INSTRUCTION FOLDING FOR A STACK-BASED MACHINE

Abstract: An instruction decoder (135, 1118) allows the folding away of JAVA virtual machine instructions pushing an operand onto the top of a stack (e.g., 423, 155, 812) merely as a precursor to a second JAVA virtual machine instruction which operates on the top of stack operand. Such an instruction decoder identifies foldable instruction sequences and supplies an execution unit with a single equivalent folded operation thereby reducing processing cycles otherwise required for execution of multiple operations corresponding to the multiple instructions of the folded instruction sequence. Instruction decoder embodiments described herein provide for folding of two, three, four, or more instruction folding. For example, in one instruction decoder embodiment described herein, two load instructions and a store instruction can be folded into execution of operation corresponding to an instruction appearing therebetween in the instruction sequence.

Description: INSTRUCTION FOLDING FOR A STACK-BASED MACHINE REFERENCE TO APPENDIX I A portion of the disclosure of this patent document including Appendix I, The JAVA Virtual Machine Specification and Appendix A thereto, contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the U.S. Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever. TECHNICAL FIELD The present invention relates to instruction decoders for a stack machine, and in particular, to methods and apparati for folding a sequence of multiple instructions into a single folded operation. BACKGROUND ART Many individuals and organizations in the computer and communications industries tout the Internet as the fastest growing market on the planet. In the 1990s, the number of users of the Internet appears to be growing exponentially with no end in sight. In June of 1995, an estimated 6,642,000 hosts were connected to the Internet; this represented an increase from an estimated 4,852,000 hosts in January, 1995. The number of hosts appears to be growing at around 75% per year. Among the hosts, there were approximately 120,000 networks and over 27,000 web servers. The number of web servers appears to be approximately doubling every 53 days. In July 1995, with over 1,000,000 active Internet users, over 12,505 usenet news groups, and over 10,000,000 usenet readers, the Internet appears to be destined to explode into a very large market for a wide variety of information and multimedia services. In addition, to the public carrier network or Internet, many corporations and other businesses are shifting their internal information systems onto an intranet as a way of more effectively sharing information within a corporate or private network. The basic infrastructure for an intranet is an internal network connecting servers and desktops, which may or may not be connected to the Internet through a firewall. These intranets provide services to desktops via standard open network protocols which are well established in the industry. Intranets provide many benefits to the enterprises which employ them, such as simplified internal information management and improved internal communication using the browser paradigm. Integrating Internet technologies with a company's enterprise infrastructure and legacy systems also leverages existing technology investment for the party employing an intranet. As discussed above, intranets and the Internet are closely related, with intranets being used for internal and secure communications within the business and the Internet being used for external transactions between the business and the outside world. For the purposes of this document, the term "networks" includes both the Internet and intranets. However, the distinction between the Internet and an intranet should be born in mind where applicable. In 1990, programmers at Sun Microsystems wrote a universal programming language. This language was eventually named the JAVA programming language. (JAVA is a trademark of Sun Microsystems of Mountain View, CA.) The JAVA programming language resulted from programming efforts which initially were intended to be coded in the C++ programming language; therefore, the JAVA programming language has many commonalities with the C++ programming language. However, the JAVA programming language is a simple, object-oriented, distributed, interpreted yet high performance, robust yet safe, secure, dynamic, architecture neutral, portable, and multi-threaded language. The JAVA programming language has emerged as the programming language of choice for the Internet as many large hardware and software companies have licensed it from Sun Microsystems. The JAVA programming language and environment is designed to solve a number of problems in modern programming practice. The JAVA programming language omits many rarely used, poorly understood, and confusing features of the C++ programming language. These omitted features primarily consist of operator overloading, multiple inheritance, and extensive automatic coercions. The JAVA programming language includes automatic garbage collection that simplifies the task of programming because it is no longer necessary to allocated and free memory as in the C programming language. The JAVA programming language restricts the use of pointers as defined in the C programming language, and instead has true arrays in which array bounds are explicitly checked, thereby eliminating vulnerability to many viruses and nasty bugs. The JAVA programming language includes objective-C interfaces and specific exception handlers. The JAVA programming language has an extensive library of routines for coping easily with TCP/IP protocol (Transmission Control Protocol based on Internet protocol), HTTP (Hypertext Transfer Protocol) and FTP (File Transfer Protocol). The JAVA programming language is intended to be used in networked/distributed environments. The JAVA programming language enabled the construction of virus-free, tamper-free systems. The authentication techniques are based on public-key encryption. DISCLOSURE OF INVENTION A JAVA virtual machine is an stack-oriented abstract computing machine, which like a physical computing machine has an instruction set and uses various storage areas. A JAVA virtual machine need not understand the JAVA programming language; instead it understands a class file format. A class file includes JAVA virtual machine instructions (or bytecodes) and a symbol table, as well as other ancillary information. Programs written in the JAVA programming language (or in other languages) may be compiled to produce a sequence of JAVA virtual machine instructions. Typically, in a stack-oriented machine, instructions typically operate on data at the top of an operand stack. One or more first instructions, such as a load from local variable instruction, are executed to push operand data onto the operand stack as a precursor to execution of an instruction which immediately follows such instruction(s). The instruction which follows, e.g., an add operation, pops operand data from the top of the stack, operates on the operand data, and pushes a result onto the operand stack, replacing the operand data at the top of the operand stack. A suitably configured instruction decoder allows the folding away of instructions pushing an operand onto the top of a stack merely as a precursor to a second instruction which operates on the top of stack operand. The instruction decoder identifies foldable instruction sequences (typically 2, 3, or 4 instructions) and supplies an execution unit with an equivalent folded operation (typically a single operation) thereby reducing processing cycles otherwise required for execution of multiple operations corresponding to the multiple instructions of the folded instruction sequence. Using an instruction decoder in accordance with the present invention, multiple load instructions and a store instruction can be folded into execution of an instruction appearing therebetween in the instruction sequence. For example, an instruction sequence including a pair of load instructions (for loading integer operands from local variables to the top of stack), an add instruction (for popping the integer operands of the stack, adding them, and placing the result at the top of stack), and an store instruction (for popping the result from the stack and storing the result in a local variable) can be folded into a single equivalent operation specifying source and destination addresses in stack and local variable storage which are randomly accessible. In accordance with an embodiment of the present invention, an apparatus includes an instruction store, an operand stack, a data store, an execution unit, and an instruction decoder. The instruction decoder is coupled to the instruction store to identify a foldable sequence of instructions represented therein. The foldable sequence includes first and second instructions, in which the first instruction is for pushing a first operand value onto the operand stack from the data store merely as a first source operand for a second instruction. The instruction decoder coupled to supply the execution unit with a single folded operation equivalent to the foldable sequence and including a first operand address identifier selective for the first operand value in the data store, thereby obviating an explicit operation corresponding to the first instruction. In a further embodiment, if the sequence of instructions represented in the instruction buffer is not a foldable sequence, the instruction decoder supplies the execution unit with an operation identifier and operand address identifier corresponding to the first instruction only. In another further embodiment, the instruction decoder further identifies a third instruction in the foldable sequence. This third instruction is for pushing a second operand value onto the operand stack from the data store merely as a second source operand for the second instruction. The single folded operation is equivalent to the foldable sequence and includes a second operand address identifier selective for the second operand value in the data store, thereby obviating an explicit operation corresponding to the third instruction. In yet another further embodiment, the instruction decoder further identifies a fourth instruction in the foldable sequence. This fourth instruction is for popping a result of the second instruction from the operand stack and storing the result in a result location of the data store. The single folded operation is equivalent to the foldable sequence and includes a destination address identifier selective for the result location in the data store, thereby obviating an explicit operation corresponding to the fourth instruction. In still yet another further embodiment, the instruction decoder includes normal and folded decode paths and switching means. The switching means are responsive to the folded decode path for selecting operation, operand, and destination identifiers from the folded decode path in response to a fold indication therefrom, and for otherwise selecting operation, operand, and destination identifiers from the normal decode path. In various further alternative embodiments, the apparatus is for a virtual machine instruction processor wherein instructions generally source operands from, and target a result to, uppermost entries of an operand stack. In one such alternative embodiment, the virtual machine instruction processor is a hardware virtual machine instruction processor and the instruction decoder includes decode logic. In another, the virtual machine instruction processor includes a just-in-time compiler implementation and the instruction decoder includes software executable on a hardware processor. The hardware processor includes the execution unit. In yet another, the virtual machine instruction processor includes a bytecode interpreter implementation and the instruction decoder including software executable on a hardware processor. The hardware processor includes the execution unit. In accordance with another embodiment of the present invention, a method includes (a) determining if a first instruction of a virtual machine instruction sequence is an instruction for pushing a first operand value onto the operand stack from a data store merely as a first source operand for a second instruction; and if the result of the (a) determining is affirmative, supplying an execution unit with a single folded operation equivalent to a foldable sequence comprising the first and second instructions. The single folded operation includes a first operand identifier selective for the first operand value, thereby obviating an explicit operation corresponding to the first instruction. In a further embodiment, the method includes supplying, if the result of the (a) determining is negative, the execution unit with an operation equivalent to the first instruction in the virtual machine instruction sequence. In another further embodiment, the method includes (b) determining if a third instruction of the virtual machine instruction sequence is an instruction for popping a result value of the second instruction from the operand stack and storing the result value in a result location of the data store and, if the result of the (b) determining is affirmative, further including a result identifier selective for the result location with the equivalent single folded operation, thereby further obviating an explicit operation corresponding to the third instruction. In a further embodiment, the method includes including, if the result of the (b) determining is negative, a result identifier selective for a top location of the operand stack with the equivalent single folded operation. In certain embodiments, the (a) determining and the (b) determining are performed substantially in parallel. In accordance with yet another embodiment of the present invention, a stack-based virtual machine implementation includes a randomly-accessible operand stack representation, a randomly-accessible local variable storage representation, and a virtual machine instruction decoder for selectively decoding virtual machine instructions and folding together a selected sequence thereof to eliminate unnecessary temporary storage of operands on the operand stack. In various alternative embodiments, the stack-based virtual machine implementation (1) is a hardware virtual machine instruction processor including a hardware stack cache, a hardware instruction decoder, and an execution unit or (2) includes software encoded in a computer readable medium and executable on a hardware processor. In the hardware virtual machine instruction processor embodiment, (a) the randomly-accessible operand stack local variable storage representations at least partially reside in the hardware stack cache, and (b) the virtual machine instruction decoder includes the hardware instruction decoder coupled to provide the execution unit with opcode, operand, and result identifiers respectively selective for a hardware virtual machine instruction processor operation and for locations in the hardware stack cache as a single hardware virtual machine instruction processor operation equivalent to the selected sequence of virtual machine instructions. In the software embodiment, (a) the randomly-accessible operand stack local variable storage representations at least partially reside in registers of the hardware processor, (b) the virtual machine instruction decoder is at least partially implemented in the software, and (c) the virtual machine instruction decoder is coupled to provide opcode, operand, and result identifiers respectively selective for a hardware processor operation and for locations in the registers as a single hardware processor operation equivalent to the selected sequence of virtual machine instructions. In accordance with still yet another embodiment of the present invention, a hardware virtual machine instruction decoder includes a normal decode path, a fold decode path, and switching means. The fold decode path is for decoding a sequence of virtual machine instructions and, if the sequence is foldable, supplying (a) a single operation identifier, (b) one or more operand identifiers; and (c) a destination identifier, which are together equivalent to the sequence of virtual machine instructions. The switching means is responsive to the folded decode path for selecting operation, operand, and destination identifiers from the folded decode path in response to a fold indication therefrom, and otherwise selecting operation, operand, and destination identifiers from the normal decode path. BRIEF DESCRIPTION OF DRAWINGS The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings. Figure 1 is a block diagram of one embodiment of virtual machine hardware processor that includes an instruction decoder for providing instruction folding in accordance with this invention. Figure 2 is an process flow diagram for generation of virtual machine instructions that are used in one embodiment of this invention. Figure 3 illustrates an instruction pipeline implemented in the hardware processor of Figure 1. Figure 4A is an illustration of the one embodiment of the logical organization of a stack structure where each method frame includes a local variable storage area, an environment storage area, and an operand stack utilized by the hardware processor of Figure I. Figure 4B is an illustration of an alternative embodiment of the logical organization of a stack structure where each method frame includes a local variable storage area and an operand stack on the stack, and an environment storage area for the method frame is included on a separate execution environment stack. Figure 4C is an illustration of an alternative embodiment of the stack management unitfor the stack and execution environment stack of Figure 4B. Figure 4D is an illustration of one embodiment of the local variables look-aside cache in the stack management unit of Figure 1. Figure 5 illustrates several possible add-ons to the hardware processor of Figure 1. Figure 6 is an illustration, in the context of a stack data structure, of data flows associated with a pair of stack instructions, wherein the first stack instruction pushes a data item onto the top of the stack only to be consumed by the second stack instruction which pops the top two stack entries off the stack and pushes their sum onto the top of the stack. Figure 7 is a contrasting illustration of folded execution of first and second stack instructions such as those depicted in Figure 6, wherein the first (push data item onto the top of the stack) operation is obviated in accordance with an exemplary embodiment of the present invention. Figure 8 is a block diagram depicting relationships between operand stack, local variable storage, and constant pool portions of memory storage together with register variables for access thereof in accordance with an exemplary embodiment of the present invention. Figures 9A-D illustrate an iload (integer load)/ iadd (integer add)/ istore (integer store) instruction sequence operating on an operand stack and local variable storage. Figures 9A, 9B, 9C, and 9D, respectively depict operand stack contents before iload instructions, after iload instructions but before an iadd instruction, after the iadd instruction but before an istore instruction, and after the istore instruction. Intermediate stages depicted in Figures 9B and 9C are eliminated by instruction folding in accordance with an exemplary embodiment of the present invention. Figures 10A, 10B and IOC illustrate an aload (object reference load)/ arraylength (integer add) instruction sequence operating on the operand stack and local variable storage. Figures 9A, 9B, and 9C, respectively depict operand stack contents before the load instruction, after the aload instruction but before an arraylength instruction (without instruction folding), and after the arraylength instruction. The intermediate stage depicted in Figure 10B is eliminated by instruction folding in accordance with an exemplary embodiment of the present invention. Figure 1 1 is a functional block diagram of a stack based processor including an instruction decoder providing instruction folding in accordance with an exemplary embodiment of the present invention. Figure 12 is a functional block diagram depicting an instruction decoder in accordance with an exemplary embodiment of the present invention and coupled to supply an execution unit with a folded operation, with operand addresses into an operand stack, local variable storage or a constant pool, and with a destination address into the operand stack or local variable storage, wherein the single operation and addresses supplied are equivalent to a sequence of unfolded instructions. Figure 13 is a functional block diagram of an instruction decoder supporting instruction folding in accordance with an exemplary embodiment of the present invention. Figure 14 is a functional block diagram of a fold decode portion of an instruction decoder supporting instruction folding in accordance with an exemplary embodiment of the present invention. Figure 15 is a flow chart depicting an exemplary sequence of operations for identifying a foldable instruction sequence in accordance with an exemplary embodiment of the present invention. Figure 16 is a functional block diagram depicting component operand and destination address generators of a fold address generator in accordance with an exemplary embodiment of the present invention. Figure 17 is a functional block diagram depicting an exemplary structure for an operand address generator in accordance with an exemplary embodiment of the present invention. Like or similar features may be designated by the same reference numeral(s) throughout the drawings. MODESfS) FOR CARRYING OUT THE INVENTION Figure 1 illustrates one embodiment of a virtual machine instruction hardware processor 100, hereinafter hardware processor 100, that includes an instruction decoder 135 for folding a sequence of ultiple instructions into a single folded operation in accordance with the present invention, and that directly executes virtual machine instructions that are processor architecture independent. The performance of hardware processor 100 in executing virtual machine instructions is much better than high-end CPUs, such as the Intel PENTIUM microprocessor or the Sun Microsystems ULTRASPARC processor, (ULTRASPARC is a trademark of Sun Microsystems of Mountain View, CA., and PENTIUM is a trademark of Intel Corp. of Sunnyvale, CA.) interpreting the same virtual machine instructions with a software JAVA interpreter, or with a JAVA just-in-time compiler; is low cost; and exhibits low power consumption. As a result, hardware processor 100 is well suited for portable applications. Hardware processor 100 provides similar advantages for other virtual machine stack-based architectures as well as for virtual machines utilizing features such as garbage collection, thread synchronization, etc. In view of these characteristics, a system based on hardware processor 100 presents attractive price for performance characteristics, if not the best overall performance, as compared with alternative virtual machine execution environments including software interpreters and just-in-time compilers. Nonetheless, the present invention is not limited to virtual machine hardware processor embodiments, and encompasses any suitable stack-based, or non-stack-based, machine implementations, including implementations emulating the JAVA virtual machine as a software interpreter, compiling JAVA virtual machine instructions (either in batch or just-in-time) to machine instruction native to a particular hardware processor, or providing hardware implementing the JAVA virtual machine in microcode, directly in silicon, or in some combination thereof. Regarding price for performance characteristics, hardware processor 100 has the advantage that the 250 Kilobytes to 500 Kilobytes (Kbytes) of memory storage, e.g., read-only memory or random access memory, typically required by a software interpreter, is eliminated. A simulation of hardware processor 100 showed that hardware processor 100 executes virtual machine instructions twenty times faster than a software interpreter running on a variety of applications on a PENTIUM processor clocked at the same clock rate as hardware processor 100, and executing the same virtual machine instructions. Another simulation of hardware processor 100 showed that hardware processor 100 executes virtual machine instructions five times faster than a just-in-time compiler running on a PENTIUM processor running at the same clock rate as hardware processor 100, and executing the same virtual machine instructions. In environments in which the expense of the memory required for a software virtual machine instruction interpreter is prohibitive, hardware processor 100 is advantageous. These applications include, for example, an Internet chip for network appliances, a cellular telephone processor, other telecommunications integrated circuits, or other low-power, low-cost applications such as embedded processors, and portable devices. Instruction decoder 135, as described herein, allows the folding away of JAVA virtual machine instructions pushing an operand onto the top of a stack merely as a precursor to a second JAVA virtual machine instruction which operates on the top of stack operand. Such an instruction decoder identifies foldable instruction sequences and supplies an execution unit with a single equivalent folded operation thereby reducing processing cycles otherwise required for execution of multiple operations corresponding to the multiple instructions of the folded instruction sequence. Instruction decoder embodiments described herein provide for folding of two, three, four, or more instruction folding For example, in one instruction decoder embodiment described herein, two load instructions and a store instruction can be folded into execution of operation corresponding to an instruction appearing therebetween in the instruction sequence As used in herein, a virtual machine is an abstract computing machine that, like a real computing machine, has an instruction set and uses various memory areas A virtual machine specification defines a set of processor architecture independent virtual machine instructions that are executed by a virtual machine implementation, e.g., hardware processor 100 Each virtual machine instruction defines a specific operation that is to be performed The virtual computing machine need not understand the computer language that is used to generate virtual machine instructions or the underlying implementation of the virtual machine Only a particular file format for virtual machine instructions needs to be understood. In an exemplary embodiment, the virtual machine instructions are JAVA virtual machine instructions Each JAVA virtual machine instruction includes one or more bytes that encode instruction identfying information, operands, and any other required information Appendix I, which is incoφorated herein by reference in its entirety, includes an illustrative set of the JAVA virtual machine instructions. The particular set of virtual machine instructions utilized is not an essential aspect of this invention. In view of the virtual machine instructions in Appendix I and this disclosure, those of skill in the art can modify the invention for a particular set of virtual machine instructions, or for changes to the JAVA virtual machine specification . A JAVA compiler JAVAC, (Fig. 2) that is executing on a computer platform, converts an application 201 written in the JAVA computer language to an architecture neutral object file format encoding a compiled instruction sequence 203, according to the JAVA Virtual Machine Specification, that includes a compiled instruction set However, for this invention, only a source of virtual machine instructions and related information is needed. The method or technique used to generate the source of virtual machine instructions and related information is not essential to this mvention. Compiled instruction sequence 203 is executable on hardware processor 100 as well as on any computer platform that implements the JAVA virtual machine using, for example, a software interpreter or just-in-time compiler. However, as described above, hardware processor 100 provides significant performance advantages over the software implementations. In this embodiment, hardware processor 100 (Fig. 1) processes the JAVA virtual machine instructions, which include bytecodes. Hardware processor 100, as explained more completely below, executes directly most of the bytecodes However, execution of some of the bytecodes is implemented via microcode One strategy for selecting virtual machine instructions that are executed directly by hardware processor 100 is described herein by way of an example. Thirty percent of the JAVA virtual machine instructions are pure hardware translations; instructions implemented in this manner include constant loading and simple stack operations. The next 50% of the virtual machine instructions are implemented mostly, but not entirely, in hardware and require some firmware assistance; these include stack based operations and array instructions. The next 10% of the JAVA virtual machine instructions are implemented in hardware, but require significant firmware support as well; these include function invocation and function return. The remaining 10% of the JAVA virtual machine instructions are not supported in hardware, but rather are supported by a firmware trap and/or microcode; these include functions such as exception handlers. Herein, firmware means microcode stored in ROM that when executed controls the operations of hardware processor 100. In one embodiment, hardware processor 100 includes an I/O bus and memory interface unit 1 10, an instruction cache unit 120 including instruction cache 125, an instruction decode unit 130, a unified execution unit 140, a stack management unit 150 including stack cache 155, a data cache unit 160 including a data cache 165, and program counter and trap control logic 170. Each of these units is described more completely below. Also, as illustrated in Figure 1, each unit includes several elements. For clarity and to avoid distracting from the invention, the interconnections between elements within a unit are not shown in Figure 1. However, in view of the following description, those of skill in the art will understand the interconnections and cooperation between the elements in a unit and between the various units. The pipeline stages implemented using the units illustrated in Figure 1 include fetch, decode, execute, and write-back stages. If desired, extra stages for memory access or exception resolution are provided in hardware processor 100. Figure 3 is an illustration of a four stage pipeline for execution of instructions in the exemplary embodiment of processor 100. In fetch stage 301, a virtual machine instruction is fetched and placed in instruction buffer 124 (Fig. 1). The virtual machine instruction is fetched from one of (i) a fixed size cache line from instruction cache 125 or (ii) microcode ROM 141 in execution unit 140. With regard to fetching, aside from instructions tableswitch and lookupswitch, (See Appendix I.) each virtual machine instruction is between one and five bytes long. Thus, to keep things simple, at least forty bits are required to guarantee that all of a given instruction is contained in the fetch. Another alternative is to always fetch a predetermined number of bytes, for example, four bytes, starting with the opcode. This is sufficient for 95% of JAVA virtual machine instructions (See Appendix I). For an instruction requiring more than three bytes of operands, another cycle in the front end must be tolerated if four bytes are fetched. In this case, the instruction execution can be started with the first operands fetched even if the full set of operands is not yet available. In decode stage 302 (Fig. 3), the virtual machine instruction at the front of instruction buffer 124 (Fig. 1 ) is decoded and instruction folding is performed if possible. Stack cache 155 is accessed only if needed by the virtual machine instruction. Register OPTOP, that contains a pointer OPTOP to a top of a stack 400 (Fig. 4), is also updated in decode stage 302 (Fig. 3). Herein, for convenience, the value in a register and the register are assigned the same reference numeral. Further, in the following discussion, use of a register to store a pointer is illustrative only of one embodiment. Depending on the specific implementation of the invention, the pointer may be implemented using hardware register, a hardware counter, a software counter, a software pointer, or other equivalent embodiments known to those of skill in the art. The particular implementation selected is not essential to the invention, and typically is made based on a price to performance trade-off. In execute stage 303, the virtual machine instruction is executed for one or more cycles. Typically, in execute stage 303, an ALU in integer unit 142 (Fig. 1) is used either to do an arithmetic computation or to calculate the address of a load or store from data cache unit (DCU) 160. If necessary, traps are prioritized and taken at the end of execute stage 303 (Fig. 3). For control flow instructions, the branch address is calculated in execute stage 303, as well as the condition upon which the branch is dependent. Cache stage 304 is a non-pipelined stage. Data cache 165 (Fig. 1) is accessed if needed during execution stage 303 (Fig. 3). The reason that stage 304 is non-pipelined is because hardware processor 100 is a stack-based machine. Thus, the instruction following a load is almost always dependent on the value returned by the load. Consequently, in this embodiment, the pipeline is held for one cycle for a data cache access. This reduces the pipeline stages, and the die area taken by the pipeline for the extra registers and bypasses. Write-back stage 305 is the last stage in the pipeline. In stage 305, the calculated data is written back to stack cache 155. Hardware processor 100, in this embodiment, directly implements a stack 400 (Fig. 4A) that supports the JAVA virtual machine stack-based architecture (See Appendix I). Sixty-four entries on stack 400 are contained on stack cache 155 in stack management unit 150. Some entries in stack 400 may be duplicated on stack cache 150. Operations on data are performed through stack cache 155. Stack 400 of hardware processor 100 is primarily used as a repository of information for methods. At any point in time, hardware processor 100 is executing a single method. Each method has memory space, i.e., a method frame on stack 400, allocated for a set of local variables, an operand stack, and an execution environment structure. A new method frame, e.g., method frame two 410, is allocated by hardware processor 100 upon a method invocation in execution stage 303 (Fig. 3) and becomes the current frame, i.e., the frame of the current method. Current frame 410 (Fig. 4A), as well as the other method frames, may contain a part of or all of the following six entities, depending on various method invoking situations: 1. Object reference; 2. Incoming arguments; 3. Local variables; 4. Invoker's method context; 5. Operand stack; and 6. Return value from method. In Figure 4A, object reference, incoming arguments, and local variables are included in arguments and local variables area 421. The invoker's method context is included in execution environment 422, sometimes called frame state, that in turn includes: a return program counter value 431 that is the address of the virtual machine instruction, e.g., JAVA opcode, next to the method invoke instruction; a return frame 432 that is the location of the calling method's frame; a return constant pool pointer 433 that is a pointer to the calling method's constant pool table; a current method vector 434 that is the base address of the current method's vector table; and a current monitor address 435 that is the address of the current method's monitor. The object reference is an indirect pointer to an object-storage representing the object being targeted for the method invocation. JAVA compiler JAVAC (See Fig. 2.) generates an instruction to push this pointer onto operand stack 423 prior to generating an invoke instruction. This object reference is accessible as local variable zero during the execution of the method. This indirect pointer is not available for a static method invocation as there is no target-object defined for a static method invocation. T e list of incoming arguments transfers information from the calling method to the invoked method. Like the object reference, the incoming arguments are pushed onto stack 400 by JAVA compiler generated instructions and may be accessed as local variables. JAVA compiler JAVAC (See Fig. 2) statically generates a list of arguments for current method 410 (Fig. 4A), and hardware processor 100 determines the number of arguments from the list. When the object reference is present in the frame for a non-static method invocation, the first argument is accessible as local variable one. For a static method invocation, the first argument becomes local variable zero. For 64-bit arguments, as well as 64-bit entities in general,, the upper 32-bits, i.e., the 32 most significant bits, of a 64-bit entity are placed on the upper location of stack 400, i.e., pushed on the stack last. For example, when a 64-bit entity is on the top of stack 400, the upper 32-bit portion of the 64-bit entity is on the top of the stack, and the lower 32-bit portion of the 64-bit entity is in the storage location immediately adjacent to the top of stack 400. The local variable area on stack 400 (Fig. 4A) for current method 410 represents temporary variable storage space which is allocated and remains effective during invocation of method 410. JAVA compiler JAVAC (Fig. 2) statically determines the required number of local variables and hardware processor 100 allocates temporary variable storage space accordingly. When a method is executing on hardware processor 100, the local variables typically reside in stack cache 155 and are addressed as offsets from the pointer VARS (Figs. 1 and 4A), which points to the position of the local variable zero. Instructions are provided to load the values of local variables onto operand stack 423 and store values from operand stack into local variables area 421. The information in execution environment 422 includes the invoker's method context. When a new frame is built for the current method, hardware processor 100 pushes the invoker's method context onto newly allocated frame 410, and later utilizes the information to restore the invoker's method context before returning. Pointer FRAME (Figs. 1 and 4 A) is a pointer to the execution environment of the current method. In the exemplary embodiment, each register in register set 144 (Fig. 1) is 32-bits wide. Operand stack 423 is allocated to support the execution of the virtual machine instructions within the current method. Program counter register PC (Fig. 1) contains the address of the next instruction, e.g., opcode, to be executed. Locations on operand stack 423 (Fig. 4A) are used to store die operands of virtual machine instructions, providing both source and target storage locations for instruction execution. The size of operand stack 423 is statically determined by JAVA compiler JAVAC (Fig. 2) and hardware processor 100 allocates space for operand stack 423 accordingly. Register OPTOP (Figs. 1 and 4A) holds a pointer to a top of operand stack 423. The invoked method may return its execution result onto the invoker's top of stack, so that the invoker can access the return value with operand stack references. The return value is placed on the area where an object reference or an argument is pushed before a method invocation. Simulation results on the JAVA virtual machine indicate that method invocation consumes a significant portion of the execution time (20-40%). Given this attractive target for accelerating execution of virtual machine instructions, hardware support for method invocation is included in hardware processor 100, as described more completely below. The beginning of the stack frame of a newly invoked method, i.e., the object reference and the arguments passed by the caller, are already stored on stack 400 since the object reference and the incoming arguments come from the top of the stack of the caller. As explained above, following these items on stack 400, the local variables are loaded and then the execution environment is loaded. One way to speed up this process is for hardware processor 100 to load the execution environment in the background and indicate what has been loaded so far, e.g., simple one bit scoreboarding. Hardware processor 100 tries to execute the bytecodes of the called method as soon as possible, even though stack 400 is not completely loaded. If accesses are made to variables already loaded, overlapping of execution with loading of stack 400 is achieved, otherwise a hardware interlock occurs and hardware processor 100 just waits for the variable or variables in the execution environment to be loaded. Figure 4B illustrates another way to accelerate method invocation. Instead of storing the entire method frame in stack 400, the execution environment of each method frame is stored separately from the local variable area and the operand stack of the method frame. Thus, in this embodiment, stack 400B contains modified method frames, e.g. modified method frame 410B having only local variable area 421 and operand stack 423. Execution environment 422 of the method frame is stored in an execution environment memory 440. Storing the execution environment in execution environment memory 440 reduces the amount of data in stack cache 155. Therefore, the size of stack cache 155 can be reduced. Furthermore, execution environment memory 440 and stack cache 155 can be accessed simultaneously. Thus, method invocation can be accelerated by loading or storing the execution environment in parallel with loading or storing data onto stack 400B. In one embodiment of stack management unit 150, the memory architecture of execution environment memory 440 is also a stack. As modified method frames are pushed onto stack 400b through stack cache 155, corresponding execution environments are pushed onto execution environment memory 440. For example, since modified method frames 0 to 2, as shown in Figure 4B, are in stack 400B, execution environments (EE) 0 to 2, respectively, are stored in execution environment memory circuit 440. To further enhance method invocation, an execution environment cache can be added to improve the speed of saving and retrieving the execution environment during method invocation. The architecture described more completely below for stack cache 155, dribbler manager unit 151, and stack control unit 152 for caching stack 400, can also be applied to caching execution environment memory 440. Figure 4C illustrates an embodiment of stack management unit 150 modified to support both stack 400b and execution environment memory 440. Specifically, the embodiment of stack management unit 150 in Figure 4C adds an execution environment stack cache 450, an execution environment dribble manager unit 460, and an execution environment stack control unit 470. Typically, execution dribble manager unit 460 transfers an entire execution environment between execution environment cache 450 and execution environment memory 440 during a spill operation or a fill operation. I/O Bus and Memory Interface Unit I/O bus and memory interface unit 1 10 (Fig. 1), sometimes called interface unit 110, implements an interface between hardware processor 100 and a memory hierarchy which in an exemplary embodiment includes external memory and may optionally include memory storage and/or interfaces on the same die as hardware processor 100. In this embodiment, I/O controller 1 1 1 interfaces with external I/O devices and memory controller 1 12 interfaces with external memory. Herein, external memory means memory external to hardware processor 100. However, external memory either may be included on the same die as hardware processor 100, may be external to the die containing hardware processor 100, or may include both on- and off- die portions. In another embodiment, requests to I/O devices go through memory controller 1 12 which maintains an address map of the entire system including hardware processor 100. On the memory bus of this embodiment, hardware processor 100 is the only master and does not have to arbitrate to use the memory bus. Hence, alternatives for the input output bus that interfaces with I/O bus and memory interface unit 1 10 include supporting memory-mapped schemes, providing direct support for PCI, PCMCIA, or other standard busses. Fast graphics (w/ VIS or other technology) may optionally be included on the die with hardware processor 100. I/O bus and memory interface unit 1 10 generates read and write requests to external memory. Specifically, interface unit 1 10 provides an interface for instruction cache and data cache controllers 121 and 161 to the external memory. Interface unit 1 10 includes arbitration logic for internal requests from instruction cache controller 121 and data cache controller 161 to access external memory and in response to a request initiates either a read or a write request on the memory bus to the external memory. A request from data cache controller 121 is always treated as higher priority relative to a request from instruction cache controller 161. Interface unit 110 provides an acknowledgment signal to the requesting instruction cache controller 121, or data cache controller 161 on read cycles so that the requesting controller can latch the data. On write cycles, the acknowledgment signal from interface unit 110 is used for flow control so that the requesting instruction cache controller 121 or data cache controller 161 does not generate a new request when there is one pending. Interface unit 110 also handles errors generated on the memory bus to the external memory. Instruction Cache Unit Instruction cache unit (ICU) 120 (Fig. 1) fetches virtual machine instructions from instruction cache 125 and provides the instructions to instruction decode unit 130. In this embodiment, upon a instruction cache hit, instruction cache controller 121, in one cycle, transfers an instruction from instruction cache 125 to instruction buffer 124 where the instruction is held until integer execution unit IEU, that is described more completely below, is ready to process the instruction. This separates the rest of pipeline 300 (Fig. 3) in hardware processor 1 0 from fetch stage 301. If it is undesirable to incur the complexity of supporting an instruction-buffer type of arrangement, a temporary one instruction register is sufficient for most purposes. However, instruction fetching, caching, and buffering should provide sufficient instruction bandwidth to support instruction folding as described below. The front end of hardware processor 100 is largely separate from the rest of hardware processor 100. Ideally, one instruction per cycle is delivered to the execution pipeline. The instructions are aligned on an arbitrary eight-bit boundary by byte aligner circuit 122 in response to a signal from instruction decode unit 130. Thus, the front end of hardware processor 100 efficiently deals with fetching from any byte position. Also, hardware processor 100 deals with the problems of instructions that span multiple cache lines of cache 125. In this case, since the opcode is always the first byte, the design is able to tolerate an extra cycle of fetch latency for the operands. Thus, a very simple de-coupling between the fetching and execution of the bytecodes is possible. In case of an instruction cache miss, instruction cache controller 121 generates an external memory request for the miss.d instruction to I/O bus and memory interface unit 1 10. If instruction buffer 124 is empty, or nearly empty, when there is an instruction cache miss, instruction decode unit 130 is stalled, i.e., pipeline 300 is stalled. Specifically, instruction cache controller 121 generates a stall signal upon a cache miss which is used along with an instruction buffer empty signal to determine whether to stall pipeline 300. Instruction cache 125 can be invalidated to accommodate self-modifying code, e.g., instruction cache controller 121 can invalidate a particular line in instruction cache 125. Thus, instruction cache controller 121 determines the next instruction to be fetched, i.e., which instruction in instruction cache 125 needs to accessed, and generates address, data and control signals for data and tag RAMs in instruction cache 125. On a cache hit, four bytes of data are fetched from instruction cache 125 in a single cycle, and a maximum of four bytes can be written into instruction buffer 124. Byte aligner circuit 122 aligns the data out of the instruction cache RAM and feeds the aligned data to instruction buffer 124. As explained more completely below, the first two bytes in instruction buffer 124 are decoded to determine the length of the virtual machine instruction. Instruction buffer 124 tracks the valid instructions in the queue and updates the entries, as explained more completely below. Instruction cache controller 121 also provides the data path and control for handling instruction cache misses. On an instruction cache miss, instruction cache controller 121 generates a cache fill request to I/O bus and memory interface unit 1 10. On receiving data from external memory, instruction cache controller 121 writes the data into instruction cache 125 and the data are also bypassed into instruction buffer 124. Data are bypassed to instruction buffer 124 as soon as the data are available from external memory, and before the completion of the cache fill. Instruction cache controller 121 continues fetching sequential data until instruction buffer 124 is full or a branch or trap has taken place. In one embodiment, instruction buffer 124 is considered full if there are more than eight bytes of valid entries in buffer 124. Thus, typically, eight bytes of data are written into instruction cache 125 from external memory in response to the cache fill request sent to interface unit 1 10 by instruction cache unit 120. If there is a branch or trap taken while processing an instruction cache miss, only after the completion of the miss processing is the trap or branch executed. When an error is generated during an instruction cache fill transaction, a fault indication is generated and stored into instruction buffer 124 along with the virtual machine instruction, i.e., a fault bit is set. The line is not written into instruction cache 125. Thus, the erroneous cache fill transaction acts like a non-cacheable transaction except that a fault bit is set. When the instruction is decoded, a trap is taken. Instruction cache controller 121 also services non-cacheable instruction reads. An instruction cache enable (ICE) bit, in a processor status register in register set 144, is used to define whether a load can be cached. If the instruction cache enable bit is cleared, instruction cache unit 120 treats all loads as non¬ cacheable loads. Instruction cache controller 121 issues a non-cacheable request to interface unit 1 10 for non- cacheable instructions. When the data are available on a cache fill bus for the non-cacheable instruction, the data are bypassed into instruction buffer 124 and are not written into instruction cache 125. In this embodiment, instruction cache 125 is a direct-mapped, eight-byte line size cache. Instruction cache 125 has a single cycle latency. The cache size is configurable to OK, IK, 2K, 4K, 8K and 16K byte sizes where K means kilo. The default size is 4K bytes. Each line has a cache tag entry associated with the line. Each cache tag contains a twenty bit address tag field and one valid bit for the default 4K byte size. Instruction buffer 124, which, in an exemplary embodiment, is a twelve-byte deep first-in, first-out (FIFO) buffer, de-links fetch stage 301 (Fig. 3) from the rest of pipeline 300 for performance reasons. Each instruction in buffer 124 (Fig. 1) has an associated valid bit and an error bit. When the valid bit is set, the instruction associated with that valid bit is a valid instruction. When the error bit is set, the fetch of the instruction associated with that error bit was an erroneous transaction. Instruction buffer 124 includes an instruction buffer control circuit (not shown) that generates signals to pass data to and from instruction buffer 124 and that keeps track of the valid entries in instruction buffer 124, i.e., those with valid bits set. In an exemplary embodiment, four bytes can be received into instruction buffer 124 in a given cycle. Up to five bytes, representing up to two virtual machine instructions, can be read out of instruction buffer 124 in a given cycle. Alternative embodiments, particularly those providing folding of multi-byte virtual machine instructions and/or those providing folding of more than two virtual machine instructions, provide higher input and output bandwidth. Persons of ordinary skill in the art will recognize a variety of suitable instruction buffer designs including, for example, alignment logic, circular buffer design, etc. When a branch or trap is taken, all the entries in instruction buffer 124 are nullified and the branch/trap data moves to the top of instruction buffer 124. In the embodiment of Figure 1 , a unified execution unit 140 is shown. However, in another embodiment, instruction decode unit 120, integer unit 142, and stack management unit 150 are considered a single integer execution unit, and floating point execution unit 143 is a separate optional unit. In still other embodiments, the various elements in the execution unit may be implemented using the execution unit of another processor. In general the various elements included in the various units of Figure 1 are exemplary only of one embodiment. Each unit could be implemented with all or some of the elements shown. Again, the decision is largely dependent upon a price vs. performance trade-off. Instruction Decode Unit As explained above, virtual machine instructions are decoded in decode stage 302 (Fig. 3) of pipeline 300. In an exemplary embodiment, two bytes, that can correspond to two virtual machine instructions, are fetched from instruction buffer 124 (Fig. 1). The two bytes are decoded in parallel to determine if the two bytes correspond to two virtual machine instructions, e.g., a first load top of stack instruction and a second add top two stack entries instruction, that can be folded into a single equivalent operation. Folding refers to supplying a single equivalent operation corresponding to two or more virtual machine instructions. In an exemplary hardware processor 100 embodiment, a single-byte first instruction can be folded with a second instruction. However, alternative embodiments provide folding of more than two virtual machine instructions, e.g., two to four virtual machine instructions, and of multi-byte virtual machine instructions, though at the cost of instruction decoder complexity and increased instruction bandwidth. In the exemplary processor 100 embodiment, if the first byte, which corresponds to the first virtual machine instruction, is a multi-byte instruction, the first and second instructions are not folded. An optional current object loader folder 132 exploits instruction folding, such as that described above and as well as in greater detail below in virtual machine instruction sequences which simulation results have shown to be particularly frequent and therefore a desirable target for optimization. In particular, method invocations typically load an object reference for the corresponding object onto the operand stack and fetch a field from the object. Instruction folding allow this extremely common virtual machine instruction sequence to be executed using an equivalent folded operation. Quick variants are not part of the virtual machine instruction set (See Chapter 3 of Appendix I), and are invisible outside of a JAVA virtual machine implementation. However, inside a virtual machine implementation, quick variants have proven to be an effective optimization. (See Appendix A in Appendix I; which is an integral part of this specification.) Supporting writes for updates of various instructions to quick variants in a non-quick to quick translator cache 131 changes the normal virtual machine instruction to a quick virtual machine instruction to take advantage of the large benefits bought from the quick variants. In particular, as described in more detail in U.S. Patent Application. Serial No. 08/xxx,xxx, entitled "NON- QUICK INSTRUCTION ACCELERATOR AND METHOD OF IMPLEMENTING SAME" naming Marc Tremblay and James Michael O'Connor as inventors, assigned to the assignee of this application, and filed on even date herewith with Attorney Docket No. SP2039 of which is incoφorated herein by reference in its entirety, when the information required to initiate execution of an instruction has been assembled for the first time, the information is stored in a cache along with the value of program counter PC as tag in non-quick to quick translator cache 131 and the instruction is identified as a quick-variant. In one embodiment, this is done with self-modifying code. Upon a subsequent call of that instruction, instruction decode unit 130 detects that the instruction is identified as a quick-variant and simply retrieves the information needed to initiate execution of the instruction from non-quick to quick translator cache 131. Non-quick to quick translator cache is an optional feature of hardware processor 100. With regard to branching, a very short pipe with quick branch resolution is sufficient for most implementations. However, an appropriate simple branch prediction mechanism can alternatively be introduced, e.g., branch predictor circuit 133. Implementations for branch predictor circuit 133 include branching based on opcode, branching based on offset, or branching based on a two-bit counter mechanism. The JAVA virtual machine specification defines an instruction invokenon virtual, opcode 183, which, upon execution, invokes methods. The opcode is followed by an index byte one and an index byte two. (See Appendix I.) Operand stack 423 contains a reference to an object and some number of arguments when this instruction is executed. Index bytes one and two are used to generate an index into the constant pool of the current class. The item in the constant pool at that index points to a complete method signature and class. Signatures are defined in Appendix I and that description is incoφorated herein by reference. The method signature, a short, unique identifier for each method, is looked up in a method table of the class indicated. The result of the lookup is a method block that indicates the type of method and the number of arguments for the method. The object reference and arguments are popped off this method's stack and become initial values of the local variables of the new method. The execution then resumes with the first instruction of the new method. Upon execution, instructions invokevirtual, opcode 182, and invokestatic, opcode 184, invoke processes similar to that just described. In each case, a pointer is used to lookup a method block. A method argument cache 134, that also is an optional feature of hardware processor 100, is used, in a first embodiment, to store the method block of a method for use, after the first call to the method, along with the pointer to the method block as a tag. Instruction decode unit 130 uses index bytes one and two to generate the pointer and then uses the pointer to retrieve the method block for that pointer in cache 134. This permits building the stack frame for the newly invoked method more rapidly in the background in subsequent invocations of the method. Alternative embodiments may use a program counter or method identifier as a reference into cache 134. If there is a cache miss, the instruction is executed in the normal fashion and cache 134 is updated accordingly. The particular process used to determine which cache entry is overwritten is not an essential aspect of this invention. A least-recently used criterion could be implemented, for example. In an alternative embodiment, method argument cache 134 is used to store the pointer to the method block, for use after the first call to the method, along with the value of program counter PC of the method as a tag. Instruction decode unit 130 uses the value of program counter PC to access cache 134. If the value of program counter PC is equal to one of the tags in cache 134, cache 134 supplies the pointer stored with mat tag to instruction decode unit 130. Instruction decode unit 139 uses the supplied pointer to retrieve the method block for the method. In view of these two embodiments, other alternative embodiments will be apparent to those of skill in the art. Wide index forwarder 136, which is an optional element of hardware processor 100, is a specific embodiment of instruction folding for instruction wide. Wide index forwarder 136 handles an opcode encoding an extension of an index operand for an immediately subsequent virtual machine instruction. In this way, wide index forwarder 136 allows instruction decode unit 130 to provide indices into local variable storage 421 when the number of local variables exceeds that addressable with a single byte index without incurring a separate execution cycle for instruction wide.. Aspects of instruction decoder 135, particularly instruction folding, non-quick to quick translator cache 131, current object loader folder 132, branch predictor 133, method argument cache 134, and wide index forwarder 136 are also useful in implementations that utilize a software inteφreter or just-in-time compiler, since these elements can be used to accelerate the operation of the software inteφreter or just-in- time compiler. In such an implementation, typically, the virtual machine instructions are translated to an instruction for the processor executing the inteφreter or compiler, e.g., any one of a Sun processor, a DEC processor, an Intel processor, or a Motorola processor, for example, and the operation of the elements is modified to support execution on that processor. The translation from the virtual machine instruction to the other processor instruction can be done either with a translator in a ROM or a simple software translator. For additional examples of dual instruction set processors, see U.S. Patent Application Serial No. 08/xxx,xxx, entitled "A PROCESSOR FOR EXECUTING INSTRUCTION SETS RECEIVED FROM A NETWORK OR SUPPLIED BY FROM A LOCAL MEMORY" naming Marc Tremblay and James Michael O'Connor as inventors, assigned to the assignee of this application, and filed on even date herewith with Attorney Docket No. SP2042, which is incoφorated herein by reference in its entirety. As explained above, one embodiment of processor 100 implements instruction folding to enhance the performance of processor 100. In general, instruction folding in accordance with the present invention can be used in any of a stack-based virtual machine implementation, including, e.g., in a hardware processor implementation, in a software inteφreter implementation, in a just-in-time compiler implementation, etc. Thus, while various embodiments of instruction folding are described in the following more detailed description in terms of a hardware processor, those of skill in the art will appreciate, in view of this description, suitable extensions of instruction folding to other stack-based virtual machine implementations. Figure 7 illustrates folded execution of first and second stack instructions, according to the principles of this invention. In this embodiment, a first operand for an addition instruction resides in top-of-stack (TOS) entry 711a of stack 710. A second operand resides in entry 712 of stack 710. Notice that entry 712 is not physically adjacent to top-of stack entry 711a and in fact, is in the interior of stack 710. An instruction stream includes a load top-of-stack instruction for pushing the second operand onto the top of stack (see description of instruction iload in Appendix I) and an addition instruction for operating on the first and second operands residing in the top two entries of stack 710 (see description of instruction iadd in Appendix I). However, to speed execution of the instruction stream, the load top-of-stack and addition instructions are folded into a single operation whereby the explicit sequential execution of the load top-of-stack instruction and the associated execution cycle are eliminated. Instead, a folded operation corresponding to the addition instruction operates on the first and second operands, which reside in TOS entry 71 la and entry 712 of stack 710. The result of the folded operation is pushed onto stack 710 at TOS entry 711b. Thus, folding according to the principles of this invention enhances performance compared to an unfolded method for executing the same sequence of instructions. Without instruction folding, a first operand for an addition instruction resides in top-of-stack (TOS) entry 611a of stack 610 (see Figure 6). A second operand resides in entry 612 of stack 610. A load to top-of- stack instruction pushes the second operand onto the top of stack 610 and typically requires an execution cycle. The push results in the second and first operands residing in TOS entry 611b and (TOS- 1 ) entry 613, respectively. Thereafter, the addition instruction operates, in another execution cycle, on the first and second operands which properly reside in the top two entries, i.e., TOS entry 611b and (TOS- 1) entry 613, of stack 610 in accordance with the semantics of a stack architecture. The result of the addition instruction is pushed onto stack 610 at TOS entry 61 lc and after the addition instruction is completed, it is as if the first and second operand data were never pushed onto stack 610. As described above, folding reduces the execution cycles required to complete the addition and so enhances the speed of execution of the instruction stream. More complex folding, e.g., folding including store instructions and folding including larger numbers of instructions, is described in greater detail below. In general, instruction decoder unit 130 (Figure 1) examines instructions in a stream of instructions. Instruction decoder unit 130 folds first and second adjacent instructions together and provides a single equivalent operation for execution by execution unit 140 when instruction decoder unit 130 detects that die first and second instructions have neither structural nor resource dependencies and the second instruction operates on data provided by the first instruction. Execution of the single operations obtains the same result as execution of an operation corresponding to the first instruction followed by execution an operation corresponding to the second instruction, except that an execution cycle has been eliminated. As described above, the JAVA virtual machine is stack-oriented and specifies an instruction set, a register set, an operand stack, and an execution environment. Although, the present invention is described in relation to the JAVA Virtual Machine, those of skill in the art will appreciate that the invention is not limited to embodiments implementing or related to the JAVA virtual machine and, instead, encompasses systems, articles, methods, and apparati for a wide variety of stack machine environments, both virtual and physical. As illustrated in Figure 4A, according to the JAVA Virtual Machine Specification, each method has storage allocated for an operand stack and a set of local variables. Similarly, in the embodiment of Figure 8 (see also Figure 4A), a series of method frames e.g., method frame 801 and method frame 802 on stack 803, each include an operand stack instance, local variable storage instance, and frame state information instance for respective methods invoked along the execution path of a JAVA program. A new frame is created and becomes current each time a method is invoked and is destroyed after the method completes execution. A frame ceases to be current if its method invokes another method. On method return, the current frame passes back the result of its method invocation, if any, to the previous frame via stack 803. The current frame is then discarded and the previous frame becomes current. Folding in accordance with the present invention, as described more completely below, is not dependent upon a particular process used to allocate or define memory space for a method, such as a frame, and can, in general, be used in any stack based architecture. This series of method frames may be implemented in any of a variety of suitable memory hierarchies, including for example register/ cache/ memory hierarchies. However, irrespective of the memory hierarchy chosen, an operand stack instance 812 (Figure 8) is implemented in randomly-accessible storage 810, i.e., at least some of the entries in operand stack instance 812 can be accessed from locations other than the top most locations of operand stack instance 812 in contrast with a conventional stack implementation in which only the top entry or topmost entries of the stack can be accessed. As described above, register OPTOP stores a pointer that identifies the top of operand stack instance 812 associated with the current method. The value stored in register OPTOP is maintained to identify the top entry of an operand stack instance corresponding to the current method. In addition, local variables for the current method are represented in randomly-accessible storage 810. A pointer stored in register VARS identifies the starting address of local variable storage instance 813 associated with the current method. The value in register VARS is maintained to identify a base address of the local variable storage instance corresponding to the current method. Entries in operand stack instance 812 and local variable storage instance 813 are referenced by indexing off of values represented in registers OPTOP and VARS, respectively, that in the embodiment of Figure 1 are included in register set 144, and in the embodiment of Figure 8 are included in pointer registers 822. Pointer registers 822 may be represented in physical registers of a processor implementing the JAVA Virtual Machine, or optionally, in randomly-accessible storage 810. In an exemplary embodiment, commonly used offsets OPTOP- 1, OPTOP-2, VARS+1, VARS+2, and VARS+3 are derived from the values in registers OPTOP and VARS, respectively Alternatively, the additional offsets could be stored in registers of pointer registers 822 Operand stack instance 812 and local variable storage instance 813 associated with the current method are preferably represented in a flat 64-entry cache, e g , stack cache 155 (see Figure 1 ) whose contents are kept updated so that a working set of operand stack and local variable storage entries are cached However, depending on the size of the current frame, the current frame including operand stack instance 812 and local variable storage instance 813 may be fully or partially represented in the cache Operand stack and local variable storage entries for frames other than the current frame may also be represented in the cache if space allows A suitable representation of a cache suitable for use with the folding of this invention is described in greater detail in U.S Patent Application Serial No 08/xxx,xxx, entitled "METHODS AND APPARATI FOR STACK CACHING" naming Marc Tremblay and James Michael O'Connor as inventors, assigned to the assignee of this application, and filed on even date herewith with Attorney Docket No SP2037, the detailed description of which is incoφorated herein by reference, and in U.S Patent Application Serial No 08/xxx,xxx, entitled "METHOD FRAME STORAGE USING MULTIPLE MEMORY CIRCUITS" naming Marc Tremblay and James Michael O'Connor as inventors, assigned to the assignee of this application, and filed on even date herewith with Attorney Docket No SP2038, the detailed descπption of which also is incoφorated herein by reference However, other representations, including separate and or uncached operand stack and local variable storage areas, are also suitable In addition to method frames and their associated operand stack and local variable storage instances, a constant area 814 is provided in the address space of a processor implementing the JAVA virtual machme for commonly-used constants, e g., constants specified by JAVA virtual machine instructions such as instruction iconst. In some cases, an operand source is represented as an index into constant area 814 In the embodiment of Figure 8, constant area 814 is represented in randomly-accessible storage 810 Optionally, entries of constant area 814 could also be cached, e.g , in stack cache 155. Although those of skill in the art will recognize the advantages of maintaining an operand stack and local variable storage instance for each method, as well as the opportunities for passing parameters and results created by maintaining the various instances of operand stack and local variable storage in a stack-oriented structure, in the interest of clarity, the description which follows focuses on the particular instances (operand stack instance 812 and local variable storage instance 813) of each associated the current memod. Hereafter, these particular instances of an operand stack and local variable storage are referred to simply as operand stack 812 and local variable storage 813. Despite this simplification for puφoses of illustration, those of skill in the art will appreciate that operand stack 812 and local variable storage 813 refer to any instances of an operand stack and variable storage associated with the current method, including representations which maintain separate instances for each method and representations which combine instances mto a composite representation Operand sources and result targets for JAVA Virtual Machine instructions typically identify entries of operand stack instance 812 or local variable storage instance 813, i.e., identify entries of the operand stack and local variable storage for the current method. By way of example, and not limitation, representative JAVA virtual machine instructions are described in Chapter 3 of The JA VA Virtual Machine Specification which is included at Appendix I. JAVA virtual machine instructions rarely explicitly designate both the source of the operand, or operands, and the result destination. Instead, either the source or the destination is implicitly the top of operand stack 812. Some JAVA bytecodes explicitly designate neither a source nor a destination. For example, instruction iconsι_0 pushes a constant integer zero onto operand stack 812. The constant zero is implicit in the instruction, although the instruction may actually be implemented by a particular JAVA virtual machine implementation using a representation of the value zero from a pool of constants, such as constant area 814, as the source for the zero operand. An instruction decoder for a JAVA virtual machine implementation that implements instruction iconst O in this way could generate, as the source address, the index of the entry in constant area 814 where the constant zero is represented. Prior to considering the various embodiments of folding in accordance with the present invention, it is informative to consider execution of JAVA virtual machine instructions, such as the iadd instruction and the arraylength instruction, without the folding process. After the operations associated with typical execution of JAVA virtual machine instructions are understood, the advantages of this invention will be more apparent. Further, this understanding will assist those of skill in the art in extending the invention to other stack-based architectures that do not rely upon the JAVA virtual machine instructions. Focusing illustratively on operand stack and local variable storage structures associated with the current method and referring now to Figures 9A-D, the JAVA virtual machine integer add instruction, iadd, generates the sum of first and second integer operands, referred to as operand 1 and operand2, respectively, that are at the top two locations of operand stack 812. The top two locations are identified, at the time of instruction iadd execution, by pointer OPTOP in register OPTOP and by pointer OPTOP- 1. The result of the execution of instruction iadd, i.e., the sum of first and second integer operands, is pushed onto operand stack 812. Figure 9A shows the state of operand stack 812 and local variable storage 813 that includes first and second values, referred to as value 1 and value2, before execution of a pair of JAVA virtual machine integer load instructions iload. In Figure 9A, pointer OPTOP has the value AACOh. Figure 9B shows operand stack 812 after execution of the pair of instructions iload that load integer values from local variable storage 813 onto operand stack 812, pushing (i.e., copying) values value 1 and value2 from locations identified by pointer VARS in register VARS and by pointer VARS+2 onto operand stack 812 as operand] at location AAC4h and operand2 at location AAC8h, and updating pointer OPTOP in die process to value AAC8h. Figure 9C shows operand stack 812 after instruction iadd has been executed. Execution of instruction iadd pops operands operand ! and operand2 off operand stack 812, calculates the sum of operands operand 1 and operancO, and pushes that sum onto operand stack 812 at location AAC4h After execution of instruction iadd, pointer OPTOP has the value AACOh and points to the operand stack 812 entry storing the sum Figure 9D shows operand stack 812 after an instruction istore has been executed Execution of instruction istore pops the sum off operand stack 812 and stores the sum in the local variable storage 813 entry at the location identified by pointer VARS+2 Variations for other instructions which push operands onto operand stack 812 and which operate on values residing at the top of operand stack 812 will be apparent to those of skill in the art For example, variations for alternate operations and for data types requiring multiple operand stack 812 entries, e g , long integer values, double-precision floating point values, etc , will be apparent to those of skill in the art in view of this disclosure The folding example of Figures 10A-C is analogous to that illustrated with reference to Figures 9A- D, though with only load folding illustrated. Execution of JAVA virtual machine length of array instruction arraylength determines the length of an array whose object reference pointer objectref is at the top of operand stack 812, and pushes the length onto operand stack 812 Figure 10A shows the state of operand stack 812 and local variable storage 813 before execution of JAVA virtual machine reference load instruction aload that is used to load an object reference from local variable storage 813 onto the top of operand stack 812. In Figure 10A, pointer OPTOP has the value AACOh Figure 10B shows operand stack 812 after execution of instruction aload pushes, l e , copies, object reference pointer objectref onto the top of operand stack 812 and updates pointer OPTOP to AAC4h m the process Figure IOC shows operand stack 812 after instruction arraylength has been executed. Execution of instruction arraylength pops object reference pointer objectref off operand stack 812, calculates the length of the array referenced thereby, and pushes that length onto operand stack 812. Suitable implementations of the instruction arraylength may supply object reference pointer objectref to an execution unit, e.g , execution unit 1 0, which subsequently overwrites the object reference pointer objectref with the value length Whether the object reference pointer objectref is popped from operand stack 812 or simply overwritten, after execution of instruction arraylength, pointer OPTOP has the value AAC4h and points to the operand stack 812 entry storing the value length. Figure 1 1 illustrates a processor 1100 wherein loads, such as those illustrated in Figures 9 A and 9B and in Figures 10A and 10B, are folded into execution of subsequent instructions, e.g., into execution of subsequent instruction iadd, or instruction arraylength. In this way, intermediate execution cycles associated with loadmg operands operand 1 and operand2 for instruction iadd, or with loading pointer objectref for instruction arraylength onto the top of operand stack 812 can be eliminated. As a result, single cycle execution of groups of JAVA virtual machine instructions e.g., the group of instructions iload, iload, iadd, and istore, or the group of instructions aload and arraylength, is provided by processor 1 100. One embodiment of processor 1 100 is presented in Figure 1 as hardware processor 100. However, hardware processor 1 100 includes other embodiments that do not include the various optimizations of hardware processor 100. Further, the folding processes described below could be implemented in a software inteφreter or a included within a just-in-time compiler. In the processor 1 100 embodiment of Figure 1 1 , stores such as that illustrated in Figure 9D, are folded into execution of prior instructions, e.g., in Figure 9D, into execution of the immediately prior instruction iadd. The instruction folding is provided primarily by instruction decoder 1 118. Instruction decoder 1 118 retrieves fetched instructions from instruction buffer 1116 and depending upon the nature of instructions in the fetched instruction sequence, supplies execution unit 1 120 with decoded operation and operand addressing information implementing the instruction sequence as a single folded operation. Unlike instructions of the JAVA virtual machine instruction set to which the instruction sequence from instruction buffer 1116 conforms, decoded operations supplied to execution unit 1120 by instruction decoder 1118 operate on operand values represented in entries of local variable storage 813, operand stack 812, and constant area 814. In the exemplary embodiment of Figure 1 1 , valid operand sources include local variable storage 813 entries identified by pointers VARS, VARS+1 , VARS+2, and VARS+3, as well as operand stack 812 entries identified by pointers OPTOP, OPTOP- 1, and OPTOP-2. Similarly, valid result targets include local variable storage 813 entries identified by operands VARS, VARS+1 , VARS+2, and VARS+3. Embodiments in accordance with Figure 1 1 may also provide for constant area 814 entries as valid operand sources as well as other locations in operand stack 812 and local variable storage 813. Referring now to Figures 11 and 12, a sequence of JAVA virtual machine instructions is fetched from memory and loaded into instruction buffer 1116. Conceptually, instruction buffer 1116 is organized as a shift register for JAVA bytecodes. One or more bytecodes are decoded by instruction decoder 1118 during each cycle and operations are supplied to execution unit 1120 in the form of a decoded operation on instruction decode bus instr dec and associated operand source and result destination addressing information on instruction address bus instr addr. Instruction decoder 1118 also provides an instruction valid signal instr_valid to execution unit 1120. When asserted, signal instr_valid indicates that the information on instruction decode bus instr dec specifies a valid operation. One or more bytecodes are shifted out of instruction buffer 1116 to instruction decode unit 1 1 18 each cycle in correspondence with the supply of decoded operations and operand addressing information to execution unit 1 120, and subsequent undecoded bytecodes are shifted into instruction buffer 1 116. For normal decode operations, a single instruction is shifted out of instruction buffer 1 116 and decoded by instruction decode unit 1 1 18, and a single corresponding operation is executed by execution unit 1 120 during each instruction cycle. In contrast, for folded decode operations, multiple instructions, e.g., a group of instructions, are shifted out of instruction buffer 1 1 16 to instruction decode unit 1 1 18. In response to the multiple instructions, instruction decode unit 1 1 18 generates a single equivalent folded operation that in turn is executed by execution unit 1 120 during each instruction cycle. Referring illustratively to the instruction sequence described above with reference to FlGS. 9A-9D, instruction decoder 1118 selectively decodes bytecodes associated with four JAVA virtual machine instructions: 1. iload value 1 ; 2. iload value2; 3. iadd; and 4. istore, that were described in the above description of Figures 9A-D. As now described, both instructions iload and the instruction istore are folded by instruction decoder 1118 into an add operation corresponding to instruction iadd. Although operation of instruction decoder 1118 is illustrated using a foldable sequence of four instructions, those of skill in the art will appreciate that the invention is not limited to four instructions. Foldable sequences of two, three, four, five, or more instructions are envisioned. For example, more than one instruction analogous to the instruction istore and more than two instructions analogous to the instructions iload may be included in foldable sequences. Instruction decoder 1118 supplies decoded operation information over bus instr dec and associated operand source and result destination addressing information over bus instr addr specifying that execution unit 1120 is to add the contents of local variable storage 813 location 0, this is identified by pointer VARS, and local variable storage 813 location 2, that is identified by pointer VARS+2, and store the result in local variable storage 813 location 2, that is identified by pointer VARS+2. In this way, the two load instructions are folded into execution of an operation corresponding to instruction iadd. Two instruction cycles and the intermediate data state illustrated in Figure 9B are eliminated. In addition, instruction istore is also folded into execution of the operation corresponding to instruction iadd, eliminating another instruction cycle, for a total of three, and the intermediate data state illustrated in Figure 9C. In various alternative embodiments, instruction folding in accordance with the present invention may eliminate loads, stores, or both loads and stores. Figure 13 depicts an exemplary embodiment of an instruction decoder 1 118 providing both folded and unfolded decoding of bytecodes. Selection of a folded or unfolded operating mode for instruction decoder 1 1 18 is based on the particular sequence of bytecodes fetched into instruction buffer 1 1 16 and subsequently accessed by instruction decoder 1 118. A normal decode portion 1302 and a fold decode portion 1304 of instruction decoder 1 118 are configured in parallel to provide support for unfolded and folded execution, respectively. In the embodiment of Figure 13, fold decode portion 1304 detects opportunities for folding execution of bytecodes in the bytecode sequence fetched into instruction buffer 1 1 16. A detection of such a foldable sequence triggers selection of the output of fold decode portion 1304, rather than normal decode portion 1302, for provision to execution unit 1 120. Advantageously, selection of folded or unfolded decoding is transparent to execution unit 1 120, which simply receives operation information over bus instr dec and associated operand source and result destination addressing information over bus instr addr, and which need not know whether the information corresponds to a single instruction or a folded instruction sequence. Normal decode portion 1302 functions to inspect a single bytecode from instruction buffer 1116 during each instruction cycle, and generates the following indications in response thereto: 1. a normal instruction decode signal n instr dec, which specifies an operation, e.g., integer addition, corresponding to the decoded instruction, is provided to a first set of input terminals of switch 1306; 2. a normal address signal n adr, which makes explicit the source and destination addresses, e.g., first operand address = OPTOP, second operand address = OPTOP- 1, and destination address = OPTOP- 1 for an instruction iadd, for the decoded instruction, is provided to a first bus input of switch 1310; 3. a net change in pointer OPTOP signal n_delta_optop, e.g., for the instruction iadd, net change = -1, which in the embodiment of Figure 13 is encoded as a component of normal address signal n_adr; and 4. an instruction valid signal instr valid, which indicates whether normal instruction decode signal n_instr_dec specifies a valid operation, is provided to a first input terminal of switch 1308. In contrast with normal decode portion 802, and as discussed in greater detail below, fold decode portion 804 of instruction decoder 618 inspects sequences of bytecodes from the instruction buffer 616 and determines whether operations corresponding to these sequences (e.g., the sequence iload valuel from local variable 0, iload value2 from local variable 2, iadd, and istore sum to local variable 2) can be folded together to eliminate unnecessary temporary storage of instruction operands and/or results on the operand stack. When fold decode portion 804 determines that a sequence of bytecodes in instruction buffer 616 can be folded together, fold decode portion 804 generates the following indications: 1. a folded instruction decode signal f_instr_dec, which specifies an equivalent operation, e.g., integer addition corresponding to the folded instruction sequence, is provided to a second set of input terminals of switch 1306; 2. a folded address signal f adr, which specifies source and destination addresses for the equivalent operation, e.g., first operand address = VARS, second operand address = VARS+2, and destination address = VARS+2, is provided to a second bus input of switch 1310; 3. a net change in pointer OPTOP signal f_delta_optop, e.g., for the above sequence net change = 0, which in the embodiment of Figure 13 is encoded as a component of normal address signal π adr; and 4. a folded instruction valid signal f valid, which indicates whether folded instruction decode signal f instr dec specifies a valid operation, is provided to a second input terminal of switch 1308. Fold decode portion 804 also generates a signal on fold line f/nf which indicates whether a sequence of bytecodes in instruction buffer 1116 can be folded together. The signal on fold line f/nf is provided to control inputs of switches 1306, 1310 and 1308. If a sequence of bytecodes in instruction buffer 1116 can be folded together, the signal on fold line f/nf causes switches 1306, 1310 and 1308 to select respective second inputs for provision to execution unit 1120, i.e., to source folded instruction decode signal f_instr_dec, folded address signal f_adr, and folded instruction valid signal f_valid from fold decode portion 804. If a sequence of bytecodes in instruction buffer 1116 cannot be folded together, the signal on fold line f/nf causes switches 1306, 1310 and 1308 to select respective first inputs for provision to execution unit 1120, i.e., to source normal instruction decode signal π_instr_dec, normal address signal n adr, and normal instruction valid signal n valid from fold decode portion 804. In some embodiments in accordance with the present invention, the operation of fold decode portion 1304 is suppressed in response to an active suppress folding signal suppress fold supplied from outside instruction decoder 1118. In response to an asserted suppress folding signal suppress fold (see Figure 14), the signal on fold line f/nf remains in a state selective for respective first inputs of switches 1306, 1310 and 1308 even if the particular bytecode sequence presented by instruction buffer 1116 would otherwise trigger folding. For example, in one such embodiment, suppress folding signal suppress fold is asserted when the local variable storage 813 entry identified by pointer VARS is not cached, e.g., when entries in operand stack 812 have displaced local variable storage 813 from a stack cache 155. In accordance with the exemplary embodiment described therein, a stack cache and cache control mechanism representing at least a portion of operand stack 812 and local variable storage 813 may advantageously assert suppress folding signal suppress fold if fold-relevant entries of local variable storage 813 or operand stack 812 are not represented in stack cache 155. Figure 14 illustrates fold decode portion 1304 of instruction decoder 1118 in greater detail. A fold determination portion 1404 selectively inspects the sequence of bytecodes in instruction buffer 1116. If the next bytecode and one or more subsequent bytecodes represent a foldable sequence of operations (as discussed below with respect to Figure 15), then fold determination portion 1404 supplies a fold-indicating signal on fold line f/nf and a folded instruction decode signal f instr dec that specifies an equivalent folded operation. Folded instruction decode signal f_instr_dec is supplied to execution unit 1120 as the decoded instruction instr_dec. In an exemplary embodiment, a foldable sequence of operations includes those associated with 2, 3, or 4 bytecodes from instruction decoder 1118 (up to 2 bytecodes loading operands onto operand stack 812, a bytecode popping the operand(s), operating thereupon, and pushing a result onto operand stack 812, and a bytecode popping the result from operand stack 812 and storing the result. The equivalent folded operation, which is encoded by the folded instruction decode signal f_instr_dec, specifies an operation, that when combined with folded execution addressing information obviates the loads to, and stores from, operand stack 812. Alternative embodiments may fold only two instructions, e.g., an instruction iload into an instruction iadd or an instruction istore back into an immediately prior instruction iadd. Other alternative embodiments may fold only instructions that push operands onto the operand stack, e.g., one or more instructions iload folded into an instruction iadd, or only instructions that pop results from the operand stack, e.g., an instruction istore back into an immediately prior instruction iadd. Further alternative embodiments may fold larger numbers of instructions that push operands onto the operand stack and/or instructions that pop results from the operand stack instructions in accordance with instructions of a particular virtual machine instruction set. In such alternative embodiments, the above described advantages over normal decoding and execution of instruction sequences are still obtained. Fold determination portion 1404 generates a series of fold address index composite signal f adr ind including component first operand index signal first adr ind, second operand index signal second adr ind, and destination index signal dest_adr_ind, which are respectively selective for a first operand address, a second operand address, and a destination address for the equivalent folded operation. Fold determination portion 1404 provides the composite signal f adr ind to fold address generator 1402 for use in supplying operand and destination addresses for the equivalent folded operation. Fold determination portion 1404 asserts a fold-indicating signal on fold line f/nf to control the switches 1306, 1310 and 1308 (see Figure 13) to provide the signals f instr dec, f adr, and f valid, as signals instr dec, instr adr, and instr valid, respectively. Otherwise respective signals are provided to execution unit 1120 from normal decode portion 1302. The operation of fold determination portion 1404 is now described with reference to the flowchart of Figure 15. At start 1501, fold determination portion 1404 begins an instruction decode cycle and transfers processing to initialize index 1502. In initialize index 1502, an instruction index instrjndex into instruction buffer 1116 is initialized to identify the next bytecode of a bytecode sequence in instruction buffer 1116. In an exemplary embodiment, instruction index instr index is initialed to one (1) and the next bytecode is the first bytecode in instruction buffer 1 116 since prior bytecodes have already been shifted out of instruction buffer 1 116, although a variety of other indexing and instruction buffer management schemes would also be suitable. Upon completion, initialize index 1502 transfers processing to first instruction check 1504. In first instruction check 1504, fold determination portion 1404 determines whether the instruction identified by index instr_index, i.e., the first bytecode, corresponds to an operation that pushes a value, e.g., an integer value, a floating point value, a reference value, etc., onto operand stack 812, Referring illustratively to a JAVA virtual machine embodiment, first instruction check 1504 determines whether the instruction identified by index instr index is one that the JAVA virtual machine specification (see Appendix I) defines as for pushing a first data item onto the operand stack. If so, first operand index signal first adr ind is asserted (at first operand address setting 1506) to identify the source of the first operand value. In an exemplary embodiment, first operand index signal first ad rjnd is selective for one of OPTOP, OPTOP- 1 , OPTOP-2, VARS, VARS+ 1 , VARS+2, and VARS+3, although alternative embodiments may encode larger, smaller, or different sets of source addresses, including for example, source addresses in constant area 814. Depending on the bytecodes which follow, this first bytecode may correspond to an operation which can be folded into the execution of a subsequent operation. However, if the first bytecode does not meet the criteria of first instruction check 1504, folding is not appropriate and fold determination portion 1404 supplies a nonfold- indicating signal on fold line f/nf, whereupon indications from normal decode portion 1302 provide the decoding. Assuming the first bytecode meets the criteria of first instruction check 1504, index instr index is incremented (at incrementing 1508) to point to the next bytecode in instruction buffer 1116. Then, at second instruction check 1510, fold determination portion 1404 determines whether instruction identified by index instr index, i.e., the second bytecode, corresponds to an operation that pushes a value, e.g., an integer value, a floating point value, a reference value, etc., onto operand stack 812. Referring illustratively to a JAVA virtual machine embodiment, second instruction check 1510 determines whether the instruction identified by index instr index is one that the JAVA virtual machine specification (see Appendix I) defines as for pushing a first data item onto the operand stack. If so, second operand index signal second adr ind is asserted (at second operand address setting 1512) to indicate the source of the second operand value and index instr index is incremented (at incrementing 1514) to point to the next bytecode in instruction buffer 1 116. As before, second operand index signal second_adr_ind is selective for one of OPTOP, OPTOP-1, OPTOP-2, VARS, VARS+1, VARS+2, and VARS+3, although alternative embodiments are also suitable. Fold determination portion 1404 continues at third instruction check 1516 with index instr index pointing to either the second or third bytecode in instruction buffer 1116. At third instruction check 1516, fold determination portion 1404 determines whether the instruction identified by index instr index, i.e., either the second or third bytecode, corresponds to an operation that operates on an operand value or values, e.g., integer value(s), floating point value(s), reference value(s), etc., from the uppermost entries of operand stack 812, effectively popping such operand values from operand stack 812 and pushing a result value onto operand stack 812. Popping of operand values may be explicit or merely a net effect of writing the result value to an upper entry of operand stack 812 and updating pointer OPTOP to identify that entry as the top of operand stack 812. Referring illustratively to a JAVA virtual machine embodiment, third instruction check 1516 determines whether the instruction identified by index instr index corresponds to an operation that the JAVA virtual machine specification (see Appendix 1) defines as for popping a data item (or items) from the operand stack, for operating on the popped data item(s), and for pushing a result of the operation onto the operand stack. If so, index instr index is incremented (at incrementing 1518) to point to the next bytecode in instruction buffer 1116. If not, folding is not appropriate and fold determination portion 1404 supplies a /.o/i/otø-indicating signal on fold line f/nf, whereupon normal decode portion 1302 provides decoding. At fourth instruction check 1520, fold determination portion 1404 determines whether the instruction identified by index instr_index, i.e., either the third or fourth bytecode, corresponds to an operation that pops a value from operand stack 812 and stores the value in a data store such as local variable storage 813. Referring illustratively to a JAVA virtual machine embodiment, fourth instruction check 1520 determines whether the instruction identified by index instr index corresponds to an operation that the JAVA virtual machine specification (see Appendix I) defines as for popping the result data item from the operand stack. If so, index signal dest adr ind is asserted (at destination address setting 1522) to identify the destination of the result value of the equivalent folded operation. Otherwise, if the bytecode at instruction buffer 1116 location identified by index instr_index does not match the criterion of fourth instruction check 1520, index signal dest adr ind is asserted (at destination address setting 1124) to identify the top of operand stack 812. Referring illustratively to a JAVA virtual machine embodiment, if the instruction identified by index instr ndex does not match the criterion of fourth instruction check 1520, index signal dest_adr_ind is asserted (at destination address setting 1124) to identify the pointer OPTOP. Whether the top of operand stack 812 or a store operation destination is selected, the folded instruction valid signal f valid is asserted (at valid fold asserting 1126) and a fold-indicating signal on line f/nf is supplied to select fold decode inputs of switches 1306, 1308, and 1310 for supply to execution unit 1120. Fold determination portion 1404 ends an instruction decode cycle at finish 1550. As a simplification, an instruction decoder for hardware processor 100, e.g., instruction decoder 135, may limit fold decoding to instruction sequences of two instructions and/or to sequences of single bytecode instructions. Those of skill in the art will appreciate suitable simplifications to fold decode portion 1304 of instruction decoder 1118. Figure 16 shows fold address generator 1402 including three component address generators, first operand address generator 1602, second operand address generator 1604, and destination address generator 1606, respectively supplying a corresponding first operand, second operand, and destination address based on indices supplied thereto and pointer VARS and pointer OPTOP values from pointer registers 822. In an exemplary embodiment, first operand address generator 1602, second operand address generator 1604, and destination address generator 1606 supply addresses in randomly-accessible storage 810 corresponding to a subset of operand stack 812 and local variable storage 813 entries. Alternative embodiments may supply identifiers selective for storage other than random access memory, e.g., physical registers, which in a particular JAVA virtual machine implementation provide underlying operand stack and local variable storage. First operand address generator 1602 receives first operand index signal first_adr_ind from fold determination portion 1404 and, using pointer VARS and pointer OPTOP values from pointer registers 822, generates a first operand address signal fιrst_op_adr for a first operand for the equivalent folded operation. The operation of second operand address generator 1604 and destination address generator 1606 is analogous. Second operand address generator 1604 receives first operand index signal first adr ind and generates a second operand address signal second_op_adr for a second operand (if any) for the equivalent folded operation. Destination address generator 1606 receives the destination index signal dest_ad_ind and generates the destination address signal dest adr for the result of the equivalent folded operation. In the embodiment of Figures 13, 14, and 16, first operand address signal first_op_adr, second operand address signal second_op_adr, and destination address signal dest adr are collectively supplied to switch 1310 as fold address signal f adr for supply to execution unit 1120 as the first operand, second operand, and destination addresses for the equivalent folded operation. Figure 17 illustrates an exemplary embodiment of first operand address generator 1602. Second operand address generator 1604 and destination address generator 1606 are analogous. In the exemplary embodiment of Figure 17, first operand address signal first_op_adr is selected from a subset of locations in local variable storage 813 and operand stack 812. Alternative embodiments may generate operand and destination addresses from a larger, smaller, or different subset of operand stack 812 and local variable storage 813 locations or from a wider range of locations in randomly-accessible storage 810. For example, alternative embodiments may generate addresses selective for location in constant area 814. Suitable modifications to the exemplary embodiment of Figure 17 will be apparent to those of skill in the art. first operand address generator 1602, second operand address generator 1604, and destination address generator 1606 may advantageously define differing sets of locations. For example, whereas locations in constant area 814 and in the interior of operand stack 812 are valid as operand sources, they are not typically appropriate result targets. For this reason, the set of locations provided by an exemplary embodiment of destination address generator 1606 is restricted to local variable storage 813 entries and uppermost entries of operand stack 812, although alternative sets are also possible. Referring to Figure 17, pointer OPTOP is supplied to register 1702, which latches the value and provides the latched value to a first input of a data selector 1750. Similarly, pointer OPTOP is supplied to registers 1704 and 1706, which latch the value minus one and minus two, respectively, and provide the latched values to second and third inputs of data selector 1750. In this way, addresses identified by values OPTOP, OPTOP- 1, and OPTOP-2 are available for selection by data selector 1750. Similarly, pointer VARS is supplied to a series of registers 1708, 1710, 1712 and 1714, which respectively latch the values VARS, VARS+1, VARS+2, and VARS+3 for provision to the fourth, fifth, sixth, and seventh inputs of data selector 1750. In this way, addresses identified by values VARS, VARS+1, VARS+2, and VARS+3 are available for selection by data selector 1750. In the exemplary embodiment described herein, offsets from pointer VARS are positive because local variable storage 813 is addressed from its base (identified by pointer VARS) while offsets to pointer OPTOP are negative because operand stack 812 is addressed from its top (identified by pointer OPTOP). Data selector 1750 selects from among the latched addresses available at its inputs. In an embodiment of fold determination portion 1404 in accordance with the Figure 17 embodiment of first operand address generator 1602, load source addresses in local variable storage 813 other than those addressed by values VARS, VARS+1, VARS+2, and VARS+3 are handled as unfoldable and decoded via normal decode portion 1302. However, suitable modifications for expanding the set of load addresses supported will be apparent to those of skill in the art. Second operand address generator 1604 and destination address generator 1606 are of analogous design, although destination address generator 1606 does not provide support for addressing into constant area 814. In one embodiment in accordance with the present invention, signal RS1_D is supplied to the zeroth input of data selector 1750. In this embodiment, additional decode logic (not shown) allows for direct supply of register identifier information to support an alternate instruction set. Addition decode logic support for such an alternate instruction set is described in greater detail in a U.S. Patent Application Serial No. 08/xxx,xxx, entitled "A PROCESSOR FOR EXECUTING INSTRUCTION SETS RECEIVED FROM A NETWORK OR FROM A LOCAL MEMORY" naming Marc Tremblay and James Michael O'Connor as inventors, assigned to the assignee of this application, and filed on even date herewith with Attorney Docket No. SP2042, the detailed description of which is incoφorated herein by reference. Referring back to Figure 13, when fold determination portion 1404 of fold decode portion 1304 identifies a foldable bytecode sequence, fold determination portion 1404 asserts a fold-indicating signal on line f/nf, supplies an equivalent folded operation as folded instruction decode signal f instr dec, and supplies, based on load and store instructions from the foldable bytecode sequence, indices into latched addresses maintained by first operand address generator 1602, second operand address generator 1604, and destination address generator 1606. Fold decode portion 1304 supplies the addresses so indexed as folded address signal f_adr. Responsive to the signal on line f/nf, switches 1306, 1308, 1310 supply decode information for the equivalent folded operation to execution unit 1120. Although fold decode portion 804 has been described above in the context of an exemplary four instruction foldable sequence, it is not limited thereto. Based on the description herein, those of skill in the art will appreciate suitable extensions to support folding of additional instructions and longer foldable instruction sequences, e.g., sequences of five or more instructions. By way of example and not of limitation, support for additional operand address signals, e.g., a third operand address signal, and/or for additional destination address signals, e.g., a second destination address signal, could be provided. Integer Execution Unit Referring again to Figure 1 , integer execution unit IEU, that includes instruction decode unit 130, integer unit 142, and stack management unit 150, is responsible for the execution of all the virtual machine instructions except the floating point related instructions The floating point related instructions are executed in floating point unit 143. Integer execution unit IEU interacts at the front end with instructions cache unit 120 to fetch instructions, with floating point unit (FPU) 143 to execute floating point instructions, and finally with data cache unit (DCU) 160 to execute load and store related instructions. Integer execution unit IEU also contains microcode ROM 149 which contains instructions to execute certain virtual machine instructions associated with integer operations. Integer execution unit IEU includes a cached portion of stack 400, i e., stack cache 155. Stack cache 155 provides fast storage for operand stack and local variable entries associated with a current method, e.g., operand stack 423 and local variable storage 421 entries Although, stack cache 155 may provide sufficient storage for all operand stack and local variable entries associated with a current method, depending on the number of operand stack and local variable entries, less than all of local variable entries or less than all of both local variable entries and operand stack entries may be represented in stack cache 155. Similarly, additional entries, e.g., operand stack and or local variable entries for a calling method, may be represented in stack cache 155 if space allows. Stack cache 155 is a sixty-four entry thirty-two-bit wide array of registers that is physically implemented as a register file in one embodiment. Stack cache 155 has three read ports, two of which are dedicated to integer execution unit IEU and one to dribble manager unit 151. Stack cache 155 also has two write ports, one dedicated to integer execution unit IEU and one to dribble manager unit 151. Integer unit 142 maintains the various pointers which are used to access variables, such as local variables, and operand stack values, in stack cache 155. Integer unit 142 also maintains pointers to detect whether a stack cache hit has taken place. Runtime exceptions are caught and dealt with by exception handlers that are implemented using information in microcode ROM 149 and circuit 170. Integer unit 142 contains a 32-bit ALU to support arithmetic operations The operations supported by the ALU include: add, subtract, shift, and, or, exclusive or, compare, greater than, less than, and bypass. The ALU is also used to determine the address of conditional branches while a separate comparator determines the outcome of the branch instruction. The most common set of instructions which executes cleanly through the pipeline is the group of ALU instructions. The ALU instructions read the operands from the top of stack 400 in decode stage 302 and use the ALU in execution stage 303 to compute the result. The result is written back to stack 400 in write- back stage 305. There are two levels of bypass which may be needed if consecutive ALU operations are accessing stack cache 155. Since the stack cache ports are 32-bits wide in this embodiment, double precision and long data operations take two cycles. A shifter is also present as part of the ALU. If the operands are not available for the instruction in decode stage 302, or at a maximum at the beginning of execution stage 303, an interlock holds the pipeline stages before execution stage 303. The instruction cache unit interface of integer execution unit IEU is a valid/accept interface, where instruction cache unit 120 delivers instructions to integer decode unit 130 in fixed fields along with valid bits. Instruction decoder 135 responds by signaling how much byte aligner circuit 122 needs to shift, or how many bytes instruction decode unit 130 could consume in decode stage 302. The instruction cache unit interface also signals to instruction cache unit 120 the branch mis-predict condition, and the branch address in execution stage 303. Traps, when taken, are also similarly indicated to instruction cache unit 120. Instruction cache unit 120 can hold integer unit 142 by not asserting any of the valid bits to instruction decode unit 130. Instruction decode unit 130 can hold instruction cache unit 120 by not asserting the shift signal to byte aligner circuit 122. The data cache interface of integer execution unit IEU also is a valid-accept interface, where integer unit 142 signals, in execution stage 303, a load or store operation along with its attributes, e.g., non-cached, special stores etc., to data cache controller 161 in data cache unit 160. Data cache unit 160 can return the data on a load, and control integer unit 142 using a data control unit hold signal. On a data cache hit, data cache unit 160 returns the requested data, and then releases the pipeline. On store operations, integer unit 142 also supplies the data along with the address in execution stage 303. Data cache unit 165 can hold the pipeline in cache stage 304 if data cache unit 165 is busy, e.g., doing a line fill etc. Floating point operations are dealt with specially by integer execution unit IEU. Instruction decoder 135 fetches and decodes floating point unit 143 related instructions. Instruction decoder 135 sends the floating point operation operands for execution to floating point unit 142 in decode state 302. While floating point unit 143 is busy executing the floating point operation, integer unit 142 halts the pipeline and waits until floating point unit 143 signals to integer unit 142 that the result is available. A floating point ready signal from floating point unit 143 indicates that execution stage 303 of the floating point operation has concluded. In response to the floating point ready signal, the result is written back into stack cache 155 by integer unit 142. Floating point load and stores are entirely handled by integer execution unit IEU, since the operands for both floating point unit 143 and integer unit 142 are found in stack cache 155. Stack Management Unit A stack management unit 150 stores information, and provides operands to execution unit 140. Stack management unit 150 also takes care of overflow and underflow conditions of stack cache 155. In one embodiment, stack management unit 150 includes stack cache 155 that, as described above, is a three read port, two write port register file in one embodiment; a stack control unit 152 which provides the necessary control signals for two read ports and one write port that are used to retrieve operands for execution unit 140 and for storing data back from a write-back register or data cache 165 into stack cache 155; and a dribble manager 151 which speculatively dribbles data in and out of stack cache 155 into memory whenever there is an overflow or underflow in stack cache 155. In the exemplary embodiment of Figure 1 , memory includes data cache 165 and any memory storage interfaced by memory interface unit 1 10. In general, memory includes any suitable memory hierarchy including caches, addressable read/write memory storage, secondary storage, etc. Dribble manager 151 also provides the necessary control signals for a single read port and a single write port of stack cache 155 which are used exclusively for background dribbling puφoses. In one embodiment, stack cache 155 is managed as a circular buffer which ensures that the stack grows and shrinks in a predictable manner to avoid overflows or overwrites. The saving and restoring of values to and from data cache 165 is controlled by dribbler manager 151 using high- and low-water marks, in one embodiment. Stack management unit 150 provides execution unit 140 with two 32-bit operands in a given cycle. Stack management unit 150 can store a single 32-bit result in a given cycle. Dribble manager 151 handles spills and fills of stack cache 155 by speculatively dribbling the data in and out of stack cache 1 5 from and to data cache 165. Dribble manager 151 generates a pipeline stall signal to stall the pipeline when a stack overflow or underflow condition is detected. Dribble manager 151 also keeps track of requests sent to data cache unit 160. A single request to data cache unit 160 is a 32-bit consecutive load or store request. The hardware organization of stack cache 155 is such that, except for long operands (long integers and double precision floating-point numbers), implicit operand fetches for opcodes do not add latency to the execution of the opcodes. The number of entries in operand stack 423 (Fig. 4A) and local variable storage 422 that are maintained in stack cache 155 represents a hardware/performance tradeoff. At least a few operand stack 423 and local variable storage 422 entries are required to get good performance. In the exemplary embodiment of Figure 1, at least the top three entries of operand stack 423 and the first four local variable storage 422 entries are preferably represented in stack cache 155. One key function provided by stack cache 155 (Fig. 1) is to emulate a register file where access to the top two registers is always possible without extra cycles. A small hardware stack is sufficient if the proper intelligence is provided to load/store values from/to memory in the background, therefore preparing stack cache 155 for incoming virtual machine instructions. As indicated above, all items on stack 400 (regardless of size) are placed into a 32-bit word. This tends to waste space if many small data items are used, but it also keeps things relatively simple and free of lots of tagging or muxing. An entry in stack 400 thus represents a value and not a number of bytes. Long integer and double precision floating-point numbers require two entries. To keep the number of read and write ports low, two cycles to read two long integers or two double precision floating point numbers are required. The mechanism for filling and spilling the operand stack from stack cache 155 out to memory by dribble manager 151 can assume one of several alternative forms. One register at a time can be filled or spilled, or a block of several registers filled or spilled at once. A simple scoreboarded method is appropriate for stack management. In its simplest form, a single bit indicates if the register in stack cache 155 is currently valid. In addition, some embodiments of stack cache 155 use a single bit to indicate whether the data content of the register is saved to stack 400, i.e., whether the register is dirty. In one embodiment, a high-water mark/low-water mark heuristic determines when entries are saved to and restored from stack 400, respectively (Fig. 4A). Alternatively, when the top-of-the-stack becomes close to bottom 401 of stack cache 155 by a fixed, or alternatively, a programmable number of entries, the hardware starts loading registers from stack 400 into stack cache 155. For other embodiments of stack management unit 150 and dribble manager unit 151 see U.S. Patent Application Serial No. 08/xxx,xxx, entitled "METHODS AND APPARATI FOR STACK CACHING" naming Marc Tremblay and James Michael O'Connor as inventors, assigned to tine assignee of this application, and filed on even date herewith with Attorney Docket No. SP2037, which is incoφorated herein by reference in its entirety, and see also U.S. Patent Application Serial No. 08/xxx,xxx, entitled "METHOD FRAME STORAGE USING MULTIPLE MEMORY CIRCUITS" naming Marc Tremblay and James Michael O'Connor as inventors, assigned to the assignee of this application, and filed on even date herewith with Attorney Docket No. SP2038, which also is incoφorated herein by reference in its entirety. In one embodiment, stack management unit 150 also includes an optional local variable look-aside cache 153. Cache 153 is most important in applications where both the local variables and operand stack 423 (Fig. 4 A) for a method are not located on stack cache 155. In such instances when cache 153 is not included in hardware processor 100, there is a miss on stack cache 155 when a local variable is accessed, and execution unit 140 accesses data cache unit 160, which in turn slows down execution. In contrast, with cache 153, the local variable is retrieved from cache 153 and there is no delay in execution. One embodiment of local variable look-aside cache 153 is illustrated in Figure 4D for method 0 to 2 on stack 400. Local variables zero to M, where M is an integer, for method 0 are stored in plane 421A 0 of cache 153 and plane 421A 0 is accessed when method number 402 is zero. Local variables zero to N, where N is an integer, for metiiod 1 are stored in plane 421 A l of cache 153 and plane 421 A l is accessed when ethod number 402 is one. Local variables zero to P, where P is an integer, for method 1 are stored in plane 421 A_2 of cache 153 and plane 421 A_2 is accessed when method number 402 is two. Notice that the various planes of cache 153 may be different sizes, but typically each plane of the cache has a fixed size that is empirically determined. When a new method is invoked, e.g., method 2, a new plane 421 A_2 in cache 153 is loaded with the local variables for that method, and method number register 402, which in one embodiment is a counter, is changed, e.g., incremented, to point to the plane of cache 153 containing the local variables for the new method. Notice that the local variables are ordered within a plane of cache 153 so that cache 153 is effectively a direct-mapped cache. Thus, when a local variable is needed for the current method, the variable is accessed directly from the most recent plane in cache 153, i.e., the plane identified by method number 402. When the current method returns, e.g., method 2, method number register 402 is changed, e.g., decremented, to point at previous plane 421 A- 1 of cache 153. Cache 153 can be made as wide and as deep as necessary. Data Cache Unit Data cache unit 160 (DCU) manages all requests for data in data cache 165. Data cache requests can come from dribbling manager 151 or execution unit 140. Data cache controller 161 arbitrates between these requests giving priority to the execution unit requests. In response to a request, data cache controller 161 generates address, data and control signals for the data and tags RAMs in data cache 165. For a data cache hit, data cache controller 161 reorders the data RAM output to provide the right data. Data cache controller 161 also generates requests to I O bus and memory interface unit 110 in case of data cache misses, and in case of non-cacheable loads and stores. Data cache controller 161 provides the data path and control logic for processing non-cacheable requests, and the data path and data path control functions for handling cache misses. For data cache hits, data cache unit 160 returns data to execution unit 140 in one cycle for loads. Data cache unit 160 also takes one cycle for write hits. In case of a cache miss, data cache unit 160 stalls the pipeline until the requested data is available from the external memory. For both non-cacheable loads and stores, data cache 161 is bypassed and requests are sent to I/O bus and memory interface unit 110. Non- aligned loads and stores to data cache 165 trap in software. Data cache 165 is a two-way set associative, write back, write allocate, 16-byte line cache. The cache size is configurable to 0, 1, 2, 4, 8, 16 Kbyte sizes. The default size is 8 Kbytes. Each line has a cache tag store entry associated with the line. On a cache miss, 16 bytes of data are written into cache 165 from external memory. Each data cache tag contains a 20-bit address tag field, one valid bit, and one dirty bit. Each cache tag is also associated with a least recently used bit that is used for replacement policy. To support multiple cache sizes, the width of the tag fields also can be varied. If a cache enable bit in processor service register is not set, loads and stores are treated like non-cacheable instructions by data cache controller 161. A single sixteen-byte write back buffer is provided for writing back dirty cache lines which need to be replaced. Data cache unit 160 can provide a maximum of four bytes on a read and a maximum of four bytes of data can be written into cache 161 in a single cycle. Diagnostic reads and writes can be done on the caches. Memory Allocation Accelerator In one embodiment, data cache unit 165 includes a memory allocation accelerator 166. Typically, when a new object is created, fields for the object are fetched from external memory, stored in data cache 165 and then the field is cleared to zero. This is a time consuming process that is eliminated by memory allocation accelerator 166. When a new object is created, no fields are retrieved from external memory. Rather, memory allocation accelerator 160 simply stores a line of zeros in data cache 165 and marks that line of data cache 165 as dirty. Memory allocation accelerator 166 is particularly advantageous with a write-back cache. Since memory allocation accelerator 166 eliminates the external memory access each time a new object is created, the performance of hardware processor 100 is enhanced. Floating Point Unit Floating point unit (FPU) 143 includes a microcode sequencer, input/output section with input/output registers, a floating point adder, i.e., an ALU, and a floating point multiply/divide unit. The microcode sequencer controls the microcode flow and microcode branches. The input output section provides the control for input/output data transactions, and provides the input data loading and output data unloading registers. These registers also provide intermediate result storage. The floating point adder-ALU includes die combinatorial logic used to perform the floating point adds, floating point subtracts, and conversion operations. The floating point multiply/divide unit contains the hardware for performing multiply/divide and remainder. Floating point unit 143 is organized as a microcoded engine with a 32-bit data path. This data path is often reused many times during the computation of the result. Double precision operations require approximately two to four times the number of cycles as single precision operations. The floating point ready signal is asserted one-cycle prior to the completion of a given floating point operation. This allows integer unit 142 to read the floating point unit output registers without any wasted interface cycles. Thus, output data is available for reading one cycle after the floating point ready signal is asserted. Execution Unit Accelerators Since the JAVA Virtual Machine Specification of Appendix I is hardware independent, the virtual machine instructions are not optimized for a particular general type of processor, e.g., a complex instruction set computer (CISC) processor, or a reduced instruction set computer (RISC) processor. In fact, some virtual machine instructions have a CISC nature and others a RISC nature. This dual nature complicates the operation and optimization of hardware processor 100. For example, the JAVA virtual machine specification defines opcode 171 for an instruction lookupswitch, which is a traditional switch statement. The data stream to instruction cache unit 120 includes an opcode 171, identifying the N-way switch statement, that is followed zero to three bytes of padding. The number of bytes of padding is selected so that first operand byte begins at an address that is a multiple of four. Herein, datastream is used generically to indicate information that is provided to a particular element, block, component, or unit. Following the padding bytes in the datastream are a series of pairs of signed four-byte quantities. The first pair is special. A first operand in the first pair is the default offset for the switch statement that is used when the argument, referred to as an integer key, or alternatively, a current match value, of the switch statement is not equal to any of the values of the matches in the switch statement. The second operand in the first pair defines the number of pairs that follow in the datastream. Each subsequent operand pair in the datastream has a first operand that is a match value, and a second operand that is an offset. If the integer key is equal to one of the match values, the offset in the pair is added to the address of the switch statement to define the address to which execution branches. Conversely, if the integer key is unequal to any of the match values, the default offset in the first pair is added to the address of the switch statement to define the address to which execution branches. Direct execution of this virtual machine instruction requires many cycles. To enhance the performance of hardware processor 100, a look-up switch accelerator 145 is included in hardware processor 100. Look-up switch accelerator 145 includes an associative memory which stores information associated with one or more lookup switch statements. For each lookup switch statement, i.e., each instruction lookupswitch, this information includes a lookup switch identifier value, i.e., the program counter value associated with the lookup switch statement, a plurality of match values and a corresponding plurality of jump offset values. Lookup switch accelerator 145 determines whether a current instruction received by hardware processor 100 corresponds to a lookup switch statement stored in the associative memory. Lookup switch accelerator 145 further determines whether a current match value associated with the current instruction corresponds with one of the match values stored in the associative memory. Lookup switch accelerator 145 accesses a jump offset value from the associative memory when the current instruction corresponds to a lookup switch statement stored in the memory and the current match value corresponds with one of the match values stored in the memory wherein the accessed jump offset value corresponds with the current match value. Lookup switch accelerator 145 further includes circuitry for retrieving match and jump offset values associated with a current lookup switch statement when the associative memory does not already contain the match and jump offset values associated with the current lookup switch statement. Lookup switch accelerator 145 is described in more detail in U.S. Patent Application Serial No. 08/xxx,xxx, entitled "LOOK¬ UP SWITCH ACCELERATOR AND METHOD OF OPERATING SAME" naming Marc Tremblay and James Michael O'Connor as inventors, assigned to the assignee of this application, and filed on even date herewith with Attorney Docket No. SP2040, which is incoφorated herein by reference in its entirety. In the process of initiating execution of a method of an object, execution unit 140 accesses a method vector to retrieve one of the method pointers in the method vector, i.e., one level of indirection. Execution unit 140 then uses the accessed method pointer to access a corresponding method, i.e., a second level of indirection. To reduce the levels of indirection within execution unit 140, each object is provided with a dedicated copy of each of the methods to be accessed by the object. Execution unit 140 then accesses the methods using a single level of indirection. That is, each method is directly accessed by a pointer which is derived from the object. This eliminates a level of indirection which was previously introduced by the method pointers. By reducing the levels of indirection, the operation of execution unit 140 can be accelerated. The acceleration of execution unit 140 by reducing the levels of indirection experienced by execution unit 140 is described in more detail in U.S. Patent Application Serial No. 08/xxx,xxx, entitled "REPLICATING CODE TO ELIMINATE A LEVEL OF INDIRECTION DURING EXECUTION OF AN OBJECT ORIENTED COMPUTER PROGRAM" naming Marc Tremblay and James Michael O'Connor as inventors, assigned to the assignee of this application, and filed on even date herewith with Attorney Docket No. SP2043, which is incoφorated herein by reference in its entirety. Getfield-putfield Accelerator Other specific functional units and various translation lookaside buffer (TLB) types of structures may optionally be included in hardware processor 100 to accelerate accesses to the constant pool. For example, the JAVA virtual machine specification defines an instruction put field, opcode 181, that upon execution sets a field in an object and an instruction getfield, opcode 180, that upon execution fetches a field from an object. In both of these instructions, the opcode is followed by an index byte one and an index byte two. Operand stack 423 contains a reference to an object followed by a value for instruction putfield, but only a reference to an object for instruction getfield. Index bytes one and two are used to generate an index into the constant pool of the current class. The item in the constant pool at that index is a field reference to a class name and a field name. The item is resolved to a field block pointer which has both the field width, in bytes, and the field offset, in bytes. An optional getfield- putfield accelerator 146 in execution unit 140 stores the field block pointer for instruction getfield or instruction putfield in a cache, for use after the first invocation of the instruction, along with the index used to identify the item in the constant pool that was resolved into the field block pointer as a tag. Subsequently, execution unit 140 uses index bytes one and two to generate the index and supplies the index to getfield-putfield accelerator 146. If the index matches one of the indexes stored as a tag, i.e., there is a hit, the field block pointer associated with that tag is retrieved and used by execution unit 140. Conversely, if a match is not found, execution unit 140 performs the operations described above. Getfield-putfield accelerator 146 is implemented without using self-modifying code that was used in one embodiment of the quick instruction translation described above. In one embodiment, getfield-putfield accelerator 146 includes an associative memory that has a first section that holds the indices that function as tags, and a second section that holds the field block pointers. When an index is applied through an input section to the first section of the associative memory, and there is a match with one of the stored indices, the field block pointer associated with the stored index that matched in input index is ouφut from the second section of the associative memory. Bounds Check Unit Bounds check unit 147 (Fig. 1) in execution unit 140 is an optional hardware circuit that checks each access to an element of an array to determine whether the access is to a location within the array. When the access is to a location outside the array, bounds check unit 147 issues an active array bound exception signal to execution unit 140. In response to the active array bound exception signal, execution unit 140 initiates execution of an exception handler stored in microcode ROM 141 that in handles the out of bounds array access. In one embodiment, bounds check unit 147 includes an associative memory element in which is stored a array identifier for an array, e.g., a program counter value, and a maximum value and a minimum value for the array. When an array is accessed, i.e., the array identifier for that array is applied to the associative memory element, and assuming the array is represented in the associative memory element, the stored minimum value is as a first input signal to a first comparator element, sometimes called a comparison element, and the stored maximum value is a first input signal to a second comparator element, sometimes called a comparison element. A second input signal to the first and second comparator elements is the value associated with the access of the array's element. If the value associated with the access of the array's element is less than or equal to the stored maximum value and greater than or equal to the stored minimum value, neither comparator element generates an output signal. However, if either of these conditions is false, the appropriate comparator element generates the active array bound exception signal. A more detailed description of one embodiment of bounds check unit 147 is provided in U.S. Patent Application Serial No. 08/xxx,xxx, entitled "PROCESSOR WITH ACCELERATED ARRAY ACCESS BOUNDS CHECKING" naming Marc Tremblay, James Michael O'Connor, and William N. Joy as inventors, assigned to the assignee of this application, and filed on even date herewith with Attorney Docket No. SP2041 which is incoφorated herein by reference in its entirety. The JAVA Virtual Machine Specification defines that certain instructions can cause certain exceptions. The checks for these exception conditions are implemented, and a hardware/software mechanism for dealing with them is provided in hardware processor 100 by information in microcode ROM 149 and program counter and trap control logic 170. The alternatives include having a trap vector style or a single trap target and pushing the trap type on the stack so that the dedicated trap handler routine determines the appropriate action. No external cache is required for the architecture of hardware processor 100. No translation lookaside buffers need be supported. Figure 5 illustrates several possible add-ons to hardware processor 100 to create a unique system. Circuits supporting any of the eight functions shown, i.e., NTSC encoder 501 , MPEG 502, Ethernet controller 503, VIS 504, ISDN 505, I/O controller 506, ATM assembly/reassembly 507, and radio link 508 can be integrated into the same chip as hardware processor 100 of this invention. While the invention has been described with reference to various embodiments, it will be understood that these embodiments are illustrative and that the scope of the invention is not limited to them. Claim terms such as first instruction, second instruction, third instruction, etc. are for identification only and should not be construed to require a particular ordering of instructions. Many variations, modifications, additions, and improvements of the embodiments described are possible. For example, although the present invention has been herein described with reference to exemplary embodiments relating to the JAVA programming language and JAVA virtual machine, it is not limited to them and, instead, encompasses systems, articles, methods, and apparati for a wide variety of stack machine environments (both virtual and physical). In addition, although certain exemplary embodiments have been described in terms of hardware, suitable virtual machine implementations (JAVA related or otherwise) incoφorating instruction folding in accordance with the above description include software providing a instruction folding bytecode inteφreter, a just-in-time (JIT) compiler producing folded operations in object code native to a particular machine architecture, and instruction folding hardware implementing the virtual machine. These and other variations, modifications, additions, and improvements may fall within the scope of the invention as defined by the claims which follow. APPE DIX I The JAVA Virtual Machine Specification ® 1993, 1994, 1995 Sun Microsystems, Inc. 2550 Garcia Avenue, Mountain View, California 94043-1100 U.S.A. All rights reserved. This BETA quality release and related documentation are protected by copyright and distributed under licenses restricting its use, copying, distribution, and decompilation. No part of this release or related documentation may be reproduced in any form by any means without prior written authorization of Sun and its licensors, if any. Portions of this product may be derived from the UNIX ® and Berkeley 4.3 BSD systems, licensed from UNIX System Laboratories, Inc. and the University of California, respectively. Third-party font software in this release is protected by copyright and licensed from Sun's Font Suppliers. RESTRICTED RIGHTS LEGEND: Use, duplication, or disclosure by the United States Government is subject to the restrictions set forth in DFARS 252.227-7013 (c) (1) (ii) and FAR 52.227-19. The release described in this manual may be protected by one or more U.S. patents, foreign patents, or pending applications. TRADEMARKS Sun, Sun Microsystems, Sun Microsystems Computer Corporation, the Sun logo, the Sun Microsystems Computer Corporation logo, WebRunner, JAVA, FirstPerson and the FirstPerson logo and agent are trademarks or registered trademarks of Sun Microsystems, Inc. The "Duke" character is a trademark of Sun Microsystems, Inc. and Copyright (c) 1992-1995 Sun Microsystems, Inc. All Rights Reserved. UNIX ® is a registered trademark in the United States and other countries, exclusively licensed through X/Open Company, Ltd. OPEN LOOK is a registered trademark of Novell, Inc. All other product names mentioned herein are the trademarks of their respective owners. All SPARC trademarks, including the SCD Compliant Logo, are trademarks or registered trademarks of SPARC International, Inc. SPARCstation, SPARCserver, SPARCengine, SPARCworks, and SPARCompiler are licensed exclusively to Sun Microsystems, Inc. Products bearing SPARC trademarks are based upon an architecture developed by Sun Microsystems, Inc. The OPEN LOOK ® and Sunδ Graphical User Interfaces were developed by Sun Microsystems, Inc. for its users and licensees. Sun acknowledges the pioneering efforts of Xerox in researching and developing the concept of visual or graphical user interfaces for the computer industry. Sun holds a non-exclusive license from Xerox to the Xerox Graphical User Interface, which license also covers Sun's licensees who implement OPEN LOOK GUIs and otherwise comply with Sun's written license agreements. X Window System is a trademark and product of the Massachusetts Institute of Technology. THIS PUBLICATION IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESS OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, OR NON-INFRINGEMENT. THIS PUBLICATION COULD INCLUDE TECHNICAL INACCURACIES OR TYPOGRAPHICAL ERRORS. CHANGES ARE PERIODICALLY ADDED TO THE INFORMATION HEREIN; THESE CHANGES WILL BE INCORPORATED IN NEW EDITIONS OF THE PUBLICATION. SUN MICROSYSTEMS, INC. MAY MAKE IMPROVEMENTS AND/OR CHANGES IN THE PRODUCT(S) AND/OR THE PROGRAM(S) DESCRIBED IN THIS PUBLICATION AT ANY TIME. Preface This document describes version 1.0 of the JAVA Virtual Machine and its instruction set. We have written this document to act as a specification for both compiler writers, who wish to target the machine, and as a speci ication for others who may wish to implement a compliant JAVA Virtual Machine. The JAVA Virtual Machine is an imaginary machine that is implemented by emulating it in software on a real machine. Code for the JAVA Virtual Machine is stored in .class files, each of which contains the code for at most one public class. Simple and efficient emulations of the JAVA Virtual Machine are possible because the machine's format is compact and efficient bytecodes. Implementations whose native code speed approximates that of compiled C are also possible, by translating the bytecodes to machine code, although Sun has not released such implementations at this time. The rest of this document is structured as follows : Chapter 1 describes the architecture of the JAVA Virtual Machine; Chapter 2 describes the .class file format; Chapter 3 describes the bytecodes; and Appendix A contains some instructions generatedinternally by Sun's implementation of the JAVA Virtual Machine. While not strictly part of the specification we describe these here so that this specification can serve as a reference for our implementation. As more implementations of the JAVA Virtual Machine become available, we may remove Appendix A from future releases. Sun will license the JAVA Virtual Machine trademark and logo for use with compliant implementations of this specification. If you are considering constructing your own implementation of the JAVA Virtual Machine please contact us, at the email address below, so that we can work together to insure 100% compatibility of your implementation. Send comments on this specification or questions about implementing the JAVA Virtual Machine to our electronic mail address: AVA®JAVA.sun.com. 1. JAVA Virtual Machine Architecture 1.1 Supported Data Types The virtual machine data types include the basic data types of the JAVA language: byte // 1-byte signed 2's complement integer short // 2-byte signed 2's complement integer int // 4-byte signed 2's complement integer long // 8-byte signed 2's complement integer float // 4-byte IEEE 754 single-precision float double // 8-byte IEEE 754 double-precision float char // 2-byte unsigned Unicode character Nearly all JAVA type checking is done at compile time. Data of the primitive types shown above need not be tagged by the hardware to allow execution of JAVA. Instead, the bytecodes that operate on primitive values indicate the types of the operands so that, for example, the iadd, Iadd, fadd, and dadd instructions each add two numbers, whose types are int, long, float, and double, respectively The virtual machine doesn't have separate instructions for boolean types. Instead, integer instructions, including integer returns, are used to operate on boolean values; byte arrays are used for arrays of boolean. The virtual machine specifies that floating point be done in IEEE 754 format, with support for gradual underflow. Older computer architectures that do not have support for IEEE format may run JAVA numeric programs very slowly. Other virtual machine data types include: object // 4-byte reference to a JAVA object returnAddress // 4 bytes, used with jsr/ret/jsr_w/ret_w instructions Note: JAVA arrays are treated as objects. This specification does not require any particular internal structure for objects. In our implementation an object reference is to a handle, which is a pair of pointers: one to a method table for the object, and the other to the data allocated for the object. Other implementations may use inline caching, rather than method table dispatch; such methods are likely to be faster on hardware that is emerging between now and the year 2000. Programs represented by JAVA Virtual Machine bytecodes are expected to maintain proper type discipline and an implementation may refuse to execute a bytecode program that appears to violate such type discipline. While the JAVA virtual Machines would appear to be limited by the bytecode deonition to running on a 32-bit address space machine, it is possible to build a version of the JAVA Virtual Machine that automatically translates the bytecodes into a 64-bit form. A description of this transformation is beyond the scope of the JAVA Virtual Machine Specification. 1.2 Registers At any point the virtual machine is executing the code of a single method, and the pc register contains the address of the next bytecode to be executed. Each method has memory space allocated for it to hold: a set of local variables, referenced by a vars register; an operand stack, referenced by an optop register; and a execution environment structure, referenced by a frame register. All of this space can be allocated at once, since the size of the local variables and operand stack are known at compile time, and the size of the execution environment structure is well-known to the interpreter. All of these registers are 32 bits wide. 1.3 Local Variables Each JAVA method uses a fixed-sized set of local variables. They are addressed as word offsets from the vars register. Local variables are all 32 bits wide. Long integers and double precision floats are considered to take up two local variables but are addressed by the index of the first local variable. (For example, a local variable with index containing a double precision float actually occupies storage at indices n and n+1. ) The virtual machine specification does not require 64-bit values in local variables to be 64-bit aligned. Implementors are free to decide the appropriate way to divide long integers and double precision floats into two words. Instructions are provided to load the values of local variables onto the operand stack and store values from the operand stack into local variables. 1.4 The Operand Stack The machine instructions all take operands from an operand stack, operate on them, and return results to the stack. We chose a stack organization so that it would be easy to emulate the machine efficiently on machines with few or irregular registers such as the Intel 486 microprocessor. The operand stack is 32 bits wide. It is used to pass parameters to methods and receive method results, as well as to supply parameters for operations and save operation results. For example, execution of instruction iadd adds two integers together. It expects that the two integers are the top two words on the operand stack, and were pushed there by previous instructions. Both integers are popped from the stack, added, and their sum pushed back onto the operand stack. Subcomputations may be nested on the operand stack, and result in a single operand that can be used by the nesting computation. Each primitive data type has specialized instructions that know how to operate on operands of that type. Each operand requires a single location on the stack, except for long and double operands, which require two locations. Operands must be operated on by operators appropriate to their type. It is illegal, for example, to push two integers and then treat them as a long. This restriction is enforced, in the Sun implementation, by the bytecode verifier. However, a small number of operations (the dup opcodes and swap) operate on runtime data areas as raw values of a given width without regard to type. In our description of the virtual machine instructions below, the effect of an instruction's execution on the operand stack is represented textually, with the stack growing from left to right, and each 32-bit word separately represented. Thus: Stack: ..., valuel, value2 X> ..., value3 shows an operation that begins by having value2 on top of the stack with valuel just beneath it. As a result of the execution of the instruction, valuel and value2 are popped from the stack and replaced by value3, which has been calculated by the instruction. The remainder of the stack, represented by an ellipsis, is unaffected by the instruction's execution. The types long and double take two 32-bit words on the operand stack: Stack: ... J» ..., value-wordl,value-word2 This specification does not say how the two words are selected from the 64-bit long or double value; it is only necessary that a particular implementation be internally consistent. 1.5 Execution Environment The information contained in the execution environment is used to do dynamic linking, normal method returns, and exception propagation. 1.5.1 Dynamic Linking The execution environment contains references to the interpreter symbol table for the current method and current class, in support of dynamic linking of the method code. The class file code for a method refers to methods to be called and variables to be accessed symbolically. Dynamic linking translates these symbolic method calls into actual method calls, loading classes as necessary to resolve as-yet-undefined symbols, and translates variable accesses into appropriate offsets in storage structures associated with the runtime location of these variables. This late binding of the methods and variables makes changes in other classes that a method uses less likely to break this code. 1.5.2 Normal Method Returns If execution of the current method completes normally, then a value is returned to the calling method. This occurs when the calling method executes a return instruction appropriate to the return type. The execution environment is used in this case to restore the registers of the caller, with the program counter of the caller appropriately incremented to skip the method call instruction. Execution then continues in the calling method's execution environment. 1.5.3 Exception and Error Propagation An exceptional condition, known in JAVA as an Error or Exception, which are subclasses of Throwable, may arise in a program because of: a dynamic linkage failure, such as a failure to find a needed class file; a run-time error, such as a reference through a null pointer; an asynchronous event, such as is thrown by Thread.stop, from another thread; and the program using a throw statement. When an exception occurs: A list of catch clauses associated with the current method is examined. Each catch clause describes the instruction range for which it is active, describes the type of exception that it is to handle, and has the address of the code to handle it An exception matches a catch clause if the instruction that caused the exception is in the appropriate instruction range, and the exception type is a subtype of the type of exception that the catch clause handles If a matching catch clause is found, the system branches to the specified handler If no handler is found, the process is repeated until all the nested catch clauses of the current method have been exhausted The order of the catch clauses in the list is important The virtual machine execution continues at the first matching catch clause Because JAVA code is structured, it is always possible to sort all the exception handlers for one method into a single list that, for any possible program counter value, can be searched in linear order to find the proper (innermost containing applicable) exception handler for an exception occuring at that program counter value. If there is no matching catch clause then the current method is said to have as its outcome the uncaught exception The execution state of the method that called this method is restored from the execution environment, and the propagation of the exception continues, as though the exception had just occurred in this caller 1.5.4 Additional Information The execution environment may be extended with additional implementation-specified information, such as debugging information. 1.6 Garbage Collected Heap The JAVA heap is the runtime data area from which class instances (objects) are allocated. The JAVA language is designed to be garbage collected - it does not give the programmer the ability to deallocate objects explicitly. The JAVA language does not presuppose any particular kind of garbage collection; various algorithms may be used depending on system requirements. 1.7 Method Area The method area is analogous to the store for compiled code in conventional languages or the text segment in a UNIX process It stores method code (compiled JAVA code) and symbol tables. In the current JAVA implementation, method code is not part of the garbage-collected heap, although this is planned for a future release. 1.8 The JAVA Instruction Set An instruction in the JAVA instruction set consists of a one-byte opcode specifying the operation to be performed, and zero or more operands supplying parameters or data that will be used by the operation. Many instructions have no operands and consist only of an opcode. The inner loop of the virtual machine execution is effectively: do { fetch an opcode byte execute an action depending on the value of the opcode } while (there is more to do) ; The number and size of the additional operands is determined by the opcode. If an additional operand is more than one byte in size, then it is stored in big-endian order - high order byte first. For example, a 16-bit parameter is stored as two bytes whose value is: first byte * 256 + second_byte The bytecode instruction stream is only byte-aligned, with the exception being the tableswitch and lookupswitch. instructions, which force alignment to a 4-byte boundary within their instructions. These decisions keep the virtual machine code for a compiled JAVA program compact and reflect a conscious bias in favor of compactness at some possible cost in performance. 1.9 Lixαitations The per-class constant pool has a maximum of 65535 entries. This acts as an internal limit on the total complexity of a single class. The amount of code per method is limited to 65535 bytes by the sizes of the indices in the code in the exception table, the line number table, and the local variable table. Besides this limit, the only other limitation of note is that the number of words of arguments in a method call is limited to 255. Class File Format This chapter documents the JAVA class (.class) file format. Each class file contains the compiled version of either a JAVA class or a JAVA interface. Compliant JAVA interpreters must be capable of dealing with all class files that conform to the following specification. A JAVA class file consists of a stream of 8-bit bytes. All 16-bit and 32-bit quantities are constructed by reading in two or four 8-bit bytes, respectively. The bytes are joined together in network (big-endian) order, where the high bytes come first. This format is supported by the JAVA JAVA.io.Datalnput and JAVA.io.DataOutput interfaces, and classes such as JAVA.io.DatalnputStream and JAVA.io.DataOutputStream. The class file format is described here using a structure notation. Successive fields in the structure appear in the external representation without padding or alignment. Variable size arrays, often of variable sized elements, are called tables and are commonplace in these structures. The types ul, u2, and u4 mean an unsigned one-, two-, or four-byte quantity, respectively, which are read by method such as readUnsignedByte, readUnsignedShort and readlnt of the JAVA.io.Datalnput interface. .1 Format The following pseudo-structure gives a top-level description of the format of a class file: ClassFile { u4 magic; u2 minor_version; u2 major_version; u2 constant_pool_count; cp_info cons ant_pool [constant_jpool_count - 1] ; u2 access_flags; u2 this_class; u2 super_class; u2 interfaces_count; u2 interfaces [interfaces_count] ; u2 fields_count; field_info fields[fields_count] ; u2 methods_count; method_info methods [methods_count] ; u2 attributes_count; attribute_info attributes [attribute_count] ,- } magic This field must have the value OxCAFEBABE. minor_version, major_version These fields contain the version number of the JAVA compiler that produced this class file. An implementation of the virtual machine will normally support some range of minor version numbers 0-n of a particular major version number. If the minor version number is incremented the new code won't run on the old virtual machines, but it is possible to make a new virtual machine which can run versions up to n+1. A change of the major version number indicates a major incompatible change, one that requires a different virtual machine that may not support the old major version in any way. The current major version number is 45; the current minor version number is 3. constant pool count This field indicates the number of entries in the constant pool in the class file. constant_pool The constant pool is a table of values. These values are the various string constants, class names, field names, and others that are referred to by the class structure or by the code. constant_j?ool [0] is always unused by the compiler, and may be used by an implementation for any purpose. Each of the constant_pool entries 1 through constant_pool_count-l is a variable-length entry, whose format is given by the first "tag" byte, as described in section 2.3. access flagβ This field contains a mask of up to sixteen modifiers used with class, method, and field declarations. The same encoding is used on similar fields in field_info and method_info as described below. Here is the encoding: thiβ_class This field is an index into the constant pool; constant_pool [this_class] must be a CONSTANT_class. super class This field is an index into the constant pool. If the value of super_class is nonzero, then constant_pool [super_class] must be a class, and gives the index of this class's superclass in the constant pool. If the value of super_class is zero, then the class being defined must be JAVA.lang.Object, and it has no superclass. interfacβs count This field gives the number of interfaces that this class implements. interfaces Each value in this table is an index into the constant pool. If a table value is nonzero (interfaces[i] != 0, where 0 <= i <interfaces_count) , then constant_pool [interfaces [i]] must be an interface that this class implements. fields_count This field gives the number of instance variables, both static and dynamic, defined by this class. The fields table includes only those variables that are defined explicitly by this class. It does not include those instance variables that are accessible from this class but are inherited from superclasses. fields Each value in this table is a more complete description of a field in the class. See section 2.4 for more information on the field_info structure. methods count This field indicates the number of methods, both static and dynamic, defined by this class. This table only includes those methods that are explicitly defined by this class. It does not include inherited methods. methods Each value in this table is a more complete description of a method in the class. See section 2.5 for more information on the method_info structure. attributβs count This field indicates the number of additional attributes about this class. attributes A class can have any number of optional attributes associated with it. Currently, the only class attribute recognized is the "SourceFile" attribute, which indicates the name of the source file from which this class file was compiled. See section 2.6 for more information on the attribute info structure. 2.2 Signatures A signature is a string representing a type of a method, field or array. The field signature represents the value of an argument to a function or the value of a variable. It is a series of bytes generated by the following grammar: <field_signature> : := <field_type> <field_type> : :=<base_type> | <object_type> | <array_type> <base_type> ::= B|C|D|F|l|J|S|Z <object_type> ::= L<fullclassname>; <array_type> : := [<optional_sizexfield_type> <optional_size> ::= [0-9] The meaning of the base types is as follows: B byte signed byte C char character D double double precision IEEE float float single precision IEEE float I int integer J long long integer L<fullclassname>; an object of the given class S short signed short Z boolean true or false [<field sig> array A return-type signature represents the return value from a method. It is a series of bytes in the following grammar: <return_signature> ::= <field_type> | V The character V indicates that the method returns no value. Otherwise, the signature indicates the type of the return value. An argument signature represents an argument passed to a method: <argument_signature> ::= <field_type> A method signature represents the arguments that the method expects, and the value that it returns. <method_signature> ::= (<arguments_signature>) <return_signature> <arguments_signature> ::= <argument_signature>* 2.3 Constant Pool Each item in the constant pool begins with a 1-byte tag: . The table below lists the valid tags and their values. Each tag byte is then followed by one or more bytes giving more information about the specific constant. 2.3.1 CONSTANT Clasβ CONSTANT_Class is used to represent a class or an interface. CONSTANT_Class_info { ul tag; u2 name_index,- } tag The tag will have the value CONSTANT_Class name_index constant_pool [name_index] is a CONSTANT_Utf8 giving the string name of the class . Because arrays are objects, the opcodes anewarray and multianewarray can reference array "classes" via CONSTANT_Class items in the constant pool. In this case, the name of the class is its signature. For example, the class name for int[] [] is [[I The class name for Thread[] is " [LJAVA.lang.Thread; " 2.3.2 CONSTANT_{Fieldref.Methodref, InterfaceMethodref} Fields, methods, and interface methods are represented by similar structures. CONSTANT_Fieldref_info { ul tag; u2 class_index; u2 name_and_type_index; } CONSTANT_Mβthodref_info { ul tag; u2 class_index; u2 name_and_type_index; } CONSTANT_InterfaceMethodref_info { ul tag; u2 class_index; u2 name_and_type_index; } tag The tag will have the value CONSTANT_Fieldref, CONSTANT_Methodref, or CONSTANT_InterfaceMethodref . class index constant_jpool [class_index] will be an entry of type CONSTANT_Class giving the name of the class or interface containing the field or method. For CONSTANT_Fieldref and CONSTANT_Methodref, the CONSTANT_Class item must be an actual class. For CONSTANT_InterfaceMethodref, the item must be an interface which purports to implement the given method. name and type index constant_pool [name_and_type_index] will be an entry of type CONSTANT_NameAndType. This constant pool entry indicates the name and signature of the field or method. 2.3.3 CONSTANT String CONSTANT_String is used to represent constant objects of the built-in type String. CONSTANT_String_info { ul tag; u2 string_index; } tag The tag will have the value CONSTANT_String string index constant_pool [string_index] is a CONSTANT_Utf8 string giving the value to which the String object is initialized. 2.3.4 CθNSTANT_Intβger and CONSTANT Float CONSTANT_Integer andCONSTANT_Float represent four-byte constants. CONSTANT_Integer_info { ul tag; u4 bytes; } CONSTANT_Float_info { ul tag; u4 bytes; } tag The tag will have the value CONSTANT_Integer or CONSTANT_Float bytes For integers, the four bytes are the integer value. For floats, they are the IEEE 754 standard representation of the floating point value. These bytes are in network (high byte first) order. 2 . 3 . 5 CONSTANT_Long and CONSTANT Double CONSTANT_Long andCONSTANT_Double represent eight-byte constants. CONSTANT_Long_info { ul tag; u4 high_bytes; u4 low_bytes; } CONSTANT_Double_info { ul tag,- u4 high_bytes; u4 low_bytes; } All eight-byte constants take up two spots in the constant pool. If this is the n item in the constant pool, then the next item will be numbered n+2. tag The tag will have the value CONSTANT_Long or CONSTANT_Double. high bytβs, low_bytes For CONSTANT_Long, the 64-bit value is (high_bytes << 32) +low_bytes. For CONSTANT_Double, the 64-bit value,high_bytes and low_bytes together represent the standard IEEE 754 representation of the double-precision floating point number. 2.3.6 CONSTANT NameλndType CONSTANT_NameAndType is used to represent a field or method, without indicating which class it belongs to. CONSTANT_NameAndType_info { ul tag; u2 name_index; u2 signature_index; } tag The tag will have the valueCONSTANT_NameAndType. name index constant_pool [name_index] is a CONSTANT_Utf8 string giving the name of the field or method. signature_index constant_pool [signature_index] is a CONSTANT_Utf8 string giving the signature of the field or method. 2.3.7 CONSTANT Utfβ and CONSTANT Unicode CONSTANT_Utf8 andCONSTANT_Unicode are used to represent constant string values. CONSTANT_Utf8 strings are "encoded" so that strings containing only non-null ASCII characters, can be represented using only one byte per character, but characters of up to 16 bits can be represented: All characters in the range 0x0001 to 0x007F are represented by a single byte: |0|7bits of dataI +-+-+-+-+-+-+-+-+ The null character (0x0000) and characters in the range 0x0080 to 0x07FF are represented by a pair of two bytes: 111110 I 5 bitst j 1110 | 6 bits | Characters in the range 0x0800 to OxFFFF are represented by three bytes: +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ |l|l|l|0|4 bits I |l|0| 6 bits | |l|θ| 6 bits | There are two differences between this format and the "standard" UTF-8 format. First, the null byte (0x00) is encoded in two-byte format rather than one-byte, so that our strings never have embedded nulls. Second, only the one-byte, two-byte, and three-byte formats are used. We do not recognize the longer formats. CONSTANT_Utf8_info { ul tag,- u2 length; ul bytes [length] ; } CONSTANT_Unicode_info { ul tag; u2 length; u2 bytes [length] ; } tag The tag will have the value CONSTANT_Utf8 or CONS ANT_Unicode. length The number of bytes in the string. These strings are not null terminated. bytes The actual bytes of the string. 2.4 Fields The information for each field immediately follows the field_count field in the class file. Each field is described by a variable length field_info structure. The format of this structure is as follows: field_info { u2 access_flags; u2 name_index; u2 signature_index; u2 attributes_count; attribute_info attributes [attribute_count] ; } access flags This is a set of sixteen flags used by classes, methods, and fields to describe various properties and how they many be accessed by methods in other classes. See the table "Access Flags" which indicates the meaning of the bits in this field. The possible fields that can be set for a field are ACC_PUBLIC, ACC_PRIVATE, ACC_PROTECTED, ACC_STATIC, ACC_FINAL, ACC_VOLATILE, and ACC TRANSIENT. At most one of ACC_PUBLIC, ACC_PROTECTED, and ACC_PRIVATE can be set for any method. name_index constant_pool [name_mdex] is a CONSTANT_Utf8 string which is the name of the field. signature_index constant_pool [signature_index] is a CONSTANT_Utf8 string which is the signature of the field. See the section "Signatures" for more information on signatures. attributes_count This value indicates the number of additional attributes about this field. attributes A field can have any number of optional attributes associated with it. Currently, the only field attribute recognized is the "ConstantValue" attribute, which indicates that this field is a static numeric constant, and indicates the constant value of that field. Any other attributes are skipped. 2.5 Methods The information for each method immediately follows the method_count field in the class file. Each method is described by a variable length τnethod_info structure. The structure has the following format: method_info { u2 access_flags; u2 name_index; u2 signature_index; u2 attributes_count; attribute_info attributes [attribute count] ; } access flags This is a set of sixteen flags used by classes, methods, and fields to describe various properties and how they many be accessed by methods in other classes. See the table "Access Flags" which gives the various bits in this field. The possible fields that can be set for a method are ACC_PUBLIC, ACC_PRIVATE, ACC_PROTECTED, ACC_STATIC, ACC_FINAL, ACC_SYNCHRONIZED, ACC_NATIVE, and ACC_ABSTRACT. At most one of ACC_PUBLIC, ACC_PROTECTED, and ACC_PRIVATE can be set for any method, name index constant_pool [name_index] is a CONSTANT_Utf8 string giving the name of the method. signature index constant_pool [signature_index] is a CONSTANT_Utf8 string giving the signature of the field. See the section "Signatures" for more information on signatures. attributes count This value indicates the number of additional attributes about this field. attributes A field can have any number of optional attributes associated with it. Each attribute has a name, and other additional information. Currently, the only field attributes recognized are the "Code" and "Exceptions" attributes, which describe the bytecodes that are executed to perform this method, and the JAVA Exceptions which are declared to result from the execution of the method, respectively. Any other attributes are skipped. . Attributes Attributes are used at several different places in the class format. All attributes have the following format: GenericAttribute_info { u2 attribute_name; u4 attribute_length; ul info[attribute_length] ; } The attribute_name is a 16-bit index into the class's constant pool; the value of constant_pool [attribute_name] is a CONSTANT_Utf8 string giving the name of the attribute. The field attribute_length indicates the length of the subsequent information in bytes. This length does not include the six bytes of the attribute_name and attribute_length. In the following text, whenever we allow attributes, we give the name of the attributes that are currently understood. In the future, more attributes will be added. Class file readers are expected to skip over and ignore the information in any attribute they do not understand. 2.6.1 SourceFile The "SourceFile" attribute has the following format: SourceFile_attribute { u2 attribute_name_index; u4 attribute_length; u2 sourcefile_index; } attribute_name_index constant_pool [attribute_name_index] is the CONSTANT_Utf8 string "SourceFile" . attribute_length The length of a SourceFile_attribute must be 2. sourcefile index constant_pool [sourcefile_index] is a CONSTANT_Utf8 string giving the source file from which this class file was compiled. 2.6.2 ConβtantValue The "ConstantValue" attribute has the following format: ConstantValue_attribute { u2 attribute_name_index; u4 attribute_length; u2 constantvalue_index; } attribute_name_index constant_pool [attribute_name_index] is the CONSTANT_Utf8 string "ConstantValue" . attribute length The length of a ConstantValue_attribute must be 2. constantvalue index constant_pool [constantvalue_index] gives the constant value for this field. The constant pool entry must be of a type appropriate to the field, as shown by the following table: 2 . 6 .3 Code The "Code" attribute has the following format: Code_attribute { u2 attribute_name_index; u4 attribute_length; u2 max_stack; u2 max_locals; u4 code length; ul code[code_length] ; u2 exception_table_length; { u2 start_pc,- u2 end_pc; u2 handler_pc; u2 catch_type; } exception_table[exception_table_length] ; u2 attributes_count; attribute_info attributes [attribute_count] ; } attribute_name_index constant_pool [attribute_name_index] is the CONSTANT_Utf8 string "Code". attributβ length This field indicates the total length of the "Code" attribute, excluding the initial six bytes. max stack Maximum number of entries on the operand stack that will be used during execution of this method. See the other chapters in this spec for more information on the operand stack. max locals Number of local variable slots used by this method. See the other chapters in this spec for more information on the local variables. code length The number of bytes in the virtual machine code for this method. code These are the actual bytes of the virtual machine code that implement the method. When read into memory, if the first byte of code is aligned onto a multiple-of-four boundary the tableswitch and tablelookup opcode entries will be aligned; see their description for more information on alignment requirements. exception_table_length The number of entries in the following exception table exception table Each entry in the exception table describes one exception handler in the code. start_pc, end pc The two fιeldsstart_pc and end_pc indicate the ranges in the code at which the exception handler is active. The values of both fields are offsets from the start of the code.start_pc is inclusive.end_pc is exclusive. handler pc This field indicates the starting address of the exception handler. The value of the field is an offset from the start of the code. catch_type If catch_type is nonzero, then constant_pool [catch_type] will be the class of exceptions that this exception handler is designated to catch. This exception handler should only be called if the thrown exception is an instance of the given class. If catch_type is zero, this exception handler should be called for all exceptions. attributes count This field indicates the number of additional attributes about code. The "Code" attribute can itself have attributes. attributes A "Code" attribute can have any number of optional attributes associated with it. Each attribute has a name, and other additional information. Currently, the only code attributes defined are the "LineNumberTable" and "LocalVariableTable, " both of which contain debugging information. 2.6.4 Exceptions Table This table is used by compilers which indicate which Exceptions a method s declared to throw: Exceptιons_attrιbute { u2 attribute_name_mdex; u4 attribute_length; u2 number_of_exceptιons; u2 exceptιon_mdex_table [number_of_ex-ceptιons] ; } attribute_name_index constant_pool [attπbute_name_ιndex] will be the CONSTANT_Utf8 string "Exceptions". attribute length This field indicates the total length of the Exceptions_attrιbute, excluding the initial six bytes. number_of_exceptions This field indicates the number of entries in the following exception index table. exception index table Each value in this table is an index into the constant pool. For each table element (exception_index_table [i] 1= 0, where 0 <= i <number_of_exceptions) , then constant_pool [exceρtion_index+table [i] ] is a Exception that this class is declared to throw. 2.6.5 LineNuxαberTable This attribute is used by debuggers and the exception handler to determine which part of the virtual machine code corresponds to a given location in the source. The LineNumberTable_attribute has the following format: LineNumberTable_attribute { u2 attribute_name_index; u4 attrιbute_length; u2 line_number_table_length; { u2 star _pc; u2 line_number; } line_number_table [line_ nu ber_table_length] ; } attribute name index constant_pool [attribute_name_index] will be the CONSTANT_Utf8 string "LineNumberTable" . attribute length This field indicates the total length of the LineNumberTable_attribute, excluding the initial six bytes. line_nuιnber_table_length This field indicates the number of entries in the following line number table. line_number_table Each entry in the line number table indicates that the line number in the source file changes at a given point in the code, start pc This field indicates the place in the code at which the code for a new line in the source begins. source_pc <<SHOULD THAT BEstart _pc? > > is an offset from the beginning of the code. line number The line number that begins at the given location in the file. 2.6.6 LocalVariableTable This attribute is used by debuggers to determine the value of a given local variable during the dynamic execution of a method. The format of the LocalVariableTable_attribute is as follows: LocalVariableTable_attribute { u2 attribute_name_index; u4 attribute_length; u2 local_variable_table_length; { u2 start_pc; u2 length; u2 name_index; u2 signature_index,- u2 slot; } local_variable_table[local_ variable_table_length] ; } attribute_name_index constant_pool [attribute_name_index] will be the CONSTANT_Utf8 string "LocalVariableTable" . attribute length This field indicates the total length of the LineNumberTable_attribute, excluding the initial six bytes. local_variable_table_length This field indicates the number of entries in the following local variable table. local_variable_tablθ Each entry in the local variable table indicates a code range during which a local variable has a value. It also indicates where on the stack the value of that variable can be found. start pc, length The given local variable will have a value at the code between start_pc andstart_pc + length. The two values are both offsets from the beginning of the code, namβ indβx, βignature_index constant_pool [name_index] and constant_pool [signature_index] are CONSTANT_Utf8 strings giving the name and signature of the local variable. slot The given variable will be the slot t local variable in the method's frame. 3. The Virtual Machine Instruction Set 3.1 Format for the Instructions JAVA Virtual Machine instructions are represented in this document by an entry of the following form. instruction name Short description of the instruction Syntax: opcode=number operandl operand2 Stack: .... , valuel, value2 _> ... , value3 A longer description that explains the functions of the instruction and indicates any exceptions that might be thrown during execution. Each line in the syntax table represents a single 8-bit byte. Operations of the JAVA Virtual Machine most often take their operands from the stack and put their results back on the stack. As a convention, the descriptions do not usually mention when the stack is the source or destination of an operation, but will always mention when it is not. For instance, instruction iload has the short description "Load integer from local variable." Implicitly, the integer is loaded onto the stack. Instruction iadd is described as "Integer add"; both its source and destination are the stack. Instructions that do not affect the control flow of a computation may be assumed to always advance the virtual machine program counter to the opcode of the following instruction. Only instructions that do affect control flow will explicitly mention the effect they have on the program counter. 3.2 Pushing Constants onto the Stack bipush Push one-byte signed integer Syntax: bipush=16 bytel Stack: ...=> ..., value bytel is interpreted as a signed 8-bitvalue. This value is expanded to an integer and pushed onto the operand stack. sipush Push two-byte signed integer Syntax: sipush=17 bytel Stack: ...=> ..., item bytel and byte2 are assembled into a signed 16-bit value. This value is expanded to an integer and pushed onto the operand stack. ldcl Push item from constant pool Syntax: ldcl=18 indexbytel Stack: ...=> ..., item indexbytel is used as an unsigned 8-bit index into the constant pool of the current class. The item at that index is resolved and pushed onto the stack. If a String is being pushed and there isn't enough memory to allocate space for it then an OutOfMemoryError is thrown. Note: A String push results in a reference to an object. ldc2 Push item from constant pool Syntax: Idc2=l9 indexbytel indexbyte2 Stack: ...=> ..., item indexbytel and indexbyte2 are used to construct an unsigned 16-bit index into the constant pool of the current class. The item at that index is resolved and pushed onto the stack. If a String is being pushed and there isn't enough memory to allocate space for it then an OutOfMemoryError is thrown. Note: A String push results in a reference to an object. ldc2w Push long or double from constant pool Syntax: ldc2w=20 indexbytel indexbyte2 Stack: constant- ordl, constant-word2 indexbytel and indexbyte2 are used to construct an unsigned 16-bit index into the constant pool of the current class. The two-word constant that index is resolved and pushed onto the stack. aeonst null Push null object reference Syntax: aconst null=l Stack: ...=> ...,null Push the null object reference onto the stack. iconst_ml Push integer constant -1 Syntax: iconst ml=2 Stack: ...=> ... , 1 Push the integer -l onto the stack. iconst_<n> Push integer constant Synta : iconst ,<n> Stack: ...=> ... , <n> Forms: iconst_θ = 3, iconst_l = 4, iconst_2 5, iconst 3 = 6, iconst_4 = 7, iconst_5 = 8 Push the integer <n> onto the stack. lconst_<l> Push long integer constant Syntax: Iconst <1> Stack: ...=> ..., <l>-wordl, <l>-word2 Forms: Iconst 0 = 9, Iconst 1 = 10 Push the long integer <1> onto the stack. fconst_<f> Push single float Syntax: fconst <f> Stack: ...=> ... , <f> Forms: fconst_0 = 11, fconst_l = 12, fconst_2 = 13 Push the single-precision floating point number <f> onto the stack. dconst_<d> Push double float Syntax: dconst «cd> Stack: ...=> ..., <d>-wordl, <d>-word2 Forms: dconst_0 = 14, dconst_l = 15 Push the double-precision floating point number <d> onto the stack. 3.3 Loading Local Variables Onto the Stack Iload Load integer from local variable Syntax: iload=21 vindex Stack: ...=> ... , value The value of the local variable at vindex in the current JAVA frame is pushed onto the operand stack. iload_<n> Load integer from local variable Syntax: iload <n> Stack : . . . => . . . , value Forms: iload_0 = 26, load_l = 27,ιload_2 = 28, ιload_3 = 29 The value of the local variable at <n> in the current JAVA frame is pushed onto the operand stack. This instruction is the same as iload with a vindex of <n>, except that the operand <n> s implicit. iload Load long integer from local variable Syntax: iload = 22 vindex Stack: ... => ..., value-wordl, value-work2 The value of the local variables at vindex and vindex+1 in the current JAVA frame is pushed onto the operand stack. lload_<n> Load long integer from local variable Syntax: iload <n> Stack: ...=> ..., value-wordl, value-word2 Forms: lload_0 = 30, lload_l = 31, lload_2 = 32, lload_3 = 33 The value of the local variables at <n> and <n>+l in the current JAVA frame is pushed onto the operand stack. This instruction is the same as Iload with a vindex of <n>, except that the operand <n> is implicit. fload Load single float from local variable Syntax: fload = 23 vindex Stack: ...=> ..., value The value of the local variable at vindex in the current JAVA frame is pushed onto the opera and stack. fload _<n> Load single float from local variable Syntax: fload <n> Stack: ...=> ...,value Forms: fload_0 = 34, fload_l = 35, fload_2 = 36, fload_3 = 37 The value of the local variable at <n> in the current JAVA frame is pushed onto the operand stack. This instruction is the same as fload with a vindex of <n>, except that the operand <n> is implicit. dload Load double float from local variable Syntax: dload = 24 vindex Stack: ...=> ... , value-wordl, value-word2 The value of the local variables at vindex and vindex+1 in the current JAVA frame is pushed onto the operand stack. dload_<n> Load double float from local variable Syntax: dload <n> Stack: ...=> ... , value-wordl, value-word2 Forms: dload_0 = 38, dload_l = 39, dload_2 = 40, dload_3 = 41 The value of the local variables at <n> and <n>+l in the current JAVA frame is pushed onto the operand stack. This instruction is the same as dload with a vindex of <n>, except that the operand <n> is implicit. aload Load object reference from local variable Syntax: aload = 25 vindex Stack: ... => ... , value The value of the local variable at vindex in the current JAVA frame is pushed onto the operand stack. aload <n> Load object reference from local variable Syntax: aload <n> Stack: ...=> ..., value Forms: aload_0 = 42,aload_l = 43, aload_2 = 44, aload_3 = 45 The value of the local variable at <n> in the current JAVA frame is pushed onto the operand stack. This instruction is the same as aload with a vindex of <n>, except that the operand <n> is implicit. 3.4 Storing Stack Values into Local Variables istore Store integer into local variable Syntax: istore = 54 vindex Stack: ..., value => ... value must be an integer. Local variable vindex in the current JAVA frame is set to value. istore_<n> Store integer into local variable Syntax: istore <n> Stack : . . . , value => . . . Forms: istore_0 = 59, istore_l = 60, istore_2 = 61, istore_3 = 62 value must be an integer. Local variable <n> in the current JAVA frame is set to value. This instruction is the same as istore with a vindex of <n>, except that the operand <n> is implicit. Istore Store long integer into local variable Syntax.- Istore = 55 vindex Stack: .../ value-wordl, value-word2 => ... value must be a long integer. Local variables vindex+1 in the current JAVA frame are set to value. lβtore_<n> Store long integer into local variable Syntax: Istore <n> Stack: ..., value-wordl, value-word2 => Forms: lstore_0 = 63, lstore_l = 64, lstore_2 = 65, lstore_3 = 66 value must be a long integer. Local variables <n> and <n>+l in the current JAVA frame are set to value. This instruction is the same as Istore with a vindex of <n>, except that the operand <n> is implicit. fβtore Store single float into local variable Syntax: fstore -=56 vindex Stack: ..., value => value must be a single-precision floating point number. Local variable vindex in the current JAVA frame is set to value. fstore_<n> Store single float into local variable Syntax: fstore <n> Stack: ..., value => ... Forms: fstore_0 = 67, fstore_l = 68, fstore_2 = 69, fεtore_3 = 70 value must be a single-precision floating point number. Local variable <n> in the current JAVA frame is set to value. This instruction is the same as fstore with a vindex of <n>, except that the operand <n> is implicit. dstore Store double float into local variable Syntax: dstore = 57 vindex Stack: ..., value-wordl, value-word2 => ... value must be a double-precision floating point number. Local variables vindex and vindex+1 in the current JAVA frame are set to value. dstore_<n> Store double float into local variable Syntax: dstore <n> Stack: ..., value-wordl, value-word2 => ... Forms: dstore_0 = 71, dstore_l = 72, dstore_2 = 73, dstore_3 = 74 value must be a double-precision floating point number. Local variables <n> and <n>+l in the current JAVA frame are set to value. This instruction is the same as dstore with a vindex of <n>, except that the operand <n> is implicit. aβtore Store object reference into local variable Syntax: astore = 58 vindex Stack: ..., value => ... value must be a return address or a reference to an object. Local variable vindex in the current JAVA frame is set to value. astore_<n> Store object reference into local variable Syntax: astore <n> Stack: ... , value => ... Forms: astore_0 = 75, astore_l = 76, astore_2 = 77, astore_3 = 78 value must be a return address or a reference to an object. Local variable <n> in the current JAVA frame is set to value. This instruction is the same as astore with a vindex of <n>, except that the operand <n> is implicit. Increment local variable by constant Syntax: line = 132 vindex const Stack: no change Local variable vindex in the current JAVA frame must contain an integer. Its value is incremented by the value const, where const is treated as a signed 8-bit quantity. 3.5 Wider index for Loading, Storing and Incrementing wide Wider index for accessing local variables in load, store and increment. Syntax: wide = 196 vindex2 Stack: no change This bytecode must precede one of the following bytecodes: iload, Iload, fload, dload, aload, istore, Istore, fstore, dstore, astore, iinc. The vindex of the following bytecode and vindex2 from this bytecode are assembled into an unsigned 16-bit index to a local variable in the current JAVA frame. The following bytecode operates as normal except for the use of this wider index. 3.6 Managing Arrays newarray Allocate new array Syntax: newarray = 188 atype Stack: ..., size => result size must be an integer. It represents the number of elements in the new array. atype is an internal code that indicates the type of array to allocate. Possible values for atype are as follows: A new array of atype, capable of holding size elements, is allocated, and result is a reference to this new object. Allocation of an array large enough to contain size items of atype is attempted All elements of the array are initialized to zero. If size is less than zero, a NegativeArraySizeException is thrown If there is not enough memory to allocate the array, anOutOfMemoryError is thrown anewarray Allocate new array of references to objects Syntax• anewarray - 189 indexbytel indexbyte2 Stack ... , size => result size must be an integer. It represents the number of elements in the new array. indexbytel and indexbyte2 are used to construct an index into the constant pool of the current class. The item at that index is resolved. The resulting entry must be a class. A new array of the indicated class type and capable of holding size elements is allocated, and result is a reference to this new object. Allocation of an array large enough to contain size items of the given class type is attempted. All elements of the array are initialized to null. If size is less than zero, a NegativeArraySizeException is thrown. If there is not enough memory to allocate the array, an OutOfMemoryError is thrown. anewarray is used to create a single dimension of an array of object references. For example, to create new Thread[7] the following code is used: bipush 7 anewarray <Class "JAVA.lang.Thread"> anewarray can also be used to create the first dimension of a multi-dimensional array. For example, the following array declaration: new int [6] [] is created with the following code: bipush 6 anewarray <Class "[I"> See CONSTANT_Class in the "Class File Format" chapter for information on array class names. multianewarray Allocate new multi-dimensional array Syntax: multianewarray = 197 indexbytel indexbyte2 dimensions Stack: ..., sizel size2...sizen => result Each size must be an integer. Each represents the number of elements in a dimension of the array. indexbytel and indexbyfce2 are used to construct an index into the constant pool of the current class. The item at that index is resolved. The resulting entry must be an array class of one or more dimensions. dimensions has the following aspects: It must be an integer ' 1. It represents the number of dimensions being .created. It must be £ the number of dimensions of the array class. It represents the number of elements that are popped off the stack. All must be integers greater than or equal to zero. These are used as the sizes of the dimension. For example, to create new int[6] [3] [] the following code is used: bipush 6 bipush 3 multianewarray <Class "[[[I"> 2 If any of the size arguments on the stack is less than zero, a NegativeArraySizeException is thrown. If there is not enough memory to allocate the array, an OutOfMemoryError is thrown. The result is a reference to the new array object. Note: It is more efficient to use newarray or anewarray when creating a single dimension. See CONSTANT_Class in the "Class File Format" chapter for information on array class names. arraylength Get length of array Syntax: arraylength = 190 Stack: ..., objectref => ..., length objectref must be a reference to an array object. The length of the array is determined and replaces objectref on the top of the stack. If the objectref is null, a NullPointerException is thrown. iaload Load integer from array Syntax: iaload = 46 Stack: ..., arrayref, index => ..., value arrayref must be a reference to an array of integers.index must be an integer. The integer value at position number index in the array is retrieved and pushed onto the top of the stack. If arrayref is null a NullPointerException is thrown. If index is not within the bounds of the array an ArraylndexOutOfBoundsException is thrown. Iaload Load long integer from array Syntax: laoad = 47 Stack: ..., arrayref, index «> ..., value-wordl, value-word2 arrayref must be a reference to an array of long integers. index must be an integer. The long integer value at position number index in the array is retrieved and pushed onto the top of the stack. If arrayref is null a NullPointerException is thrown. If index is not within the bounds of the array an ArraylndexOutOfBoundsException is thrown. faload Load single float from array Syntax: faload 48 Stack: ..., arrayref, index => ..., value arrayref must be a reference to an array of single-precision floating point numbers. index must be an integer. The single-precision floating point number value at position number index in the array is retrieved and pushed onto the top of the stack. If arrayref is null a NullPointerException is thrown. If index is not within the bounds of the array an ArraylndexOutOfBoundsException is thrown. daload Load double float from array Syntax: daload = 49 Stack: ..., arrayref, index => ..., value-wordl, value-word2 arrayref must be a reference to an array of double-precision floating point numbers. index must be an integer. The double-precision floating point number value at position number index in the array is retrieved and pushed onto the top of the stack. If arrayref is null a NullPointerException is thrown. If index is not within the bounds of the array an ArraylndexOutOfBoundsException is thrown. aaload Load object reference from array Syntax: aaload = 50 Stack ...» arrayref, index => ... , value arrayref must be a reference to an array of references to objects. index must be an integer. The object reference at position number index in the array is retrieved and pushed onto the top of the stack. If arrayref is null a NullPointerException is thrown. If index is not within the bounds of the array an ArraylndexOutOfBoundsException is throw . baload Load signed byte from array. Syntax: baload = 51 Stack: ..., arrayref, index => ..., value arrayref must be a reference to an array of signed bytes. index must be an integer. The signed byte value at position number index in the array is retrieved, expanded to an integer, and pushed onto the top of the stack. If arrayref is null a NullPointerException is thrown. If index is not within the bounds of the array an ArraylndexOutOfBoundsException is thrown. caload Load character from array Syntax: caload = 52 Stack: ..., arrayref, index => ...,value arrayref must be a reference to an array of characters. index must be an integer. The character value at position number index in the array is retrieved, zero-extended to an integer, and pushed onto the top of the stack. If arrayref is null a NullPointerException is thrown. If index is not within the bounds of the array an ArraylndexOutOfBoundsException is thrown. saload Load short from array Syntax: saload = 53 Stack: ..., arrayref, index => ..., value arrayref must be a reference to an array of short integers. index must be an integer. The ;signed short integer value at position number index in the array is retrieved, expanded to an integer, and pushed onto the top of the stack. If arrayref is null, a NullPointerException is thrown. If index is not within the bounds of the array an ArraylndexOutOfBoundsException is thrown. iastore Store into integer array Syntax: iastore = 79 Stack: ..., arrayref, index, value -> ... arrayref must be a reference to an array of integers, index must be an integer, and value an integer. The integer value is stored at position index in the array. If arrayref is null, a NullPointerException is thrown. If index is not within the bounds of the array an ArraylndexOutOfBoundsException is throw . Iastore Store into long integer array Syntax: Iastore = 80 Stack: ..., arrayref, index, value-wordl, value-word2 => ... arrayref must be a reference to an array of long integers, index must be an integer, and value a long integer. The long integer value is stored at position index in the array. If arrayref is null, a NullPointerException is thrown. If index is not within the bounds of the array, an ArraylndexOutOfBoundsException is thrown. fastore Store into single float array Syntax: fastore = 81 Stack: ... , arrayref, index, value => ... arrayref must be an array of single-precision floating point numbers, index must be an integer, and value a single-precision floating point number. The single float value is stored at position index in the array. If arrayref is null, a NullPointerException is thrown. If index is not within the bounds of the array an ArraylndexOutOfBoundsException is thrown. dastore Store into double float array Syntax: dastore = 82 Stack: ..., arrayref, index, value-wordl, value-word2=> ... arrayref must be a reference to an array of double-precision floating point numbers, index must be an integer, and value a double-precision floating point number. The double float value is stored at position index in the array. If arrayref is null, a NullPointerException is thrown. If index is not within the bounds of the array an ArraylndexOutOfBoundsException is thrown. aastore Store into object reference array Syntax: aastore = 83 Stack: ..., arrayref, index, value ■> ... arrayref must be a reference to an array of references to objects, index must be an integer, and value a reference to an object. The object reference value is stored at position index ih the array. If arrayref is null, a NullPointerException is thrown. If index s not within the bounds of the array, an ArraylndexOutOfBoundsException is thrown. The actual type of value must be conformable with the actual type of the elements of the array. For example, it is legal to store an instance of class Thread in an array of class Object, but not vice versa. An ArrayStoreException is thrown if an attempt is made to store an incompatible object reference. baβtore Store into signed byte array Syntax: astore = 84 Stack: ..., arrayref, index, value => ... arrayref must be a reference to an array of signed bytes, index must be an integer, and value an integer. The integer value is stored at position index in the array. If value is too large to be a signed byte, it is truncated. If arrayref is null, a NullPointerException is thrown. If index is not within the bounds of the array an ArraylndexOutOfBoundsException is thrown. castore Store into character array Syntax: castore = 85 Stack: ..., arrayref, index, value => ... arrayref must be an array of characters, index must be an integer, and value an integer. The integer value is stored at position index in the array. If value is too large to be a character, it is truncated. If arrayref is null, a NullPointerException is thrown. If index is not within the bounds of [the array an ArraylndexOutOfBoundsException is thrown. sastore Store into short array Syntax: sastore = 86 Stack: ..., array, index, value => ... arrayref must be an array of shorts, index must be an integer, and value an integer. The integer value is stored at position index in the array. If value is too large to be an short, it is truncated. If arrayref is null, a NullPointerException is thrown. If index is not within the bounds of the array an ArraylndexOutOfBoundsException is thrown. 3.7 Stack Instructions nop Do nothing Syntax: nop - 0 Stack: no change Do nothing. pop Pop top stack word Syntax: pop 87 Stack: ... , any => ... Pop the top word from the stack. pop2 Pop top two stack words Syntax: pop2 = 89 Stack: ..., any2, anyl => ... Pop the top two words from the stack. dup Dupl: :e top stack word Synt dup = 8 Stao ...» any = > ..., any, any DupliM e the top word on the stack. dup2 Dupl :.= e top two stack words SyntEL: dup2 = 92 Stacj ..., any2,anyl => ..., any2, an 1, ny , anyl Dupl_.r-i e the top two words on the stack. dup_xl Dupl_.r3._e top stack word and put two down Synt : dup xl = 90 Stacj ..., any2, anyl >>..., anyl, any2, anyl Dupl_.:3 e the top word on the stack and insert the copy two words down in th_ ≡ ack. dup2_xl Duplι__._e top two stack words and put two down Syntε:_ dup_xl = 93 Staci ..., any3, any2, anyl => ..., any2, anyl, any3, any2, anyl Dupl_.__= e the top two words on the stack and insert the copies two words down ___ the stack. dup x2 Dupl r e top stack word and put three down Syntax : dup_x2 = 91 Stack: .-., any3, any2, anyl => ..., anyl, any3, any2, anyl Duplicate the top word on the stack and insert the copy three words down in the stac . dup2_x2 Duplicate top two stack words and put three down Syntax: du 2 x2 = 94 Stack: ..., any4, any3, any2, anyl => ..., any2, anyl, any4, any3, any2, anyl Duplicate the top two words on the stack and insert the copies three words down in the stack. swap Swap top two stack words Syntax: swap = 95 Stack: ..., any2, anyl => ..., any2, anyl Swap the top two elements on the stack. 3.8 Arithmetic Instructions iadd Integer add Syntax: iadd = 96 Stack: ..., valuel, value2 => ..., result valuel and value 2 must be integers. The values are added and are replaced on the stack by their integer sum. ladd Long integer add Syntax: Iadd = 97 Stack: ..., valuel-wordl, valuel-word2, value2-wordl, value2-word2 => ..., result-wordl, result-word2 valuel and value 2 must be long integers. The values are added and are replaced on the stack by their long integer sum. fadd Single floats add Syntax: fadd = 98 Stack: ..., valuel, value2 => ..., result valuel and value 2 must be single-precision floating point numbers. The values are added and are replaced on the stack by their single-precision floating point sum. dadd Double floats add Syntax: dadd = 99 Stack: ..., valuel-wordl, valuel-word2, value2-wordl, value2-word2 ■> ..., result-wordl, result-word2 valuel and value 2 must be double-precision floating point numbers. The values are added and are replaced on the stack by their double-precision floating point sum. isub Integer subtract Syntax: isub 100 Stack: ..., valuel, value2 => ..., result valuel and value 2 must be integers . value2 is subtracted from valuel, and both values are replaced on the stack by their integer difference. Isub Long integer subtract Syntax: Isub = 101 Stack: ..., valuel-wordl, valuel-word2, value2-wordl, value2-word2 => ..., result-wordl, result-word2 valuel and value 2 must be long integers. value2 is subtracted from valuel, and both values are replaced on the stack by their long integer difference. fsub Single float subtract Syntax: fsub = 102 Stack: ..., valuel, value2 => ..., result valuel and value 2 must be single-precision floating point numbers. value2 is subtracted from valuel, and both values are replaced on the stack by their single-precision floating point difference. dsub Double float subtract Syntax: dsub = 103 Stack: ..., valuel-wordl, valuel-word , value2-wordl, value2-word2 => ..., result-wordl, result-word2 valuel and value 2 must be double-precision floating point numbers. value2 is subtracted from valuel, and both values are replaced on the stack by their double-precision floating point difference. imul Integer multiply Synta : imul - 104 Stack: ..., valuel, value2 => ..., result valuel and value 2 must be integers. Both values are replaced on the stack by their integer product. Imul Long integer multiply Syntax: imul = 105 Stack: ..., valuel-wordl, valuel-word2, value2-wordl, value2-word2 => ..., result-wordl, result-word2 valuel and value 2 must be long integers. Both values are replaced on the stack by their long integer product. fmul Single float multiply Syntax: fmul = 106 Stack: ..., valuel, value2 => ..., result valuel and value 2 must be single-precision floating point numbers. Both values are replaced on the stack by their single-precision floating point product. daul Double float multiply Syntax: dmul = 107 Stack: ..., valuel-wordl, valuel-word2, value2-wordl, value2-word2 ., result-wordl, resul -word2 valuel and value 2 must be double-precision floating point numbers Both values are replaced on the stack by their double-precision floating point product. idiv Integer divide Syntax: idiv - 108 Stack: - .., valuel, value2 => ..., result valuel and value 2 must be integers. valuel is divided by value2, and both values are replaced on the stack by their integer quotient. The result is truncated to the nearest integer that is between it and 0. An attempt to divide by zero results in a "/ by zero" ArithmeticException being thrown. Idiv Long integer divide Syntax: Idiv = 109 Stack: ..., valuel-wordl, valuβl-word2, value2-wordl, value2-word2 => ..., result-wordl, result-word2 valuel and value 2 must be long integers. valuel is divided by value2, and both values are replaced on the stack by their long integer quotient. The result is truncated to the nearest integer that is between it and 0. An attempt to divide by zero results in a "/ by zero" ArithmeticException being thrown. fdiv Single float divide Syntax: fdiv = 110 Stack: ..., valuel, value2 => ..., result valuel and value 2 must be single-precision floating point numbers. valuel is divided by value2, and both values are replaced on the stack by their single-precision floating point quotient. Divide by zero results in the quotient being NaN. ddiv Double float divide Syntax: ddiv = 111 Stack: ..., valuel-wordl, valuel-word2, value2-wordl, value2-word2 => ..., result-wordl, result-word2 valuel and value 2 must be double-precision floating point numbers. valuel is divided by value2, and both values are replaced on the stack by their double-precision floating point quotient. Divide by zero results in the quotient being NaN. irem Integer remainder Syntax: irem = 112 Stack: ..., valuel, value2 => ..., result valuel and value 2 must both be integers. valuel is divided by value2, and both values are replaced on the stack by their integer remainder. An attempt to divide by zero results in a "/ by zero" ArithmeticException being thrown. Irem Long integer remainder Syntax: Irem = 113 Stack: ..., valuel-wordl, valuel-word2, value2-wordl, value2-word2 . , result-wordl, resul -word2 valuel and value 2 must both be long integers. valuel is divided by value2, and both values are replaced on the stack by their long integer remainder. An attempt to divide by zero results in a "/ by zero" ArithmeticException being thrown. Single float remainder Syntax: frem = 114 Stack: ..., valuel, value2 => ..., result valuel and value 2 must both be single-precision floating point numbers. valuel is divided by value2, and the quotient is truncated to an integer, and then multiplied by value2. The product is subtracted from valuel. The result, as a single-precision floating point number, replaces both values on the stack. result = valuel - (integral_part (valuel/value2) *value2) , where integral_part () rounds to the nearest integer, with a tie going to the even number. An attempt to divide by zero results in NaN. drem Double float remainder Syntax: drem = 115 Stack: ..., valuel-wordl, valuel-word2, value2-wordl, value2-word2 => ... , result-wordl, result-word2 valuel and value 2 must both be double-precision floating point numbers . valuel is divided by value2, and the quotient is truncated to an integer, and then multiplied by value2. The product is subtracted from valuel. The result, as a double-precision floating point number, replaces both values on the stack. result - valuel - (integral_par (valuel/value2) * value2) , where integral_part () rounds to the nearest integer, with a tie going to the even number. An attempt to divide by zero results in NaN. meg Integer negate Syntax: ineg = 116 Stack: ..., value => ..., result value must be an integer. It is replaced on the stack by its arithmetic negation. lneg Long integer negate Syntax: lneg = 117 Stack: ..., value-wordl, value-word2 => ..., result-wordl, result-word2 value must be a long integer. It is replaced on the stack by its arithmetic negation. fneg Single float negate Syntax: fneg = 118 Stack: ..., value=> ..., result value must be a single-precision floating point number, It is replaced on the stack by its arithmetic negation. Double float negate Syntax: Stack: value-wordl, value-word2 -> . . . . result-wordl. r sult-word2 value must be a double-precision floating point number. It is replaced on the stack by its arithmetic negation. 3.9 Logical Instructions ishl Integer shift left Syntax: ishl 120 Stack: ...,valuel, value2 => ..., result valuel and value 2 must be integers. valuel is shifted left by the amount indicated by the low five bits of value2. The integer result replaces both values on the stack. ishr Integer arithmetic shift right Syntax: ishr = 122 Stack: ..., valuel, value2 => ..., result valuel and value 2 must be integers, valuel is shifted right arithmetically (with sign extension) by the amount indicated by the low five bits of value2. The integer result replaces both values on the stack. iushr Integer logical shift right Syntax: iushr = 124 Stack: ..., valuel, value2 »> ..., result valuel and value 2 must be integers, valuel is shifted right logically (with no sign extension) by the amount indicated by the low five bits of value2. The integer result replaces both values on the stack. Ishl Long integer shift left Syntax Ishl = 121 Stack ..., valuel-wordl, valuel-word2, value2 => .... result-wordl, result-word2 valuel must be a long integer and value 2 must be an integer valuel is shifted left by the amount indicated by the low six bits of value2 The long integer result replaces both values on the stack Ishr Long integer arithmetic shift right Syntax: Ishr - 123 Stack- ..., valuel-wordl, valuel-word2, value2 => ..., result-wordl, result-word2 valuel must be a long integer and value 2 must be an integer. valuel is shifted right arithmetically (with sign extension) by the amount indicated by the low six bits of value2. The long integer result replaces both values on the stack. Iushr Long integer logical shift right Syntax: Iushr = 125 Stack: ..., valuel-wordl, valuel-word2, value2-wordl, valuβ2-word2 => ..., result-wordl, result-word2 valuel must be a long integer and value 2 must be an integer. valuel is shifted right logically (with no sign extension) by the amount indicated by the low six bits of value2. The long integer result replaces both values on the stack. iand Integer boolean AND Syntax: Stack: ..., valuel, value2 => ..., result valuel and value 2 must both be integers. They are replaced on the stack by their bitwise logical and (conjunction) . land Long integer boolean AND Syntax: land = 127 Stack: ..., valuel-wordl, valuel-word2, value2-wordl, value2-word2 => ..., result-wordl, result-word2 valuel and value 2 must both be long integers. They are replaced on the stack by their bitwise logical and (conjunction) . ior Integer boolean OR Syntax: ior = 128 Stack: ..., valuel, value2 => ..., result valuel and value 2 must both be integers. They are replaced on the stack by their bitwise logical or (disjunction) . lor Long integer boolean OR Syntax: lor 129 Stack: ..., valuel-wordl, valuel-word2, value2-wordl, value2-word2 => ..., result-wordl, result-word2 valuel and value 2 must both be long integers . They are replaced on the stack by their bitwise logical or (disjunction) . lxor Integer boolean XOR Syntax : ixor = 130 Stack: ..., valuel, value2 => ..., result valuel and value 2 must both be integers. They are replaced on the stack by their bitwise exclusive or (exclusive disjunction) . lxor Long integer boolean XOR Syntax: lxor = 131 Stack: ..., valuel-wordl, valuel-word2, value2-wordl, value2-word2 => ..., result-wordl, result-word2 valuel and value 2 must both be long integers. They are replaced on the stack by their bitwise exclusive or (exclusive disjunction) . 3.10 Conversion Operations 121 Integer to long integer conversion Syntax: 121 133 Stack: ..., value ■> ..., result-wordl, result-word2 value must be an integer. It is converted to a long integer. The result replaces value on the stack. i2f Integer to single float Syntax: i2f 134 Stack: ..., value => ..., result value must be an integer. It is converted to a single-precision floating point number. The result replaces value on the stack. i2d Integer to double float Syntax: i2d = 135 Stack: ..., value => ..., result-wordl, result-word2 value must be an integer. It is converted to a double-precision floating point number. The result replaces value on the stack. 12i Long integer to integer Syntax: 12i 136 Stack: ..., value-wordl, value-word2 => ..., result value must be a long integer. It is converted to an integer by taking the low-order 32 bits. The result replaces value on the stack. 12f Long integer to single float Syntax: 12f 137 Stack: ..., value-wordl, value-word2 => ..., result value must be a long integer. It is converted to a single-precision floating point number. The result replaces value on the stack. 12d Long integer to double float Syntax: 12d = 138 Stack: ..., value-wordl, value-word2 => ..., result-wordl, result-word2 value must be a long integer. It is converted to a double-precision floating point number. The result replaces value on the stack. f2i Single float to integer Syntax: f2i = 139 Stack: ..., value => ..., result value must be a single-precision floating point number. It is converted to an integer. The result replaces value on the stack. f21 Single float to long integer Syntax: f21 = 140 Stack: ..., value => ..., result-wordl, result-word2 value must be a single-precision floating point number. It is converted to a long integer. The result replaces value on the stack. f2d Single float to double float Syntax: £2d - 141 Stack: ..., value => ..., result-wordl, result-word2 value must be a single-precision floating point number. It is converted to a double-precision floating point number. The result replaces value on the stack. d2i Double float to integer Syntax: 2di 142 Stack: ..., value-wordl, value-word2 => ..., result value must be a double-precision floating point number. It is converted to an integer. The result replaces value on the stack. d21 Double float to long integer Syntax: d21 143 Stack: ..., value-wordl, value-word2 => ..., result-wordl, result-word2 value must be a double-precision floating point number. It is converted to a long integer. The result replaces value on the stack. d2f Double float to single float Syntax: 2df = 144 Stack: ..., value-wordl, value-word2 => ..., result value must be a double-precision floating point number. It is converted to a single-precision floating point number. If overflow occurs, the result must be infinity with the same sign as value. The result replaces value on the stack. int2byte Integer to signed byte Syntax: int2byte = 157 Stack: ..., value => ..., result value must be an integer. It is truncated to a signed 8-bit result, then sign extended to an integer. The result replaces value on the stack. int2char Integer to char Synta : int2char 146 Stack: , value => ... , result value must be an integer. It is truncated to an unsigned 16-bit result, then zero extended to an integer. The result replaces value on the stack. int2short Integer to short Syntax: int2short = 147 Stack: value result value must be an integer. It is truncated to a signed 16-bit result, then sign extended to an integer. The result replaces value on the stack. 3.11 Control Transfer Instructions ifeg Branch if equal to 0 Syntax: ifeg = 153 branchbytel branchbyte2 Stack: ..., value => ... value must be an integer. It is popped from the stack. If value is zero, branchbytel and branchbyte2 are used to construct a signed 16-bit offset. Execution proceeds at that offset from the address of this instruction. Otherwise execution proceeds at the instruction following the ifeq. ifnull Branch if null Syntax: ifnull = 198 branchbytel branchbyte2 Stack : . . . , value => . . . value must be a reference to an object. It is popped from the stack. If value is null, branchbytel and branchbyte2 are used to construct a signed 16-bit offset. Execution proceeds at that offset from the address of this instruction. Otherwise execution proceeds at the instruction following the ifnull . iflt Branch if less than 0 Syntax: iflt = 155 branchbytel branchbyte2 Stack: ..., value => ... value must be an integer. It is popped from the stack. If value is less than zero, branchbytel and branchbyte2 are used to construct a signed 16-bit offset. Execution proceeds at that offset from the address of this instruction. Otherwise execution proceeds at the instruction following the iflt. ifle Branch if less than or equal to 0 Syntax: ifle=158 branchbytel branchbyte2 Stack: ..., value => ... value must be an integer. It is popped from the stack. If value is less than or equal to zero, branchbytel and branchbyte2 are used to construct a signed 16-bit offset. Execution proceeds at that offset from the address of this instruction. Otherwise execution proceeds at the instruction following the ifle. ifne Branch if not equal to 0 Syntax: ifne=154 branchbytel branchbyte2 Stack: ..., value => ... value must be an integer. It is popped from the stack. If value is not equal to zero, branchbytel and branchbyte2 are used to construct a signed 16-bit offset. Execution proceeds at that offset from the address of this instruction. Otherwise execution proceeds at the instruction following the ifne. ifnonnull Branch if not null Syntax: ifnonnull=199 branchbytel branchbyte2 Stack: ..., value => ... value must be a reference to an object. It is popped from the stack. If value is notnull, branchbytel and branchbyte2 are used to construct a signed 16-bit offset. Execution proceeds at that offset from the address of this instruction. Otherwise execution proceeds at the instruction following the ifnonnull. ifgt Branch if greater than 0 Syntax: ifft=157 branchbytel branchbyte2 Stack: ..., value => ... value must be an integer. It is popped from the stack. If value is greater than zero, branchbytel and branchbyte2 are used to construct a signed 16-bit offset. Execution proceeds at that offset from the address of this instruction Otherwise execution proceeds at the instruction following the ifgt ifge Branch if greater than or equal to 0 Syntax ifge=156 branchbytel branchbyte2 Stack: ..., value => ... value must be an integer It is popped from the stack. If value is greater than or equal to zero, branchbytel and branchbyte2 are used to construct a signed 16-bit offset Execution proceeds at that offset from the address of this instruction. Otherwise execution proceeds at the instruction following instruction ifge if icmpeq Branch if integers equal Syntax: if_icmpeq=159 branchbytel branchbyte2 Stack- ..., valuel, value2 => ... valuel and value2 must be integers. They are both popped from the stack. If valuel is equal to value2, branchbytel and branchbyte2 are used to construct a signed 16-bit offset. Execution proceeds at that offset from the address of this instruction. Otherwise execution proceeds at the instruction following instruction if_icmpβq. if icmpnβ Branch if integers not equal Syntax: if icmpne=160 branchbytel branchbyte2 Stack: ..., valuel, value2 => ... valuel and value2 must be integers. They are both popped from the stack. If valuel is not equal to value2, branchbytel and branchbyte2 are used to construct a signed 16-bit offset. Execution proceeds at that offset from the address of this instruction. Otherwise execution proceeds at the instruction following instruction if icmpne. if_icmplt Branch if integer less than Syntax: if_icιnplt=161 branchbytel branchbyte2 Stack: ... , valuel, value2 => ... valuel and value2 must be integers. They are both popped from the stack. If valuel is less than value2, branchbytel and branchbyte2 are used to construct a signed 16-bit offset. Execution proceeds at that offset from the address of this instruction. Otherwise execution proceeds at the instruction following instruction if icmplt. if_icmpgt Branch if integer greater than Syntax: if icmpgt-163 branchbytel branchbyte2 Stack: ..., valuel, valuβ2 -> . . . valuel and value2 must be integers. They are both popped from the stack. If valuel is greater than value2, branchbytel and branchbyte2 are used to construct a signed 16-bit offset. Execution proceeds at that offset from the address of this instruction. Otherwise execution proceeds at the instruction following instruction if_icmpgt. if icmple Branch if integer less than or equal to Syntax: if_icmple=164 branchbytel branchbyte2 Stack: ..., valuel, value2 => ... valuel and value2 must be integers. They are both popped from the stack. If valuel is less than or equal to value2, branchbytel and branchbyte2 are used to construct a signed 16-bit offset. Execution proceeds at that offset from the address of this instruction. Otherwise execution proceeds at the instruction following instruction if_icmple. if icmpge Branch if integer greater than or equal to Syntax: if_icmpge=162 branchbytel branchbyte2 Stack: ..., valuel, value2 -> . . . valuel and value2 must be integers. They are both popped from the stack. If valuel is greater than or equal to value2, branchbytel and branchbyte2 are used to construct a signed 16-bit offset. Execution proceeds at that offset from the address of this instruction. Otherwise execution proceeds at the instruction following instruction if_icmpge. lcmp Long integer compare Syntax: lcmp=148 Stack: ..., valuel-wordl, valuel-word2,value2-wordl, value2-wordl => ... , result valuel and value2 must be long integers. They are both popped from the stack and compared. If valuel is greater than value2, the integer valuel is pushed onto the stack. If valuel is equal to value2, the value 0 is pushed onto the stack. If valuel is less than value2, the value -l is pushed onto the stack. fcmpl Single float compare ( 1 on NaN) Syntax: fcmpl=149 Stack: ..., valuel, value2=> ...,result valuel and value2 must be single-precision floating point numbers. They are both popped from the stack and compared. If valuel is greater than value2, the integer value 1 is pushed onto the stack. If valuel is equal to value2, the value 0 is pushed onto the stack. If valuel is less than value2, the value -1 is pushed onto the stack. If either valuel or value2 is NaN, the value -1 is pushed onto the stack. fcmpg Single float compare (1 on NaN) Syntax: fcmpg=150 Stack: ...,valuel, value2=> ..., result valuel and value2 must be single-precision floating point numbers. They are both popped from the stack and compared. If valuel is greater than value2, the integer value 1 is pushed onto the stack. If valuel is equal to value2, the value 0 is pushed onto the stack. If valuel is less than value2, the value -1 is pushed onto the stack. If either valuel or value2 is NaN, the value 1 is pushed onto the stack. dcmpl Double float compare (-1 on NaN) Syntax: dcmpl-151 Stack: ..., valuel-wordl, valuel-word2, valuβ2-wordl, value2-wordl=> ..., result valuel and value2 must be double-precision floating point numbers. They are both popped from the stack and compared. If valuel is greater than value2, the integer value 1 is pushed onto the stack. If valuel is equal to value2, the value 0 is pushed onto the stack. If valuel is less than value2, the value 1 is pushed onto the stack. If either valuel or value2 is NaN, the value 1 is pushed onto the stack. dcmpg Double float compare (1 on NaN) Syntax: dcmpg=152 Stack: ..., valuel-wordl, valuel-word2, value2-wordl, value2-wordl => ... , result valuel and value2 must be double-precision floating point numbers. They are both popped from the stack and compared. If valuel is greater than value2, the integer value 1 is pushed onto the stack. If valuel is equal to value2, the value 0 is pushed onto the stack. If valuel is less than value2, the value -1 is pushed onto the stack. If either valuel or value2 is NaN, the value 1 is pushed onto the stack. if acmpeq Branch if object references are equal Syntax: if acmpeq=165 branchbytel branchbyte2 Stack: ...,valuel,value2 => ... valuel and value2 must be references to objects. They are both popped from the stack. If the objects referenced are not the same, branchbytel and branchbyte2 are used to construct a signed 16-bit offset. Execution proceeds at that offset from the Address of this instruction. Otherwise execution proceeds at the instruction following the if_acmpeq. if acmpnβ Branch if object references not equal Syntax: if acmpne=166 branchbytel branchbyte2 18 Stack: ... , valuel, value2 => ... valuel and value2 must be references to objects. They are both popped from the stack. If the objects referenced are not the same, branchbytel and branchbyte2 are used to construct a signed 16-bit offset. Execution proceeds at that offset from the address of this instruction. Otherwise execution proceeds at the instruction following instruction if_acmpne. goto Branch always Syntax : goto=167 branchbytel branchbyte2 Stack: no change branchbytel and branchbyte2 are used to construct a signed 16-bit offset. Execution proceeds at that offset from the address of this instruction. goto_w Branch always (wide index) Syntax: goto_w= 00 branchbytel branchbyte2 branchbytβ3 branchbyte4 Stack: no change branchbytel, branchbyte2, branchbyte3, and branchbyte4 are used to construct a signed 32-bit offset. Execution proceeds at that offset from the address of this instruction. Jump subroutine Syntax: j sr=168 branchbytel branchbyte2 Stack: ...=> ..., return-address branchbytel and branchbyte2 are used to construct a signed 16-bit offset. The address of the instruction immediately following the jsr is pushed onto the stack. Execution proceeds at the offset from the address of this instruction. sr_w Jump subroutine (wide index) Syntax-. jsr_w=201 branchbytel branchbyte2 branchbyte3 branchbyte4 Stack: ...=> ..., return-address branchbytel, branchbyte2, branchbyte3, and branchbyte4 are used to construct a signed 32-bit offset. The address of the instruction immediately following the jsr_w is pushed onto the stack. Execution proceeds at the offset from the address of this instruction. ret Return from subroutine Syntax; ret=169 vindex Stack: no change Local variable vindex in the current JAVA frame must contain a return address. The contents of the local variable are written into the pc. Note that jsr pushes the address onto the stack, and ret gets it out of a local variable. This asymmetry is intentional. ret w Return from subroutine (wide index) Syntax: ret w=209 vindexbytel vindexbyte2 Stack: no change vindexbytel and vindexbyte2 are assembled into an unsigned 16-bit index to a local variable in the current JAVA frame. That local variable must contain a return address. The contents of the local variable are written into the pc. See the ret instruction for more information. .12 Function Return ireturn Return integer from function Syntax: ireturn=172 Stack: ..., value => [empty] value must be an integer. The value value is pushed onto the stack of the previous execution environment. Any other values on the operand stack are discarded. The interpreter then returns control to its caller. Ireturn Return long integer from function Syntax: lreturn=173 Stack: ..., value-wordl, value-word2 => [empty] value must be a long integer. The value value is pushed onto the stack of the previous execution environment. Any other values on the operand stack are discarded. The interpreter then returns control to its caller. freturn Return single float from function Syntax: freturn=174 Stack: ..., value=> [empty] value must be a single-precision floating point number. The value value is pushed onto the stack of the previous execution environment. Any other values on the operand stack are discarded. The interpreter then returns control to its caller. dreturn Return double float from function Syntax: dreturn=175 Stack: ..., value-wordl, value-word2 => [empty] value must be a double-precision floating point number. The value value is pushed onto the stack of the previous execution environment . Any other values on the operand stack are discarded. The interpreter then returns control to its caller. aretun. Return object reference from function Syntax: areturn=176 Stack: ..., value => [empty] value must be a reference to an object. The value value is pushed onto the stack of the previous execution environment. Any other values on the operand stack are discarded. The interpreter then returns control to its caller. return Return (void) from procedure Syntax: return*177 Stack: ...=> [empty] All values on the operand stack are discarded. The interpreter then returns control to its caller. breakpoint Stop and pass control to breakpoint handler Syntax: breakpoint=202 Stack: no change 3.13 Table Jumping tableswitch Access jump table by index and jump Syntax: tableswitch=170 .0-3 byte pad.. default-offsetl default-offsβt2 default-offset3 default-offβet4 lowl low2 low3 low4 highl high2 high3 high4 jump offsets. Stack: ... , index=> ... tableswitch is a variable length instruction. Immediately after the tableswitch opcode, between zero and three 0 's are inserted as padding so that the next byte begins at an address that is a multiple of four. After the padding follow a series of signed 4-byte quantities: default-offset, low, high, and then high-low+l further signed 4-byte offsets. The high-low+1 signed 4-byte offsets are treated as a 0-based jump table. The index must be an integer. If index is less than low or index is greater than high, then default-offeet is added to the address of this instruction. Otherwise, low is subtracted from index, and the index-low'th element of the jump table is extracted, and added to the address of this instruction. lookupswitch Access jump table by key match and jump Syntax: lookupswitch=171 ...0-3 byte pad. defaul -offsetl default-offset2 default-offset3 default-offset4 npairsl npairs2 npairβ3 npairβ4 .match-offset pairs. Stack: ..., key=> ... lookupswitch is a variable length instruction. Immediately after the lookupswitch opcode, between zero and three 0's are inserted as padding so that the next byte begins at an address that is a multiple of four. Immediately after the padding are a series of pairs of signed 4-byte quantities. The first pair is special. The first item of that pair is the default offset, and the second item of that pair gives the number of pairs that follow. Each subsequent pair consists of a match and an offset. The key must be an integer. The integer key on the stack is compared against each of the matches. If it is equal to one of them, the offset is added to the address of this instruction. If the key does not match any of the matches, the default offset is added to the address of this instruction. 3.14 Manipulating Object Fields putfield Set field in object Syntax: putfiθld=181 indexbytel indexbyte2 Stack: ..., objectref, value=> ... OR Stack: ..., objectref, value-wordl, value-word2=> ... indexbytel and indexbyte2 are used to construct an index into the constant pool of the current class. The constant pool item will be a field reference to a class name and a field name. The item is resolved to a field block pointer which has both the field width (in bytes) and the field offset (in bytes) . The field at that offset from the start of the object referenced by object refwill be set to the value on the top of the stack. This instruction deals with both 32-bit and 64-bit wide fields. If object ref is null, aNullPointerException is generated. If the specified field is a static field, anlncompatibleClassChangeError is thrown. getfield Fetch field from object Syntax: getfield=180 indexbytel indβxbyte2 Stack: ..., alue OR Stack: ..., objectref=> ..., value-wordl, value-word2 indexbytel and indexbyte2 are used to construct an index into the constant pool of the current class . The constant pool item will be a field reference to a class name and a field name. The item is resolved to a field block pointer which has both the field width (in bytes) and the field offset (in bytes) . objectref must be a reference to an object. The value at offset into the object referenced by objectref replaces objectref on the top of the stack. This instruction deals with both 32-bit and 64-bit wide fields. If objectref is null, a NullPointerException is generated. If the specified field is a static field, an IncompatibleClassChangeError is thrown. putstatic Set static field in class Syntax: putstatic-179 indexbytel indexbyte2 Stack: ..., value=> ... OR Stack: ..., value-wordl, value-word2=> ... indexbytel and indexbyte2 are used to construct an index into the constant pool of the current class. The constant pool item will be a field reference to a static field of a class. That field will be set to have the value on the top of the stack. This instruction works for both 32-bit and 64-bit wide fields. If the specified field is a dynamic field, an IncompatibleClassChangeError is thrown. getstatic Get static field from class Syntax: getstatic=178 indexbytel indβxbyte2 Stack: ... , => ... , value OR Stack: ..., => ..., value-wordl, value-word2 indexbytel and indexbyte2 are used to construct an index into the constant pool of the current class. The constant pool item will be a field reference to a static field of a class. This instruction deals with both 32-bit and 64-bit wide fields. If the specified field is a dynamic field, an IncompatibleClassChangeError is generated. 3.15 Method Invocation There are four instructions that implement method invocation. invokevirtual Invoke an instance method of an object, dispatching based on the runtime (virtual) type of the object. This is the normal method dispatch in JAVA, invokenonvirtual Invoke an instance method of an object, dispatching based on the compile-time (non-virtual) type of the object. This is used, for example, when the keywordsuper or the name of a superclass is used as a method qualifier, invokestatic Invoke a class (static) method in a named class. invokeinterface Invoke a method which is implemented by an interface, searching the methods implemented by the particular run-time object to find the appropriate method. invokevirtual Invoke instance method, dispatch based on run-time type Syntax: invokevirtual-182 indexbytel indexbytβ2 Stack: ..., objectref, [argl, [arg2 ...]], ...=> ... The operand stack must contain a reference to an object and some number of arguments.indexbytel and indexbyte2 are used to construct an index into the constant pool of the current class. The item at that index in the constant pool contains the complete method signature. A pointer to the object's method table is retrieved from the object reference. The method signature is looked up in the method table. The method signature is guaranteed to exactly match one of the method signatures in the table. The result of the lookup is an index into the method table of the named class, which is used with the object's dynamic type to look in the method table of that type, where a pointer to the method block for the matched method is found. The method block indicates the type of method (native, synchronized, and so on) and the number of arguments expected on the operand stack. If the method is marked synchronized the monitor associated with objectref is entered. The objectref and arguments are popped off this method's stack and become the initial values of the local variables of the new method. Execution continues with the first instruction of the new method. If the object reference on the operand stack is null, a NullPointerException is thrown. If during the method invocation a stack overflow is detected, a StackOverflowError is thrown. invokenonvirtual Invoke instance method, dispatching based on compile-time type Syntax: invokenonvirtual = 183 indexbytel indexbyte2 Stack: ..., objectref, [argl, [arg2 ...]], ... => ... The operand stack must contain a reference to an object and some number of arguments.indexbytel and indexbyte2 are used to construct an index into the constant pool of the current class. The item at that index in the constant pool contains a complete method signature and class. The method signature is looked up in the method table of the class indicated. The method signature is guaranteed to exactly match one of the method signatures in the table. The result of the lookup is a method block. The method block indicates the type of method (native, synchronized, and so on) and the number of arguments (nargs) expected on the operand stack. If the method is marked synchronized the monitor associated with objectref is entered. The objectref and arguments are popped off this method's stack and become the initial values of the local variables of the new method. Execution continues with the first instruction of the new method. If the object reference on the operand stack is null, a NullPointerException is thrown. If during the method invocation a stack overflow is detected, a StackOverflowError is thrown. invokestatic Invoke a class (static) method Syntax: invokestatis = 184 indexbytel indexbyte2 Stack: ..., [argl, [arg2 ...]], ... => ... The operand stack must contain some number of arguments.indexbytel and indexbyte2 are used to construct an index into the constant pool of the current class. The item at that index in the constant pool contains the complete method signature and class. The method signature is looked up in the method table of the class indicated. The method signature is guaranteed to exactly match one of the method signatures in the class's method table. The result of the lookup is a method block. The method block indicates the type of method (native, synchronized, and so on) and the number of arguments (nargs) expected on the operand stack. If the method is marked synchronized the monitor associated with the class is entered. The arguments are popped off this method's stack and become the initial values of the local variables of the new method. Execution continues with the first instruction of the new method. If during the method invocation a stack overflow is detected, a StackOverflowError is thrown. invokeinterfacβ Invoke interface method Syntax: invokeinterface = 185 indexbytel indexbyte2 nargs reserved Stack: ..., objectref, [argl, [arg2 ...]], ... => ... The operand stack must contain a reference to an object and nargs-1 arguments . indexbytel and indexbyte2 are used to construct an index into the constant pool of the current class . The item at that index in the constant pool contains the complete method signature. A pointer to the object's method table is retrieved from the object reference. The method signature is looked up in the method table . The method signature is guaranteed to exactly match one of the method signatures in the table. The result of the lookup is a method block. The method block indicates the type of method (native, synchronized, and so on) but unlike invokevirtual and invokenonvirtual, the number of available arguments (nargs) is taken from the bytecode. If the method is markedsynchronized the monitor associated with objectref is entered. The objectref and arguments are popped off this method's stack and become the initial values of the local variables of the new method. Execution continues with the first instruction of the new method. If the objectref on the operand stack is null, a NullPointerException is thrown. If during the method invocation a stack overflow is detected, a StackOverflowError is thrown. 3.16 Exception Handling athrow Throw exception or error Syntax: athrow = 191 Stack: ..., objectref => [undefined] objectref must be a reference to an object which is a subclass of Throwable, which is thrown. The current JAVA stack frame is searched for the most recent catch clause that catches this class or a superclass of this class. If a matching catch list entry is found, the pc is reset to the address indicated by the catch-list entry, and execution continues there. If no appropriate catch clause is found in the current stack frame, that frame is popped and the object is rethrown. If one is found, it contains the location of the code for this exception. The pc is reset to that location and execution continues. If no appropriate catch is found in the current stack frame, that frame is popped and the objectref is rethrown. If objectref is null, then a NullPointerException is thrown instead. 3.17 Miscellaneous Object Operations new Create new object Syntax: new = 187 indexbytel indexbyte2 Stack: ... => ..., objectref indexbytel and indexbyte2 are used to construct an index into the constant pool of the current class. The item at that index must be a class name that can be resolved to a class pointer, class. A new instance of that class is then created and a reference to the object is pushed on the stack. checkcaβt Make sure object is of given type Syntax: checkcast = 192 indexbytel indexbyte2 Stack: ... , objectref => ... , objectref indexbytel and indexbyte2 are used to construct an index into the constant pool of the current class. The string at that index of the constant pool is presumed to be a class name which can be resolved to a class pointer, class. objectref must be a reference to an object. checkcast determines whether objectref can be cast to be a reference to an object of class class. A null objectref can be cast to any class. Otherwise the referenced object must be an instance of class or one of its superclasses. If objectref can be cast to class execution proceeds at the next instruction, and the objectref remains on the stack. If objectref cannot be cast to class, a ClassCastException is thrown. instanceof Determine if an object is of given type Syntax: instanceof = 193 indexbytel indexbyte2 Stack: ..., objectref => ..., result indexbytel and indexbyte2 are used to construct an index into the constant pool of the current class. The string at that index of the constant pool is presumed to be a class name which can be resolved to a class pointer, class. objectref must be a reference to an object. instanceof determines whether objectref can be cast to be a reference to an object of the class class. This instruction will overwrite objectref with 1 if objectref is an instance of class or one of its superclasses. Otherwise, objectref is overwritten by 0. If objectref is null, it's overwritten by 0. 3.18 Monitors xαonitorenter Enter monitored region of code Syntax: monitorenter = 194 Stack: ..., objectref => ... objectref must be a reference to an object. The interpreter attempts to obtain exclusive access via a lock mechanism to objectref. If another thread already has objectref locked, than the current thread waits until the object is unlocked. If the current thread already has the object locked, then continue execution. If the object is not locked, then obtain an exclusive lock. If objectref is null, then a NullPointerException is thrown instead. monitorexit Exit monitored region of code Syntax: monitorexit = 195 Stack: .... objectref => ... objectref must be a reference to an object. The lock on the object released. If this is the last lock that this thread has on that object (one thread is allowed to have multiple locks on a single object) , then other threads that are waiting for the object to be available are allowed to proceed. If objectref is null, then a NullPointerException is thrown instead. Appendix A: An Optimization The following set of pseudo-instructions suffixed by _guick are variants of JAVA virtual machine instructions . They are used to improve the speed of interpreting bytecodes. They are not part of the virtual machine specification or instruction set, and are invisible outside of an JAVA virtual machine implementation. However, inside a virtual machine implementation they have proven to be an effective optimization. A compiler from JAVA source code to the JAVA virtual machine instruction set emits only non-_quick instructions. If the _quick pseudo-instructions are used, each instance of a non-_quick instruction with a _quick variant is overwritten on execution by its_quick variant. Subsequent execution of that instruction instance will be of the_quick variant. In all cases, if an instruction has an alternative version with the suffix_quick, the instruction references the constant pool. If the_quick optimization is used, each non-_quick instruction with a_quick variant performs the following: Resolves the specified item in the constant pool; Signals an error if the item in the constant pool could not be resolved for some reason; Turns itself into the _quick version of the instruction. The instructions putstatic, getstatic, putfield, and getfield each have two quick versions; and Performs its intended operation. This is identical to the action of the instruction without the _quick optimization, except for the additional step in which the instruction overwrites itself with its _quick variant. The _quick variant of an instruction assumes that the item in the constant pool has already been resolved, and that this resolution did not generate any errors. It simply performs the intended operation on the resolved item. Note: some of the invoke methods only support a single-byte offset into the method table of the object; for objects with 256 or more methods some invocations cannot be "quicked" with only these bytecodes. This Appendix doesn't give the opcode values of the pseudo-instructions, since they are invisible and subject to change. A.l Constant Pool Resolution When the class is read in, an array constant_pool [] of size n constants is created and assigned to a field in the class .constant pool [0] is set to point to a dynamically allocated array which indicates which fields in the constant_pool have already been resolved.constant_pool [1] through constant_pool [nconstants - 1] are set to point at the "type" field that corresponds to this constant item. When an instruction is executed that references the constant pool, an index is generated, and constant_pool [0] is checked to see if the index has already been resolved. If so, the value of constant_pool [index] is returned. If not, the value of constant_pool [index] is resolved to be the actual pointer or data, and overwrites whatever value was already in constant_pool [index] . A.2 Pushing Constants onto the Stack ( quick variants) ldcl ςruick Push item from constant pool onto stack Syntax: ldcl quick indexbytel Stack: ...=>..., item indexbytel is used as an unsigned 8-bit index into the constant pool of the current class. The item at that index is pushed onto the stack. Idc2_quick Push item from constant pool onto stack Syntax: ldc2_quick indexbytel indexbyte2 Stack: ...=>... ,item indexbytel and indexbyte2 are used to construct an index into the constant pool of the current class. The constant at that index is resolved and the item at that index is pushed onto the stack. ldc2w_quick Push long integer or double float from constant pool onto stack Syntax: ldc2w quick indexbytel indexbyte2 Stack: ...=>... ,constant-wordl,constant-word2 indexbytel and indexbyte2 are used to construct an index into the constant pool of the current class. The constant at that index is pushed onto the stac . A.3 Managing Arrays ( quick variants) anewarray_guick Allocate new array of references to objects Syntax: anewarray_quick indexbytel indexbyte2 Stack: ...,size=>result size must be an integer. It represents the number of elements in the new array. indexbytel and indexbyte2 are used to construct an index into the constant pool of the current class. The entry must be a class. A new array of the indicated class type and capable of holding size elements is allocated, and result is a reference to this new array. Allocation of an array large enough to contain size items of the given class type is attempted. All elements of the array are initialized to zero. If size is less than zero, a NegativeArraySizeException is thrown. If there is not enough memory to allocate the array, an OutOfMemoryError is thrown. multianβwarray quick Allocate new multi-dimensional array Syntax : multianewarray quick indexbytel indexbyte2 dimensions Stack: ... ,sizel,size2, ...sizen=>result Each size must be an integer. Each represents the number of elements in a dimension of the array. indexbytel and indexbyte2 are used to construct an index into the constant pool of the current class . The resulting entry must be a class . dimensions has the following aspects : It must be an integer J l. It represents the number of dimensions being created. It must be £ the number of dimensions of the array class . It represents the number of elements that are popped off the stack. All must be integers greater than or equal to zero. These are used as the sizes of the dimension. If any of the size arguments on the stack is less than zero, a NegativeArraySizeException is thrown. If there is not enough memory to allocate the array, an OutOfMemoryError is thrown. The result is a reference to the new array object. A.4 Manipulating Object Fields (_quick variants) putfield_quick Set field in object Syntax: putfield2_quick offset unused Stack: ... ,objectref,value=>... objectref must be a reference to an object, value must be a value of a type appropriate for the specified field. offset is the offset for the field in that object, value is written at offset into the object. Both objectref and value are popped from the stack. If objectref is null, a NullPointerException is generated. putfield2_quick Set long integer or double float field in object Syntax: putfield2_quick offset unused Stack: ... ,objectref,value-wordl,value-word2=>... objectref must be a reference to an object. value must be a value of a type appropriate for the specified field. offset is the offset for the field in that object. value is written at offset into the object. Both objectref and value are popped from the stack. If objectref is null, a NullPointerException is generated. getfield quick Fetch field from object Syntax: getfield2_quick offset unused Stack: ... ,objectref=>... ,value objectref must be a handle to an object. The value at offset into the object referenced by objectref replaces objectref on the top of the stack. If objectref is null, a NullPointerException is generated. getfield2_quick Fetch field from object Syntax: gβtfield2 quick offset unused Stack: ... ,objectref=>... ,value-wordl,value-word2 ob;==tref must be a handle to an object. The value at offset into the objer: referenced by objectref replaces objectref on the top of the stack. I: i jectref is null, a NullPointerException is generated. putstat:.: quick Sε: static field in class Sλ _=_X: putstatic quick indexbytel indexbyte2 St__._: ...,values>... in_u_xbytel and indexbyte2 are used to construct an index into the constant rrol of the current class. The constant pool item will be a field rε:_ren e to a static field of a class.value must be the type appropriate r.3 that field. That field will be set to have the value value. putstatic! quick Se static field in class putβtatic2_quick indexbytel indexbyte2 Stϊ-r..: ... ,value-wordl,value- ord2=>... in___xbytel and indexbyte2 are used to construct an index into the constant tool of the current class. The constant pool item will be a field re:__rence to a static field of a class. That field must either be a long intfrer or a double precision floating point number, value must be the type appropriate to that field. That field will be set to have the value v_.l__e. getstat_.:_quick Ge.: static field from class Syntax : getstatic_quick indexbytel indexbyte2 Stack: ...,=>...,value indexbytel and indexbyte2 are used to construct an index into the constant pool of the current class . The constant pool item will be a field reference to a static field of a class. The value of that field will replace handle on the stack. getstatic2_quick Get static field from class Syntax: getstatic2_quick indexbytel indexbyte2 Stack: ...,=>... ,value-wordl,value-word2 indexbytel and indexbyte2 are used to construct an index into the constant pool of the current class. The constant pool item will be a field reference to a static field of a class. The field must be a long integer or a double precision floating point number. The value of that field will replace handle on the stack A.5 Method Invocation ( quick variants) invokevirtual_quick Invoke instance method, dispatching based on run-time type Syntax: invokβvirtual quick offset nargs Stack: ... ,objectref, [argl, [arg ...]]=>... The operand stack must contain objectref, a reference to an object and nargs-1 arguments. The method block at offset in the object's method table, as determined by the object's dynamic type, is retrieved The method block indicates the type of method (native, synchronized, etc ) If the method is marked synchronized the monitor associated with the object is entered The base of the local variables array for the new JAVA stack frame is set to point to objectref on the stack, making objectref and the supplied arguments (argl,arg2, ...) the first nargs local variables of the new frame The total number of local variables used by the method is determined, and the execution environment of the new frame is pushed after leaving sufficient room for the locals The base of the operand stack for this method invocation is set to the first word after the execution environment Finally, execution continues with the first instruction of the matched method If objectref is null, a NullPointerException is thrown If during the method invocation a stack overflow is detected, a StackOverflowError is thrown invokevirtualobject_quick Invoke instance method of class JAVA lang Object, specifically for benefit of arrays Syntax: invokevirtualobject quick offset nargs Stack: ... ,objectref, [argl, [arg2...]}=>... The operand stack must contain objectref, a reference to an object or to an array and nargs-1 arguments. The method block at offset in JAVA.lang.Object's method table is retrieved The method block indicates the type of method (native, synchronized, etc.) If the method is marked synchronized the monitor associated with handle is entered. The base of the local variables array for the new JAVA stack frame is set to point to objectref on the stack, making objectref and the supplied arguments (argl,arg2, ... ) the first nargs local variables of the new frame. The total number of local variables used by the method is determined, and the execution environment of the new frame is pushed after leaving sufficient room for the locals The base of the operand stack for this method invocation is set to the first word after the execution environment. Finally, execution continues with the first instruction of the matched method. If objectref is null, a NullPointerException is thrown. If during the method invocation a stack overflow is detected, a StackOverflowError is thrown. invokenonvirtual_quick Invoke instance method, dispatching based on compile-time type Syntax: invokenonvirtual_quick indexbytel indexbyte2 Stack: ... ,objectref, [argl, [arg2...]]=>... The operand stack must contain objectref, a reference to an object and some number of arguments. indexbytel and indexbyte2 are used to construct an index into the constant pool of the current class. The item at that index in the constant pool contains a method slot index and a pointer to a class. The method block at the method slot index in the indicated class is retrieved. The method block indicates the type of method (native, synchronized, etc.) and the number of arguments (nargs) expected on the operand stack. If the method is marked synchronized the monitor associated with the object is entered. The base of the local variables array for the new JAVA stack frame is set to point to objectref on the stack, making objectref and the supplied arguments (argl, arg2,...) the first nargs local variables of the new frame. The total number of local variables used by the method is determined, and the execution environment of the new frame is pushed after leaving sufficient room for the locals. The base of the operand stack for this method invocation is set to the first word after the execution environment. Finally, execution continues with the first instruction of the matched method. If objectref is null, a NullPointerException is thrown. If during the method invocation a stack overflow is detected, a StackOverflowError is thrown. invokes atic_quiek Invoke a class (static) method Syntax: invokestatic_quick indexbytel indexbytβ2 Stack: ..., [argl, [arg2...]]=>... The operand stack must contain some number of arguments . indexbytel and indexbyte2 are used to construct an index into the constant pool of the current class . The item at that index in the constant pool contains a method slot index and a pointer to a class. The method block at the method slot index in the indicated class is retrieved. The method block indicates the type of method (native, synchronized, etc.) and the number of arguments (nargs) expected on the operand stack. If the method is marked synchronized the monitor associated with the method's class is entered. The base of the local variables array for the new JAVA stack frame is set to point to the first argument on the stack, making the supplied arguments (argl,arg2, ... ) the first nargs local variables of the new frame. The total number of local variables used by the method is determined, and the execution environment of the new frame is pushed after leaving sufficient room for the locals. The base of the operand stack for this method invocation is set to the first word after the execution environment. Finally, execution continues with the first instruction of the matched method. If the object handle on the operand stack is null, a NullPointerException is thrown. If during the method invocation a stack overflow is detected, a StackOverflowError is thrown. invokeinterface quick Invoke interface method Syntax: invokeinterface quick idbytel idbyte2 nargs Stac : ... ,objectref, [argl, [arg2...]]=>... The operand stack must contain objectref, a reference to an object, and nargs-1 arguments. idbytel and idbyte2 are used to construct an index into the constant pool of the current class . The item at that index in the constant pool contains the complete method signature. A pointer to the object's method table is retrieved from the object handle. The method signature is searched for in the object's method table. As a short-cut, the method signature at slot guess is searched first. If that fails, a complete search of the method table is performed. The method signature is guaranteed to exactly match one of the method signatures in the table. The result of the lookup is a method block. The method block indicates the type of method (native, synchronized, etc.) but the number of available arguments (nargs) is taken from the bytecode. If the method is marked synchronized the monitor associated with handle is entered. The base of the local variables array for the new JAVA stack frame is set to point to handle on the stack, making handle and the supplied arguments (argl,arg2, ... ) the first nargs local variables of the new frame. The total number of local variables used by the method is determined, and the execution environment of the new frame is pushed after leaving sufficient room for the locals. The base of the operand stack for this method invocation is set to the first word after the execution environment. Finally, execution continues with the first instruction of the matched method. If objectref is null, a NullPointerException is thrown. If during the method invocation a stack overflow is detected, a StackOverflowError is thrown. guess is the last guess. Each time through, guess is set to the method offset that was used. A.6 Miscellaneous Object Operations ( quick variants) new_quick Create new object Syntax: new_quick indexbytel indexbyte2 Stack: . .=>... ,objectref indexbytt and indexbyte2 are used to construct an index into the constant pool i: the current class. The item at that index must be a class. A new _.__.ranee of that class is then created and objectref, a reference to tr * -: object is pushed on the stack. checkcast quicr. Make sur. r ject is of given type Syntax: checkcast quick indexbytel indexbyte2 Stack: . . ,objectref=>... ,objectref objectre rust be a reference to an object, indexbytel and indexbyte2 are _ _ed to construct an index into the constant pool of the current class. Tne object at that index of the constant pool must have already been re_rived. checkcaε: ..len determines whether objectref can be cast to a reference to ar. object of class class. A null reference can be cast to any class, and r ___erwise the superclasses of objectrefs type are searched for class. If zlass is determined to be a superclass of objectref's type, or if objectref s null, it can be cast to objectref cannot be cast to class, a Class: stException is thrown. instanceof quiet Determin. : f object is of given type Syntax: instanceof quick indexbytel indexbyte2 Stack: ....objectref=>... ,result objectref must be a reference to an object. indexbytel and indexbyte2 are used to construct an index into the constant pool of the current class . The item of class class at that index of the constant pool must have already been resolved. Instance of determines whether objectref can be cast to an object of the class class. A null objectref can be cast to any class, and otherwise the superclasses of objectrefs type are searched for class. If class is determined to be a superclass of objectrefs type, result is 1 (true) . Otherwise, result is 0 (false) . If handle is null, result is 0 (false) .

Claims: WHAT IS CLAIMED IS: 1. In a virtual machine instruction processor wherein instructions generally source operands from, and target a result to, uppermost entries of an operand stack, an apparatus comprising: an instruction store; an operand stack; a data store; an execution unit; and an instruction decoder coupled to the instruction store to identify a foldable sequence of instructions represented therein, the foldable sequence including first and second instructions, the first instruction for pushing a first operand value onto the operand stack from the data store merely as a first source operand for a second instruction, the instruction decoder coupled to supply the execution unit with a single folded operation equivalent to the foldable sequence and including a first operand address identifier selective for the first operand value in the data store, thereby obviating an explicit operation corresponding to the first instruction. 2. An apparatus, as recited in claim 1, wherein the data store includes local variable storage. 3. An apparatus, as recited in claim 1, wherein the data store includes constant storage. 4. An apparatus, as recited in claim 1, wherein the operand stack and the data store are represented in a storage hierarchy including a stack cache, and wherein the stack cache caches at least a portion of entries in the operand stack and the data store. 5. An apparatus, as recited in claim 4, wherein the instruction decoder selectively disables supply of the equivalent folded operation if the first operand value is not represented in the stack cache portion of the storage hierarchy, and instead supplies the execution unit with an operation identifier and operand address identifier corresponding to the first instruction only. 6. An apparatus, as recited in claim 1 , wherein if the sequence of instructions represented in the instruction buffer is not a foldable sequence, the instruction decoder supplies the execution unit with an operation identifier and operand address identifier corresponding to the first instruction only. 7. An apparatus, as recited in claim 1 , wherein the instruction decoder further identifies a third instruction in the foldable sequence, the third instruction being for pushing a second operand value onto the operand stack from the data store merely as a second source operand for the second instruction; and wherein the single folded operation equivalent to the foldable sequence includes a second operand address identifier selective for the second operand value in the data store, thereby obviating an explicit operation corresponding to the third instruction. 8. An apparatus, as recited in claim 1 , wherein the instruction decoder further identifies a fourth instruction in the foldable sequence, the fourth instruction being for popping a result of the second instruction from the operand stack and storing the result in a result location of the data store; and wherein the single folded operation equivalent to the foldable sequence includes a destination address identifier selective for the result location in the data store, thereby obviating an explicit operation corresponding to the fourth instruction. 9. An apparatus, as recited in claim 1 , wherein the instruction decoder further identifies third and fifth instructions in the foldable sequence, the third and fifth instructions respectively being for pushing second and third operand values onto the operand stack from the data store merely as a respective second and third source operands for the second instruction; and wherein the single folded operation equivalent to the foldable sequence includes second and third operand address identifiers respectively selective for the second and third operand value in the data store, thereby obviating explicit operations corresponding to the third and fifth instructions. 10. An apparatus, as recited in claim 1 , wherein the instruction decoder further identifies fourth and sixth instructions in the foldable sequence, the fourth and sixth instructions respectively being for popping first and second results of the second instruction from the operand stack and storing the first and second results in respective first and second result locations of the data store; and wherein the single folded operation equivalent to the foldable sequence includes first and second destination address identifiers respectively selective for the first and second result locations in the data store, thereby obviating explicit operations corresponding to the fourth and sixth instructions. 11. An apparatus, as recited in claim 1 , wherein the foldable sequence comprises two or more instructions. 12. An apparatus, as recited in claim 1, wherein the foldable sequence comprises four instructions. 13. An apparatus, as recited in claim 1 , wherein the foldable sequence comprises five instructions. 14. An apparatus, as recited in claim 1, wherein the foldable sequence comprises more than five instructions. 15. An apparatus, as recited in claim 1 , wherein the instruction decoder further comprises: normal and folded decode paths; and switching means responsive to the folded decode path for selecting operation, operand, and destination identifiers from the folded decode path in response to a fold indication therefrom, and for otherwise selecting operation, operand, and destination identifiers from the normal decode path. 16. An apparatus, as recited in claim 1, wherein the virtual machine instruction processor is a hardware virtual machine instruction processor and the instruction decoder comprises decode logic. 17. An apparatus, as recited in claim 1 , wherein the virtual machine instruction processor includes a just-in-time compiler implementation and the instruction decoder comprises software executable on a hardware processor, the hardware processor including the execution unit. 18. An apparatus, as recited in claim 1, wherein the virtual machine instruction processor includes a bytecode inteφreter implementation and the instruction decoder comprises software executable on a hardware processor, the hardware processor including the execution unit. 19. A method for decoding virtual machine instructions in a virtual machine instruction processor wherein generally source operands from, and target a result to, uppermost entries of an operand stack, the method comprising: (a) determining if a first instruction of a virtual machine instruction sequence is an instruction for pushing a first operand value onto the operand stack from a data store merely as a first source operand for a second instruction; and if the result of the (a) determining is affirmative, supplying an execution unit with a single folded operation equivalent to a foldable sequence comprising the first and second instructions, the single folded operation including a first operand identifier selective for the first operand value, thereby obviating an explicit operation corresponding to the first instruction. 20. A method as recited in claim 19, further comprising: if the result of the (a) determining is negative, supplying the execution unit with an operation equivalent to the first instruction in the virtual machine instruction sequence. 21. A method as recited in claim 19, further comprising: (b) determining if a third instruction of the virtual machine instruction sequence is an instruction for popping a result value of the second instruction from the operand stack and storing the result value in a result location of the data store; and if the result of the (b) determining is affirmative, further including a result identifier selective for the result location with the equivalent single folded operation, thereby further obviating an explicit operation corresponding to the third instruction. 22. A method as recited in claim 21 , further comprising: if the result of the (b) determining is negative, including a result identifier selective for a top location of the operand stack with the equivalent single folded operation. 23. A method as recited in claim 19, wherein the (a) determining includes: (al ) determining if the first instruction is for pushing the first operand value onto the operand stack from the data store; and (a2) determining if the second instruction is for operating on first operand value on operand stack and pushing a result value of the second instruction onto the operand stack such the first operand value is no longer represented in the uppermost entries of the operand stack. 24. A method as recited in claim 23, further comprising: performing the (al ) determining and the (a2) determining substantially in parallel. 25. A method as recited in claim 23, further comprising: perfor ing the (a) determining and the (b) determining substantially in parallel. 26. A stack-based virtual machine implementation comprising: a randomly-accessible operand stack representation; a randomly-accessible local variable storage representation; and a virtual machine instruction decoder for selectively decoding virtual machine instructions and folding together a selected sequence thereof to eliminate unnecessary temporary storage of operands on the operand stack. 27 A stack-based virtual machine implementation, as recited in claim 26, further comprising a hardware virtual machine instruction processor, including a hardware stack cache, a hardware instruction decoder, and an execution unit, wherein the randomly-accessible operand stack local variable storage representations at least partially reside in the hardware stack cache, wherein the virtual machme instruction decoder comprises the hardware instruction decoder coupled to provide the execution unit with opcode, operand, and result identifiers respectively selective for a hardware virtual machine instruction processor operation and for locations in the hardware stack cache as a single hardware virtual machine instruction processor operation equivalent to the selected sequence of virtual machine instructions 28 A stack-based virtual machine implementation, as recited in claim 26, further comprising software encoded in a computer readable medium and executable on a hardware processor, wherein the randomly-accessible operand stack local variable storage representations at least partially reside in registers of the hardware processor, wherein the virtual machme instruction decoder is at least partially implemented in the software, and wherein the virtual machine instruction decoder is coupled to provide opcode, operand, and result identifiers respectively selective for a hardware processor operation and for locations in the registers as a single hardware processor operation equivalent to the selected sequence of virtual machine instructions 29 A hardware virtual machme instruction decoder comprising a normal decode path, a fold decode path for decoding a sequence of virtual machine instructions and, if the sequence is foldable, supplying a smgle operation identifier, one or more operand identifiers, and a destination identifier, together equivalent to the sequence of virtual machme instructions; and switching means responsive to the folded decode path for selecting operation, operand, and destination identifiers from the folded decode path in response to a fold indication therefrom, and for otherwise selecting operation, operand, and destination identifiers from the normal decode path 30. A hardware virtual machme mstruction processor comprismg an mstruction cache unit including an mstruction cache; and an mstruction buffer, an integer execution unit including an instruction decode unit comprising: a normal decode path; a fold decode path for decoding a sequence of virtual machine instructions from the instruction buffer and, if the sequence is foldable, supplying a single operation identifier, operand identifiers, and a destination identifier together equivalent to the sequence of virtual machine instructions; and a switch complex responsive to the folded decode path for selecting operation, operand, and destination identifiers from the folded decode path in response to a foldable sequence indication therefrom, and otherwise selecting operation, operand, and destination identifiers from the normal decode path; an integer unit coupled to the switch complex for executing in accordance with operation, operand and destination identifiers selected thereby; and a stack management unit coupled to the integer execution unit for representing operand stack and data store locations identified by the selected operand and destination identifiers.

Inventors: O'CONNOR JAMES MICHAEL, TREMBLAY MARC, O'CONNOR, JAMES, MICHAEL, TREMBLAY, MARC
Applicants: SUN MICROSYSTEMS INC, SUN MICROSYSTEMS, INC.
Application year: 1997
Main classification: G06F9
IPC codes: G06F9, G06F12