stored (partly) as data stored in combination with the conversion means The selection data 430 may also take the form of a register m the converter 132 loaded from a memory Various forms of selection data may be used In a further embodiment, the address range of the instruction memory 120 is divided into several sub-ranges An address sub-range is reserved for storing virtual machine instructions of only one virtual machine Preferably, an address sub-range is also reserved for storing native instructions Such native instructions may, for instance, be used to initialise the system or to allow certain software modules, such as drivers or special parts of the embedded software application, to be compiled to native instructions for optimal performance In this embodiment the selection data indicates for each of the defined address sub-ranges which one of the conversion means 400, 410 or 420 should be used for converting an instruction fetched by the instruction fetcher 134 to native instructions To this end, the converter 132 comprises a detector 440 for selectively directing an instruction fetched from a location in the instruction memory 120 to one of the conversion means 400, 410 or 420, based on the selection data. If also native instructions are stored in the instruction memory 120, the detector 440 ensures that these instructions are directly supplied to the feeder 136 for supply to the microcontroller core 114. As an alternative to basing the decision on the address of the fetched instruction, the information may also be stored in direct association with the instruction. For instance one or more bits of each entry in the instruction memory 120 may be reserved to differentiate between virmal machines and/or between native code or virtual code. For instance, if only two virtual machines are used, where the instructions require 7 bit, an eighth bit can be used to indicate the virmal machine to which the instruction belongs and the associated conversion means. Obviously, this technique may be combined with differentiating based on addresses. As a further alternative, the selection data may be stored in a register where the register is set to indicate a different conversion means each time a switch occurs between virtual machines. To set the register a special instruction (e.g. , a form of a jump instruction) may be used. Such implementation-supporting instructions can be mixed with the virtual machine/native instructions in the instruction memory. As described above, typically a virtual machine instruction is converted to a sequence of native instructions. To regulate the supply of instructions to the core, the processing unit 100 comprises a sequencer 450 coupled in between the converter 132 and the microcontroller core 1 14 for sequentially feeding the sequence of native instructions to the microcontroller core 114. The sequencer 450 may be implemented using conventional components, such as a counter, which may, for instance, be incremented as a result of a trigger from the microcontroller core indicating that a new instruction is required (e.g. , an increment of the instruction counter of the core). In a conventional processing unit, a change of the instruction pointer (also referred to as program counter) of the microcontroller core results in the fetcher 134 fetching an instruction from the instruction memory 120 and the feeder 136 supplying the instruction to the microcontroller core 114. To break the link between automatically fetching and feeding, the processing unit 100 further comprises inhibiting means 460 for inhibiting fetching an instruction from the instruction memory during the feeding of the sequence. It will be appreciated that the sequencing and inhibiting may be performed in several ways. In a further embodiment, the inhibiting means 460 is operative to perform the inhibiting by obstructing an increment of the instruction pointer of the microcon- troller core 114. This may require a small modification to the core 114. For instance, an additional control line into the core 114 may allow selective inhibiting of the instruction counter, where the inhibiting means 460 inhibits the increment whenever instructions from the sequence are being supplied and enables an increment when a sequence has been fully supplied. In an alternative embodiment, the instruction fetcher 134 maintains its own instruction counter 135, separate from the instruction pointer of the microcontroller core 112. The inhibiting means 460 controls when an increment (or more general a change) of the instruction pointer of the microcontroller results in a change of the instruction counter 135 of the instruction fetcher 134. Similar as described above, the inhibiting means 460 inhibits a change of the instruction counter 135 whenever instructions from the sequence are being supplied and enables a change when a sequence has been fully supplied. As a result of a change of the instruction counter 135, the instruction fetcher 134 will normally fetch a new instruction from the instruction memory 1 12, from an address indicated by the instruction counter. Advantageously, particularly in view of the compact representation of virtual machine instructions, the instruction fetcher 134 may fetch several instructions from the memory in one read operation (e.g. , four one-byte instructions may be read as one 32-bit word). In this way, not every change of the instruction counter 135 needs to result in actually fetching a new instruction. In a further embodiment according to the invention, a program module expressed in virtual machine instructions of a further virtual machine is received, e.g.. via a network or from a local background memory. An example of such a program module is a Java applet. The received program module is stored in the instruction memory 120. To enable conversion of the further virtual machine instructions into native instructions also conversion data is received. The conversion data may for instance specify a conversion table for a table based conversion means or E-PLD programming data for a PLD based conversion means. The conversion data is stored in the processing unit for subsequent use by a further conversion means of the converter 132. To be able to select the appropriate conversion means during execution, also selection data is stored in the processing unit associating each further virmal machine instruction with the conversion data. During execution, for a fetched further virtual machine instruction the further conversion means is operated under control of the conversion data indicated by the selection data. As described above, the converter 132 may comprise a table for converting a virmal machine instruction to a sequence of native instructions. A one dimensio- nal table may be used, where each cell of the table comprises a sequence of native instructions for one corresponding virmal machine instruction. The cell number may correspond to the value of the corresponding virmal machine instruction. As an example, the sequence of native instructions for the Java integer addition (0x60) may be located in cell 96 ( = 0x60 in hexadecimal notation). Since the length of the sequence of native instructions may vary considerably for the various virtual instructions, preferably the sequences are located in a one-dimensional table without any explicit cells where the sequences immediately follow each other. Such a translation table 500 is shown in Fig. 5, where the implicit cell boundaries are indicated using dotted lines. In order to be able to locate a sequence for a virtual machine instruction a code index table 510 may be used, which for each virtual machine instruction (VMI 1 to VMI N) indicates the starting point of the corresponding sequence in the translation table 500. For the cell of the translation table 500 which corresponds to VMI 3 the related sequence 520 of native instruction NI 1 to NI M are shown. A further example of a conversion is given for the Java byte code bipush /i (used for sign extending byte n and placing the result on top of the stack). This virtual machine instruction consists of two bytes {0x16 and 72} , where the first byte specifies the operation and the second byte provides the parameter 72. The instruction may be converted to the following sequence of native MIPS instructions: ori $a0, $0, n/* Load register SaO with constant n */ sll SaO, SaO, 24/* Shift left by 24 bits */ sra $a0. SaO, 24/* Arithmetic shift right, causing sign extension, by /* replicating last left-most bit */ sw SaO, 0 (Stosp)/* Store result at new top of stack */ addi Stosp, -4/* Increment stack size */ This example illustrates that a virtual machine instruction may be parameter ised, where an operation code is followed by at least one operand. Advantageously, the converter 132 comprises a conversion table 500, where native instructions are represented either by the full code or by an instruction skeleton. As an example, the instruction addi Stosp, -4 (last instruction of the sequence of the previous example) contains no variable parts and may be located in full as a 4-byte entry in the table. The instruction ori $a0, $0, n (first instruction of the sequence of the previous example) contains a variable part and may be located in the table as a skeleton, not specifying the variable part (being n). Preferably, the entry in the table for an instruction skeleton is the same width as a full instruction (e.g. , 4-bytes for a MIPS processor), allowing a uniform table. Further information may be located in the table (or in separate table(s)) for indicating how the unspecified part of the native instruction skeleton should be filled in Advantageously, microprogramming is used to fill in the unspecified parts The further information may then comprise or indicate micro code It will be appreciated that it is advantageous to use for an instruction skeleton a same structure (width and composition) as for a full native instruction However, other structures may be used as well. If the virtual machine is a stack oriented machine, preferably the stack or at least the top elements of the stack are mapped onto registers of the microcontroller 1 10 In this way the memory stack (with the virtual machine stack) is mapped to the register stack Assuming that registers $rl , Sr2 and $r3 contain three successive elements ot the memory stack, where initially Sri corresponds to the first empty location of the memory stack (above the top of the stack), Sr2 contains the top of the memorv stack, and $r3 contains the second element of the memory stack, the Ja a byte code bipush n ma\ be converted to the following sequence of native MIPS instructions on Sri , SO, n sll $rl , Sri, 24 sra Sri, Sri, 24 After this operation, $rl contains the top of the memorv stack Similarly, the Jav a byte code (a virtual machine instruction) for integer addition (0x60) may be converted to the following sequence of MIPS instructions, starting from the same position where initially Sri corresponds to the first empty location of the memorv stack (above the top of the stack), Sr2 contains the top of the memorv stack, and $r3 contains the second element of the memory stack add Sr3, $r2, $r3 After this operation, $r3 contains the top of the memory stack In above examples, preferably the position of the top of the memory stack (I e , which register contains the top of the memory stack) is indicated using a register 138 of the converter 132 The converter uses the register 138, referred to as the Register Stack Pointer (RSP), to generate the appropriate native instructions Preferably, micropro- grammmg is used to specify the register operands of the native instructions In this way also fixed native instructions have become variable, since the register operands need to be specified by the converter 132 Preferably, such operands are also stored in the translation table 500 using instruction skeletons Assuming that RSP points to the first free register, the Java byte code bψush n may be converted to the following sequence of native MIPS instructions under control of the corresponding micro code: Micro code Instructions rsp -= 1; f Ig = rsp + lori $(rsp + 1), $0, n f- g = rsp 4- 1; f„ = rsp + lsll $(rsp + 1), $(rsp + 1), 24 f tg = rsp + 1; f ao = rsp + lsra $(rsp + 1), $(rsp + 1), 2 where f lg indicates the target register for the instruction, and f a0 and f al indicate the first and, respectively, second argument register for the instruction. A following Java byte code iadd for adding the two top elements of the stack would result in the following micro code and instruction: f [g = rsp + 2; f ao = rsp + 2; f al = rsp + 1 ; rsp + = ladd $(rsp + 2), $(rsp + 2), S(rsp + 1) Annex A. Basic virtual machine instructions Operator Type Suffixes Operation ADDRF P address of a parameter ADDRG P address of a global ADDRL F address of a local CNST CSIUPFD constant BCOM U bitwise complement CVC IU convert from char CVD IF convert from double CVF D convert from float CVI CSUD convert from int CVP U convert from pointer CVS IU convert from short CVU CSIP convert from unsigned INDIR CSIPFDB fetch NEG IFD negation ADD IUPFD addition BAND U bitwise AND BOR U bitwise inclusive OR BXOR LT bitwise exclusive OR DIV IUFD division LSH IU left shift MOD IU modulus MUL IUFD multiplication RSH IU right shift SUB IUPFD subtraction ASGN CSIPFDB assignment EQ IFD jumpi if equal GE IUFD jump if greater than or equal GT IUFD jumpi if greater than LE IUFD jump if less than or equal LT IUFD jump if less than NE IUFD jump if not equal ARG IPFDB argument CALL IFDBV function call RET IFD remrn from function JUMP V unconditional jump LABEL V definition ADDSP increment stack pointer POP pop stack Suffix B Boolean C Char D Double I Integer F Floating point P Pointer S Short U Unsigned integer V Label Annex B. Sample "C" program int up[15], down[15], rows[8], x[8]; int queensQ, print(); main() { int i; { char *i; printf (" %s" , i); } for (i = 0: i < 15; i + +) up[i] = down[i] = 1 ; for (i = 0; i < 8; i+ +) rowsfi] = 1 queens(O); remrn 0: queens(c) { int r; for (r = 0; r < 8; r+ +) if (rows[r] && up[r-c+7] && down[r + c]) { rows[r] = up[r-c + 7] = down[r+c] = 0; x[c] = r; if (c = = 7) print(); else queens(c + 1); rows[r] = up[r-c+7] = down[r+c] = 1; } printO { int k; for(k = 0; k < 8; k++) printf (" c ", x [k] + '1'); printf("\n"); } Annex C. Sample program expressed in default virmal machine 1 .text 30 addp 59 addrlp(-4) 2 addrgp(_main) 31 addrlp(- 12) 60 indiri 3 jumpv 32 indiri 61 cnsti(l) 4 .globl _main 33 asgni 62 addi 5 _main: 34 L.3 : 63 asgni 6 addSP (-12) 35 addrlp(-4) 64 addrlp(-4) 7 addrlp(-4) 36 addrlp(-4) 65 indiri 8 cnsti(O) 37 indiri 66 cnsti(8) 9 asgni 38 cnsti( l) 67 lti(L.ό) 10 L.2: 39 addi 68 cnsti(O 1 1 addrlp(-8) 40 asgni 69 argi 12 addrlp(-4) 41 addrlp(-4) 70 addrgpf queens) 13 indiri 42 indiri 71 calli 14 cnsti(2) 43 cnsti( 15) 72 addSP(4) 15 lshi 44 lti(L.2) 73 pop 16 asgni 45 addrlp(-4) 74 cnsti(O) 17 addrlp(- 12) 46 cnsti(O) 75 ret 18 cnsti(l) 47 asgni 76 L. l : 19 asgni 48 L.6: 77 cnsti(99) 20 addrlp(-8) 49 addrlp(-4) 78 ret 21 indiri 50 indiri 79 .globl queens 22 addrgp(_down) 51 cnsti(2) 80 _queens: 23 addp 52 lshi 81 addSP (-52) 24 addrlp(-12) 53 addrgp(_rows) 82 addrlp(-4) 25 indiri 54 addp 83 cnsti(O) 26 asgni 55 cnsti(l) 84 asgni 27 addrlp(-S) 56 asgni 85 L.13: 28 indiri 57 L.7: 86 addrlp(-8) 29 addrgp(_up) 58 addrlp(-4) 87 addrlp(-4) 88 indiri 119 addrgp(_up+28) 150 addrlp(-36) 89 asgni 120 addp 151 cnsti(O) 90 addrlp(-12) 121 indiri 152 asgni 91 cnsti(2) 122 addrlp(-lό) 153 addrlp(-24) 92 asgni 123 indiri 154 indiri 93 addrlp(-lό) 124 eqi(L.17) 155 addrlpι-28) 94 cnsti(O) 125 addrlp(-8) 156 indiri 95 asgni 126 indiri 157 addi 96 addrlp(-8) 127 addrlp(-20) 158 addrlp(-32) 97 indiri 128 indiri 159 indiri 98 addrlp(-12) 129 addi 160 lshi 99 indiri 130 addrlp(-12) 161 addrgp(_down) 100 lshi 131 indiri 162 addp 101 addrgp(_rows) 132 lshi 163 addrlp(-36) 102 addp 133 addrgp(_down) 164 indiri 103 indiri 134 addp 165 asgni 104 addrlp(-lό) 135 indiri 166 addrlp(-24) 105 indiri 136 addrlp(-lό) 167 indiri 106 eqi(L.17) 137 indiri 168 addrlp(-28) 107 addrlp(-20) 138 eqi(L.17) 169 indiri 108 addrfp(8) 139 addrlp(-24) 170 subi 109 indiri 140 addrlp(-4) 171 addrlp(-32) 110 asgni 141 indiri 172 indiri 111 addrlp(-8) 142 asgni 173 lshi 112 indiri 143 addrlp(-28) 174 addrgp(_up+28) 113 addrlp(-20) 144 addrfp(8) 175 addp 114 indiri 145 indiri 176 addrlp(-36) 115 subi 146 asgni 177 indiri 116 addrlp(-12) 147 addrlp(-32) 178 asgni 117 indiri 148 cnsti(2) 179 addrlp(-24) 118 lshi 149 asgni 180 indiri 181 addrlp(-32) 212 addi 243 addrlp(-52) 182 indiri 213 argi 244 indiri 183 lshi 214 addrgp(_queens) 245 asgni 184 addrgp(_rows) 215 calli 246 addrlp(-40) 185 addp 216 addSP(4) 247 indiri 186 addrlp(-36) 217 pop 248 addrlp(-44) 187 indiri 218 L.22: 249 indiri 188 asgni 219 addrlp(-40) 250 subi 189 addrfp(8) 220 addrlp(-4) 251 addrlp(-48) 190 indiri 221 indiri 252 indiri 191 cnsti(2) 222 asgni 253 lshi 192 lshi 223 addrlp(-44) 254 addrgp(_up + 28) 193 addrgp(_x) 224 addrfp(8) 255 addp 194 addp 225 indiri 256 addrlp(-52) 195 addrlp(-4) 226 asgni 257 indiri 196 indiri 227 addrlp(-48) 258 asgni 197 asgni 228 cnsti(2) 259 addrlp(-40) 198 addrfp(8) 229 asgni 260 indiri 199 indiri 230 addrlp(-52) 261 addrlp(-48) 200 cnsti(7) 231 cnsti(l) 262 indiri 201 nei(L.2D 232 asgni 263 lshi 202 addrgp(j)rint) 233 addrlp(-40) 264 addrgp(_rows) 203 calli 234 indiri 265 addp 204 addSP(O) 235 addrlp(-44) 266 addrlp(-52) 205 pop 236 indiri 267 indiri 206 addrgp(L.22) 237 addi 268 asgni 207 jumpv 238 addrlp(-48) 269 L.17: 208 L.21 : 239 indiri 270 L.14: 209 addrfp(8) 240 lshi 271 addrlp(-4) 210 indiri 241 addrgp(_down) 272 addrlp(-4) 211 cnsti(l) 242 addp 273 indiri 274 cnsti(l) 305 addSP(8) 336 .space 32 275 addi 306 pop 337 .globl _down 276 asgni 307 L.38: 338 .align 4 277 addrlp(-4) 308 addrlp(-4) 339 _down: 278 indiri 309 addrlp(-4) 340 .space 60 279 cnsti(8) 310 indiri 341 .globl _up 280 lti(L.13) 311 cnsti(l) 342 .align 4 281 L.12: 312 addi 343 _up: 282 cnsti(99) 313 asgni 344 .space 60 283 ret 314 addrlp(-4) 345 .dataEnd 284 .globl jirint 315 indiri 346 .data 285 _print: 316 cnsti(8) 347 .align 1 286 addSP (-4) 317 lti(L.37) 348 L.42: 287 addrlp(-4) 318 addrgp(L.42) 349 .byte 10 288 cnsti(O) 319 argp 350 .byte 0 289 asgni 320 addrgp (_printf) 351 .align 1 290 L.37: 321 calli 352 L.41 : 291 addrlp(-4) 322 addSP(4) 353 .byte 37 292 indiri 323 pop 354 .byte 99 293 cnsti(2) 324 L.36: 355 .byte 32 294 lshi 325 cnsti(99) 356 .byte 0 295 addrgp(_x) 326 ret 357 .dataEnd 296 addp 327 . textEnd 297 indiri 328 .data 298 cnsti(49) 329 .globl _x 299 addi 330 .align 4 300 argi 331 _x: 301 addrgp(L.41) 332 .space 32 302 argp 333 .globl _rows 303 addrgp(jprintf) 334 .align 4 304 calli 335 rows: Annex D Superinstructions 1 instrl l2 : ADDRLP [-4] 2 instrl23I INDIRI(ADDRLP [#]) 3 instr298 : EQI [L.17] (INDIRI (ADDP(*, *)), INDIRI(ADDRLP[-16])) 4 instrl l3 : CNSTI [0] 5 instrl33 : CNSTI [1] 6 instr315 : INDIRI (ADDRFP [8]) 7 instrl24 : CNSTI [2] 8 instr249U : ASGNI (ADDP(LSHI(*, *), ADDRGP[#]), *) 9 instr 121 : ADDRLP [-8] 10 instrl32 : ADDRLP [-12] 11 instr259U : LTI [#](INDIRI(ADDRLP [-4]) ,CNSTI[8]) 12 instr 180 : ASGNI(ADDRLP[-4]ADDI(INDIRI(ADDRLP [-4]), *)) 13 instr 160 : ASGNI(ADDP(*, ADDRGP[_down]), INDIRI(*)) 14 instr355 : ADDRLP [-36] 15 instr471 : ADDRLP [-52] 16 instr243 : ASGNI(ADDP(LSHI(INDIRI(*), *), ADDRGP[_rows]), *) 17 instr352 : ADDRLP [-24] 18 instr468 : ADDRLP [-40] 19 instr354 : ADDRLP [-32] 0 instr470 : ADDRLP [-48] 1 instr514 : ADDI (INDIRI(ADDP(*. ADDRGP[_x])), CNSTI[49]) 2 instr211 : LTI [L.2](INDIRI(ADDRLP[-4]), CNSTI[15]) 3 instr313 : ADDRLP [-20] 4 instr353 : ADDRLP [-28] 5 instr461 : NEI [L.21](INDIRI(ADDRFP[8]), CNSTI[7]) 6 instr469 : ADDRLP [-44] 7 instr 114 : ASGNI (ADDRLP[-4], CNSTI [0]) 8 instr267 : ADDRGP [ queens] 9 instr509 : ADDRGP [_printf] 0 instr 122 : INDIRI ( ADDRLP [-4] ) 1 instr336 : ADDRGP up+28] 2 instr 146 : INDIRI ( ADDRLP [-8] ) instr 147 : ADDRGP [_down] instr206 : ADDRGP [_up] instr219 : ADDRGP [_rows] instr276 : ADDRLP [-16] instr473 : ADDRGP [_print] instr474 : ADDRGP [L.22] instr515 : ADDRGP [L.41] instr523 : ADDRGP [L.42] Annex E. Sample program expressed m program-specific virtual machine 1 .text 31 L.6: 61 asgni 2 addrgp(_main) 32 instr 112 62 mstr276 3 jumpv 33 mstrl 24 63 instr 1 13 4 .globl _main 34 instr 133 64 asgni 5 main: 35 ιnstr243 65 instr 146 6 addSP (-12) 36 L.7: 66 ιnstrl32 7 mstrl 14 37 instr 133 67 indiri 8 L.2: 38 instr 180 68 lshi 9 instr 121 39 ιnstr259U(L.6) 69 ιnstr219 10 mstrl 22 40 instr 1 13 70 mstr298 11 mstrl 24 41 argi 71 ιnstr313 12 lshi 42 ιnstr267 72 ιnstr315 13 asgni 43 calli 73 asgni 14 instr 132 44 addSP(4) 74 ιnstrl46 15 instr 133 45 pop 75 ιnstr313 16 asgni 46 mstrl 13 76 indiri 17 instr 146 47 ret 77 subi 18 mstrl 32 48 L. l : 78 ιnstrl32 19 instr 160 49 cnstι(99) 79 indiri 20 ιnstrl46 50 ret 80 lshi 21 instr206 51 .globl _queens 81 ιnstr336 22 addp 52 _queens: 82 ιnstr298 23 instr 132 53 addSP (-52) 83 instr 146 24 indiri 54 instr 114 84 ιnstr313 25 asgni 55 L.13 : 85 indiri 26 L.3: 56 instr 121 86 addi 27 instr 133 57 instr 122 87 instr 132 28 instr 180 58 asgni 88 indiri 29 instr211 59 mstrl32 89 lshi 30 instr 114 60 instr 124 90 instr 147 91 instr298 123 addp 155 instr 122 92 instr352 124 instr355 156 asgni 93 instr 122 125 indiri 157 instr469 94 asgni 126 asgni 158 instr315 95 instr353 127 instr352 159 asgni 96 instr315 128 instr354 160 instr470 97 asgni 129 indiri 161 instr 124 98 instr354 130 instr355 162 asgni 99 instr 124 131 indiri 163 instr471 100 asgni 132 instr243 164 instr 133 101 instr355 133 instr315 165 asgni 102 instr 113 134 instrl24 166 instr468 103 asgni 135 instr 122 167 indiri 104 instr352 136 instr249U(_x) 168 instr469 105 indiri 137 instr461 169 indiri 106 instr353 138 instr473 170 addi 107 indiri 139 calli 171 instr470 108 addi 140 addSP(O) 172 indiri 109 instr354 141 pop 173 lshi 110 indiri 142 instr474 174 instr471 1 11 lshi 143 jumpv 175 instr 160 112 instr355 144 L.21 : 176 instr468 113 instr 160 145 instr315 177 indiri 114 instr352 146 instrl33 178 instr469 115 indiri 147 addi 179 indiri 116 instr353 148 argi 180 subi 117 indiri 149 instr267 181 instr470 118 subi 150 calli 182 indiri 119 instr354 151 addSP(4) 183 lshi 120 indiri 152 pop 184 instr336 121 lshi 153 L.22: 185 addp 122 instr336 154 instr468 186 instr471 187 indiri 213 instr515 239 .align 4 188 asgni 214 argp 240 _rows: 189 instr468 215 instr509 241 .space 32 190 instr470 216 calli 242 .globl _down 191 indiri 217 addSP(8) 243 .align 4 192 instr471 218 pop 244 _down: 193 indiri 219 L.38: 245 .space 60 194 instr243 220 instrl33 246 .globl _up 195 L.17: 221 instr 180 247 .align 4 196 L.14: 222 instr259U(L.37) 248 _up: 197 instr 133 223 addrgp(L.42) 249 .space 60 198 instr 180 224 argp 250 .dataEnd 199 instr259U(L.13) 225 instr509 251 .data 200 L. 12: 226 calli 252 .align 1 201 cnsti(99) 227 addSP(4) 253 L.42: 202 ret 228 pop 254 .byte 10 203 .globl jirint 229 L.36: 255 .byte 0 204 _print: 230 cnsti(99) 256 .align 1 205 addSP (-4) 231 ret 257 L.41 : 206 instr 114 232 .textEnd 258 .byte 37 207 L.37: 233 .data 259 .byte 99 208 instr 122 234 .globl _x 260 .byte 32 209 instr 124 235 .align 4 261 .byte 0 210 lshi 236 _x: 262 .dataEnd 211 instr514 237 .space 32 212 argi 238 .globl _rows

Claims: CLAIMS: 1. A method for executing a source program on a processing unit comprising a predetermined microcontroller core for executing native instructions from a predetermined set of micro-controller specific instructions; the method comprising: a pre-processing step of expressing program statements of the source program in a sequence of instructions comprising virtual machine instructions; and storing the sequence of instructions in an instruction memory; and an execution step of: fetching instructions from the instruction memory; using conversion means of the processing unit to convert virtual instructions fetched from the instruction memory into native instructions; and feeding the native instructions to the microcontroller core for execution; characterised in that the pre-processing step comprises the steps of: defining for the program statements of the source program a program-specific virtual machine with a corresponding set of virtual machine instructions, such that the expression of the program statements in the sequence of instructions requires less storage space in the instruction memory compared to using only native instructions for expressing the program statements; defining for the program-specific virtual machine an associated conversion means for converting virmal machine instructions of the program-specific virtual machine into native instructions of the microcontroller core; and representing the associated conversion means in the processing unit. 2. A method for executing a source program on a processing unit as claimed in claim 1, characterised: in that the pre-processing step comprises the steps of: - identifying a plurality of groups of program statements in the source program: for each of the groups of program statements: defining a program group specific virmal machine with a corresponding set of virtual machine instructions; generating for the program group specific virtual machine an associated conversion means for converting virmal machine instructions into native instructions of the microcontroller core; representing the associated conversion means in the processing unit; and storing selection data in the processing unit associating each generated virtual machine instruction with the group-specific conversion means; and in that the execution step comprises the step of selecting for a fetched instruction the associated conversion means indicated by the selection data. 3. A method as claimed in claim 1, characterised in that the method comprises: receiving an instruction module comprising instructions including virmal machine instructions of a further virtual machine, and receiving conversion data enabling a further conversion means of the processing unit to convert the further virtual machine instructions into native instructions; storing the instruction module in the instruction memory; storing the conversion data in the processing unit; storing selection data in the processing unit associating each further virtual machine instruction with the conversion data; and in that the execution step comprises the steps of: selecting for a fetched further virmal machine instruction the associated conversion data indicated by the selection data; and operating the further conversion means under control of the selected conversion data. 4. A processing unit for executing instructions of a virtual machine, the instructions being referred to as virtual machine instructions: the processing unit comprising: a predetermined microcontroller core for executing native instructions from a predetermined set of micro-controller specific instructions; the native instructions being different from the virtual machine instructions; an instruction memory for storing instructions including at least one of the virtual machine instructions; a converter comprising conversion means for converting a virmal machine instruction fetched from the instruction memory into at least one native instruction for execution by the microcontroller core; characterised in that the converter is operative to perform the conversion for a plurality of different virtual machines. 5. A processing unit as claimed in claim 4, wherein the conversion means is of a re-programmable type. 6. A processing unit as claimed in claim 4, wherein the converter comprises conversion means for each one of the plurality of the virtual machines for converting virtual machine instructions of the corresponding virtual machine. 7. A processing unit as claimed in claim 6, wherein the processing unit comprises selection data associating at least two disjunct groups of locations in the instruction memory with respective conversion means, and wherein the converter comprises a detector for selectively directing an instruction fetched from a location in the instruction memory to conversion means based on the selection data. 8. A processing unit as claimed in claim 4, wherein the processing unit is operative to fetch for an instruction associated selection data from a memory for differentiating between the native and the virmal machine and/or between different virtual machines; and wherein the converter comprises a detector for selectively directing the fetched instruction to conversion means in dependence on the selection data. 9. A processing unit as claimed in claim 4, wherein the converter is operative to convert one virtual machine instruction into one corresponding native instruction, where the virmal machine instruction is coded more compactly with respect to requiring storage space in the instruction memory than the corresponding native instruction. 10. A processing unit as claimed in claim 4, wherein the converter is operative to convert one virtual machine instruction into a predetermined sequence of a plurality of native instructions; and the processing unit comprises: a sequencer coupled in between the converter and the microcontroller core for sequentially feeding the sequence of native instructions to the microcontroller core; and inhibiting means for inhibiting fetching an instruction from the instruction memory during the feeding. 11. A processing unit as claimed in claim 10, wherein the inhibiting means is operative to perform the inhibiting by obstructing an increment of an instruction pointer of the microcontroller core. 12. A processing unit as claimed in claim 11, wherein processing unit comprises an instruction fetcher for fetching an instruction from the instruction memory in response to a change of an instruction counter of the instruction fetcher, where the instruction counter is set to a different value in response to a change of an instruction pointer of the microcontroller core, and the inhibiting means is operative to perform the inhibiting by obstructing a change of value of the instruction counter.

Inventors: AUGUSTEIJN ALEXANDER, DIJKSTRA EELCO J, GORISSEN PAULUS M H M A, MEULENBROEKS FRANCISCUS J H M, STRAVERS PAUL, TRESCHER JOACHIM A, AUGUSTEIJN, ALEXANDER, DIJKSTRA, EELCO, J., GORISSEN, PAULUS, M., H., M., A., MEULENBROEKS, FRANCISCUS, J., H., M., STRAVERS, PAUL, TRESCHER, JOACHIM, A.
Applicants: KONINKL PHILIPS ELECTRONICS NV, PHILIPS SVENSKA AB, KONINKLIJKE PHILIPS ELECTRONICS N.V., PHILIPS AB
Application year: 1998
Main classification: G06F
IPC codes: G06F9