Logical substitution of processor control in an emulated computing environment

0001The present invention relates in general to the field of computer system emulation and, more particularly, to a method for transparently decoupling the host operating system from the processor of the computer system in favor of control by the processor by the guest operating system.

0002Computers include general purpose central processing units (CPUs) that are designed to execute a specific set of system instructions. A group of processors that have similar architecture or design specifications may be considered to be members of the same processor family. Examples of current processor families include the Motorola 680X0 processor family, manufactured by Motorola, Inc. of Phoenix, Arizona; the Intel 80X86 processor family, manufactured by Intel Corporation of Sunnyvale, California; and the PowerPC processor family, which is manufactured by Motorola, Inc. and used in computers manufactured by Apple Computer, Inc. of Cupertino, California. Although a group of processors may be in the same family because of their similar architecture and design considerations, processors may vary widely within a family according to their clock speed and other performance parameters.

0003Each family of microprocessors executes instructions that are unique to the processor family. The collective set of instructions that a processor or family of processors can execute is known as the processor's instruction set. As an example, the instruction set used by the Intel 80X86 processor family is incompatible with the instruction set used by the PowerPC processor family. The Intel 80X86 instruction set is based on the Complex Instruction Set Computer (CISC) format. The Motorola PowerPC instruction set is based on the Reduced Instruction Set Computer (RISC) format. CISC processors use a large number of instructions, some of which can perform rather complicated functions, but which require generally many clock cycles to execute. RISC processors use a smaller number of available instructions to perform a simpler set of functions that are executed at a much higher rate.

0004The uniqueness of the processor family among computer systems also typically results in incompatibility among the other elements of hardware architecture of the computer systems. A computer system manufactured with a processor from the Intel 80X86 processor family will have a hardware architecture that is different from the hardware architecture of a computer system manufactured with a processor from the PowerPC processor family. Because of the uniqueness of the processor instruction set and a computer system's hardware architecture, application software programs are typically written to run on a particular computer system running a particular operating system.

0005A computer manufacturer will want to maximize its market share by having more rather than fewer applications run on the microprocessor family associated with the computer manufacturer's product line. To expand the number of operating systems and application programs that can run on a computer system, a field of technology has developed in which a given computer having one type of CPU, called a host, will include an emulator program that allows the host computer to emulate the instructions of an unrelated type of CPU, called a guest. Thus, the host computer will execute an application that will cause one or more host instructions to be called in response to a given guest instruction. Thus, the host computer can both run software designed for its own hardware architecture and software written for computers having an unrelated hardware architecture. As a more specific example, a computer system manufactured by Apple Computer, for example, may run operating systems and programs written for PC-based computer systems. It may also be possible to use an emulator program to operate concurrently on a single CPU multiple incompatible operating systems. In this arrangement, although each operating system is incompatible with the other, an emulator program can host one of the two operating systems, allowing the otherwise incompatible operating systems to run concurrently on the same computer system.

0006When a guest computer system is emulated on a host computer system, the guest computer system is said to be a virtual machine, as the host computer system exists only as a software representation of the operation of the hardware architecture of the guest computer system. The terms emulator and virtual machine are sometimes used interchangeably to denote the ability to mimic or emulate the hardware architecture of an entire computer system. As an example, the Virtual PC software created by Connectix Corporation of San Mateo, California emulates an entire computer that includes an Intel 80X86 Pentium processor and various motherboard components and cards. The operation of these components is emulated in the virtual machine that is being run on the host machine. An emulator program executing on the operating system software and hardware architecture of the host computer, such as a computer system having a PowerPC processor, mimics the operation of the entire guest computer system. The emulator program acts as the interchange between the hardware architecture of the host machine and the instructions transmitted by the software running within the emulated environment.

0007A hypervisor is a control program that exists near the kernel level of a host operating system and operates to allow one or more secondary operating systems, other than the host operating system, to use the hardware of the computer system, including the processor of the computer system. A hypervisor of an operating system emulates the operating environment of the secondary operating system so that the secondary operating system believes that it is operating in its customary hardware and/or operating system environment and that it is in logical control of the computer system, when it may in fact be operating in another hardware and/or operating system environment and the host operating system may be in logical control of the computer system. Many operating systems function such that the operating system must operate as though it is in exclusive logical control of the hardware of the computer system. For multiple operating systems to function simultaneously on a single computer system, the hypervisor of each operating system must function to mask the presence of the other operating systems such that each operating system functions as though it has exclusive control over the entire computer system.

0008If it is desired to emulate the entire hardware architecture of the guest system, it is often useful or desirable to make maximum use of the hardware in the host computer system, including, for example, the memory management unit, the context switching hardware, the exception handling vectors, the interrupt routine pointers, and the floating point units of the host processor, and the memory of the host computer system. This hardware, however, is normally under the control of the host operating system and is shielded from access by user-level or application software. An exception is an event occurring in the computer system that cannot be resolved by the processor. An interrupt is a request sent to the processor to temporarily interrupt the current instruction stream to perform some other operation

0009One option to avoid this difficulty is to employ an emulation program that does not attempt to use the lower level hardware of the host computer system. This sort of approach is often used in the case of an emulation program that resides as an application program and runs on the host operating system. These application emulation programs, of course, are subject to the performance penalties of being an application program that runs on the host operating system of a host computer system. Further, these application emulation programs only gain access to the hardware features that are exposed to the application emulation program by the host operating system. As an example, an emulation program that runs as an application on the host operating system can include an exception handler. The difficulty is that the exception handler of the emulation program is separated from the processor of the host computer system by several software layers, creating a latency between the time that the host processor issues an exception and the time that the exception is identified to the exception handler of the emulation program. Another option is for the emulation program to share the hardware with the host operating system, assuming that support for the sharing of a computer system by an emulation program can be written into the hypervisor of the host operating system.

0010Another difficulty of using a hypervisor in an emulated computing environment is managing the memory space of the emulated computer system together with the memory space of the hypervisor itself. To improve the operation of the emulated computer system, the hypervisor will typically seek to operate the page tables of the host computer system such that the page tables of the host computer system largely mirror the page tables of the guest operating system. One difficulty of this approach is that the hypervisor code and its associated data is not mapped in to the page tables of the guest operating system. This is necessarily the case, as the emulated computer system is not aware that a hypervisor is hosting the system on the hardware of the host computer system. As such, the page table of the hypervisor does not completely match the page table of the host computer system. The page table of the hypervisor includes the mapping of the code and data of the hypervisor itself.

0011The mirroring by the hypervisor of the pages tables of the guest operating system raises the possibility that the address mapping of the guest operating system will conflict with the address mapping of the hypervisor, as the address mapping of the guest operating system may include the mapping of addresses over the mapped locations of the code and data of the hypervisor itself. One solution to this problem of conflicting page mappings is to alter the page table of the guest operating system when the mapping of the logical memory of the guest operating system conflicts with the mapping prescribed by the nearly identical page table of the hypervisor. Altering the page table of the emulated computer system is undesirable, however, in that it would introduce to the emulated computer system an unexpected change to its page tables, possibly introducing other incompatibilities or faults in the guest operating system.

0012<patcit id="pcit0001" dnum="DE4217444A"><text>DE 42 17 444 A</text></patcit> describes a virtual machine system having a plurality of virtual machines, in which each virtual machine can be relocated to a new storage area of main storage without interfering with any other virtual machine. The relocation is carried out by move instructions issued by a service processor upon receipt of a relocate command. These instructions pass control to a controlling section which instructs a logical instruction processor controlling section to temporarily stop the logical instruction processor of the particular virtual machine. Next, a check is made by the resource management section to determine whether the virtual machine can be moved. Once that has been checked, the virtual machine is relocated on the main storage area in accordance with a designated address by the relocate command and the controlling section restores operation of the virtual machine.

0013<patcit id="pcit0002" dnum="FR2587519A"><text>FR-A-2 587 519</text></patcit> relates to a virtual machine system in which a mode-changing arrangement is used to change the operation mode of the system. A mode memory is used for storing mode data indicating one of three different modes, namely a non-virtual machine mode, a virtual machine monitoring mode or a virtual machine operating system mode. An instruction register provides an instruction to select one of the mode data which is then loaded into a mode register so as to operate the virtual machine system in the mode indicated by the mode data stored in the mode register.

0014It is the object of the present invention to provide a method for an emulator for logically decoupling a host operating system of a host computer system from the processor of the host computer system.

0015The object is solved by the subject matter of the independent claim. Preferred embodiments of the present invention are defined by the dependent claims.

0016The present invention concerns a technique for logically decoupling a host operating system from a processor of the computer system. According to the method of the present invention, a hypervisor of an emulation program reads in a set of the processor settings of the processor. These processor settings are saved by the hypervisor to main memory. The hypervisor replaces these processor settings with a set of processor settings associated with software routines or data structures provided by the guest operating system. These software routines or data structures may include exception and interrupt handler routines and page tables. As a result of the substitution of processor settings, the host operating system is decoupled from processor control in that at least some of the processor settings of the processor are associated with software routines or data structures provided by the guest operating system, and are no longer associated with software routines or data structures provided by the host operating system. To recouple the host operating system to the processor, the hypervisor reads in the saved processor settings from memory and writes these processor settings to the appropriate registers of the processor.

0017During the period that the hypervisor is coupled to the processor, the page table of the hypervisor largely mirrors the page table of the guest operating system, with the primary difference being that the page table of the hypervisor includes the mapped in locations of the hypervisor code itself. To avoid conflicts between the page tables of the guest operating system and the page tables of the hypervisor, the page table of the hypervisor is reallocated if a change to the page table of the guest operating system results in mapped in memory location in the guest operating system overlapping with the mapped in location of the hypervisor code in the page table of the hypervisor.

0018The logical decoupling and recouping method disclosed herein is advantageous in that it allows the guest operating system to be more closely aligned with the processor for some functionality that can be provided by the guest operating system. In this manner, exception handler calls, interrupt handler calls, and memory management calls need not be passed from the processor to the host operating system before being passed to the guest operating system. Rather, these calls from the processor are passed directly to the guest operating system via the hypervisor of the emulation program.

0019The method disclosed herein is also advantageous in that the hypervisor or decoupling tool of the emulation program need not have access to the source code of the host operating system. Rather, the hypervisor decouples the host operating system from the processor in a manner that is transparent to the host operating system, which has no awareness that it has been decoupled or recoupled to the processor.

0020The method disclosed herein is also advantageous in that even if the host operating system is decoupled from the processor with respect to some functionality normally provided by the host operating system, the host operating system remains coupled to the host processor for the remainder of the functionality of the host processor. During this period, the host operating system continues to manage much of the hardware functionality of the computer system, allowing the guest operating system to route a print request through the host operating system.

0021An advantage of reallocating the page table of the hypervisor in the event of a conflict with the guest computer system is that the reallocation of the page table allows the page table of the hypervisor to shadow the page table of the guest operating system, while at the same time mapping in the code of the hypervisor itself. In this environment, changes to the page table of the guest operating system do not result in a conflict between the page tables of the guest operating system and the hypervisor. In addition, the reallocation of the page table of the hypervisor is transparent to the operation of the guest operating system, avoiding the possibility of creating system-level conflicts in the guest operating system.

0022Other technical advantages of the present invention will be readily apparent to one skilled in the art from the following figures, descriptions, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

0023A more complete understanding of the present invention and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein: <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Figure 1</figref> is a diagram of the logical relationship of the elements of an emulated computer system running in a host computer system;</li><li><figref idref="f0002">Figure 2</figref> is a flow diagram of the method for logically coupling a hypervisor of an emulation program to the processor of a computer system;</li><li><figref idref="f0003">Figure 3</figref> is a diagram of the logical relationship of the elements of an emulated computer system in which a hypervisor is logically coupled to the processor of the computer system; and</li><li><figref idref="f0004">Figure 4</figref> is a flow diagram of a method for mapping the page tables of a guest operating system.</li></ul>

DETAILED DESCRIPTION OF THE INVENTION

0024The present invention provides an emulation environment that allows the emulation software access to the lower level hardware components of the host computer system. The invention involves an emulation program that includes a hypervisor that is transparent to the host operating system. The hypervisor of the emulation program logically disconnects or decouples the operating system from exclusive control of the host computer system for brief periods, during which time the hypervisor of the emulation program is placed in control of the hardware of the host computer system, allowing the emulation program to be logically near, for brief periods, certain hardware features of the host computer system.

0025The processor state of the processor is a snapshot of all of the processor settings at a single point in time. These settings include both data that the processor is manipulating as well as the program counter, pointers, and other operational flags used by the processor. Because the data being manipulated by the processor and many other processor settings may change with each processor cycle, the processor state may likewise change with each processor cycle, and may change as often as many millions of times per second.

0026One subset of the processor state is the user state. Taken together, the data being manipulated by the processor and the processor's program counter comprise the user state of the processor. The user state is so named because the values that comprise the user state can be manipulated by an application-level program. The remainder of the processor settings of the processor state comprise the supervisor state or privileged state of the processor. The values of the supervisor state can only be modified by software that is part of the operating system of the computer system. The settings of the supervisor state or privileged state of the processor state cannot be accessed by an application-level program. The separation of processor settings between a user state, which is accessible by application-level programs, and a supervisor state, which is not accessible by application-level programs, allows the operating system to remain functional even if the application level program crashes or causes a fatal error.

0027The settings of the supervisor state of the processor generally fall into one of several categories. One category of the supervisor state of the processor is the processor's mode flags, which include the settings that instruct the processor to operate according to one of several modes to support certain computational situations, exception behavior, support for legacy programs, or the selective use of newer features of the processor. Another category of settings of the processor state are those settings that control external hardware access. Processors often support several operational settings for communication between the processor and hardware external to the processor, such as caches, memory, and other input and output devices. Because these operational settings, including the communication speed and bandwidth, for example, can only be modified by privileged or supervisor level code, they are considered to be part of the supervisor state of the processor.

0028Another category of settings of the privileged state of the processor are the exception and interrupt routine pointers. When the processor encounters an exception or is asked for an interrupt, the processor stops execution at the current program counter address and begins execution at a predetermined exception routine address. The location of these exception routines are identified through the use of exception routine pointers. For some processors, the exception routines are located at hard-coded memory addresses. On most processors, however, their locations are programmable and the value of the locations are considered to be part of the supervisor state of the processor.

0029A set of data structure pointers make up another category of the privileged state of the processor. Because some data structures of the processor state are too large to be stored in the processor, these data structures are stored in main memory and a pointer to the data structure is stored at the processor. An example of this sort of large data structure is the processor's page tables. These tables defme for the processor the logical-to-physical translation of memory addresses that are necessary for the conversion of virtual memory accesses to physical memory locations. Both the page table and the pointer to the page table, which is stored at the processor, are considered to be part of the supervisor state of the processor. Another example of a data structure of the processor's supervisor state that is stored in main memory is an exception routine pointer table. Rather than storing the pointer for each exception routine at the processor, a single pointer stored at the processor can point to a table of exception routine pointers. The exception pointer or vector table will include a listing of address pointers to the exception handlers accessible by the processor. Both the exception routine pointer that is stored at the processor and the exception routine pointer table are considered part of the supervisor state.

0030The host operating system typically establishes for the processor the vectors or pointers to the exception and interrupt handler routines. If, for example, the processor is instructed to process a set of commands that will cause the processor to divide by zero, the processor will call the exception handler routine that handles the divide-by-zero exception. In doing so, the processor may call an exception handler routine that is pointed to by a vector stored at the processor. Alternatively, the processor may scan the exception vector table for the correct exception handler for the exception condition and then call the exception handler using the exception vector or pointer associated with the applicable exception handler.

0031When the host operating system is in logical control of the host processor, the host operating system defines for the host processor the exception vectors and all other vectors that will be needed by and are supported by the host processor. Should the processor encounter an exception condition or need to access another routine that is referenced by a vector, such as the page table routines and the page table vector, the processor executes the host operating system's exception handler.

0032In the case of an emulated computer system, an emulation program provides an emulated operating environment in the host computer system. Shown in <figref idref="f0001">Figure 1</figref> is a diagram of the logical layers of the hardware and software architecture for an emulated operating environment in a computer system 10. An emulation program 14 runs on a host operating system that executes on the host computer system hardware or processor 11. Emulation program 14 emulates a guest computer system 16, including a guest operating system 18. Guest application programs are able to execute on guest operating system 18. In the emulated operating environment of <figref idref="f0001">Figure 1</figref>, because of the operation of emulation program 14, guest application 20 can run on the computer system 10 even though guest application 20 is designed to run on an operating system that is generally incompatible with host operating system 12 and host computer system hardware 11. In the architecture of <figref idref="f0001">Figure 1</figref>, guest operating system 18 is separated from the host computer system hardware 11 across several logical and software layers, including the host operating system 12. This logical separation introduces latency and performance difficulties in those instances in which the guest operating system attempts to communicate directly with the host computer system hardware 11.

0033In the example of <figref idref="f0001">Figure 1</figref>, host operating system 12 is in exclusive operating control of the host computer system hardware 11, including the processor of the host computer system. The host operating system will have established for the processor of the host computer system a number of the settings of the processor state. For example, the host operating system 12 may have established a set of exception handler vectors, whether located in a vector table or in the registers of the host processor, a set of interrupt handlers, and a page table vector. Thus, when the processor makes exception handler calls, interrupt handler calls, or memory management calls, the processor settings for these calls is provided on an exclusive basis by the host operating system. Because the host operating system established the settings of the supervisor or privileged state of the processor, the host operating system is said to be in logical control of the processor of the host computer system.

0034According to the emulation technique of the present invention, the emulation program itself includes a hypervisor that is able to logically disconnect or decouple the host operating system from exclusive control of the processor and other host computer system hardware 11. The hypervisor of emulation program 14 accomplishes the logical disconnection step by reading in and saving to a memory location the vectors and other address pointers that are used by the host processor. A flow diagram of the steps taken by the hypervisor to logically disconnect the host operating system from the exclusive control of the host computer system and to logically connect or couple the hypervisor of the emulation program to the host computer system is shown in <figref idref="f0002">Figure 2</figref>. When the hypervisor of the emulation program wants to assert control over at least some of the processor settings of the processor, the hypervisor at step 24 reads in some or all of the settings of the supervisor or privileged state of the processor. According to one embodiment of the invention, the processor reads in the address pointers from the applicable registers of the processor, including the exception vectors or the pointer to the exception vector table, the interrupt pointers or the pointer to the interrupt routine table, and the pointer to the page table. At step 26, the hypervisor saves these processor settings, together with an identification of their associated storage location on the host processor, to memory. The hypervisor next substitutes at step 28 the user-level processor state of the guest operating system and the privileged-level state of the hypervisor of the emulation program. As a result, the processor settings of the host processor are replaced with a combination of the user-level processor state of the guest operating system and the privileged-level state associated with the hypervisor of the emulation program. In doing so, the hypervisor stores in the appropriate registers of the processor the vectors and other address pointers to the exception handlers, interrupt routines, and page maps of the hypervisor of the emulation program and the guest operating system.

0035Because the processor settings that are substituted by the hypervisor of the emulation program are supervisor or privileged level settings, the hypervisor of the emulation program has logical control of the processor for all of the substituted processor settings. In this manner, the emulated computer system is logically in control of the hardware functionality of the host computer system for the functionality governed by the substituted processor settings. A diagram of the logical relationship of the emulation program 14 following substitution of the processor settings is shown in <figref idref="f0003">Figure 3</figref>. Hypervisor 22 in <figref idref="f0003">Figure 3</figref> is shown as being a component of emulation program 14, which is shown as having a logical communication link to the processor and host hardware 11.

0036As an example of the emulation program's logical control of host hardware 11, if an exception were to occur during this period, the processor would call an exception handler provided by the hypervisor of the emulation program. Similarly, if the processor is called on to translate a virtual memory address to a physical memory address, the processor will access the page table associated with the guest operating system. Thus, many settings of the processor states have been replaced so that the processor is calling upon or accessing software or data structures provided directly by the guest operating system. This allows the guest operating system to logically align itself more closely with the processor of the computer system, and eliminates the latency caused by separating the guest operating system from the processor of the computer system across the host operating system. In the case of an exception, the exception handler of the hypervisor may handle the exception itself or the exception handler of the hypervisor may invoke the exception handler of the guest operating system, allowing the guest operating system to directly handle the exception without the necessity of having the exception call pass through the host operating system software layer.

0037As shown in <figref idref="f0003">Figure 3</figref>, host operating system 12 is not permanently disconnected form the hardware of the host computer system. The logical control asserted by the hypervisor over a set of processor settings does not result in hypervisor 22 assuming responsibility over all the hardware interaction in the computer system. During those periods when the host operating system is handling a function request relating to the hardware of the computer system, the host operating system must be in logical control of the computer system. To accomplish this, the host operating system must be temporarily logically recoupled to the hardware of the computer system. Once the host operating system has completed the functions requested by the emulation program, the emulation program can reassume its logical control over the exception and interrupt handlers associated with the processor setting of the emulation program.

0038With regard to the flow diagram of <figref idref="f0002">Figure 2</figref>, at step 30, hypervisor 22 returns the logical control over the processor settings to the host operating system. Hypervisor 22 retrieves from memory the vectors and other pointers that were saved to memory. These addresses are written back to the appropriate register locations, returning logical control over these processor settings to the host operating system. During the period that the host operating system is logically disconnected from the processor, the logical disconnection of host operating system 12 is not apparent to host operating system 12. Host operating system 12 is not aware that a set of processor settings has been substituted, resulting in the transfer of logical control for some hardware functionality to the emulation program. When logical control is returned to host operating system 12, host operating system 12 is similarly not aware that logical control of a set of hardware calls has been returned to it.

0039The substitution of some of the processor settings of the processor supervisor states results in a performance gain by the emulation program as compared with the option of the handling by the emulation program of exceptions, interrupts, and memory management functions as filtered through the host operating system. In those cases in which the emulation program operates as an application program that resides on the host operating system, logically disconnecting the host operating system from exclusive control over the functionality of the processor is a performance advantage for the emulated computer system. Once the host operating system has been logically decoupled from at least some of the functionality of the processor, the emulation program, by substituting its processor settings for the processor settings of the host operating system, can operate much more closely with the host processor. Once the processor settings have been substituted, the emulation program can direct the processor to the emulation program's own set of exception vectors, interrupt routines, and page tables, thereby avoiding the latency problems caused when the host operating system separates the processor from the emulation program.

0040The interrupt and exception handling functions of the hypervisor of the present invention also permit the handling of exceptions and interrupts as expected by the guest operating system. An operating system may choose to handle certain exceptions at the operating system level without passing these exceptions to the application that initiated the exception. One example is the divide-by-zero exception, which is often handled by the operating system without being passed to the application program that initiated the exception. The hypervisor of the emulation program can continue to pass exceptions to the guest operating system without the necessity of initiating an exception handler in the hypervisor. Thus, as expected, when a guest application program causes the initiation of a divide-by-zero exception at a time when the emulation program is in logical control of the processor with respect to the substituted processor settings, the hypervisor of the emulation program is able to pass the exception to the guest operating system.

0041Shown in <figref idref="f0004">Figure 4</figref> is a flow diagram of a method for mapping the page tables of a guest operating system. At step 40, the hypervisor identifies a write to the page table of the guest operating system. At step 42, the hypervisor determines if the write to the page table of the guest operating system resulted in mapping conflict between the page tables of the guest operating system and the page table of the hypervisor. The page table of the hypervisor closely mirrors the page table of the guest operating system, with the principal difference being that the page table of the hypervisor maps in to its page table the code and data of the hypervisor. The hypervisor determines that there is a conflict, for example, if mapped pages in the page table of the guest operating system overlap with the mapped in locations of the hypervisor in the page tables of the hypervisor.

0042If the hypervisor determines that there will be no conflict caused by the change to the page tables of the guest operating system, then no change is necessary to the page tables of the hypervisor (step 44). If the hypervisor determines that a change to the page table of the guest operating system causes a conflict with the page table of the host computer system, the hypervisor reallocates its page table at step 46 so that the code and data of the hypervisor itself is moved or mapped in to another location in the page table of the hypervisor. This process of identifying a change to the page table of the guest operating system and, if necessary, reallocating the page table of the hypervisor in response, is transparent to the operation of the guest operating system. Once the page table of the hypervisor has been reallocated, if necessary, the page table of the guest operating system no longer conflicts with the page table of hypervisor.

0043The present invention is not limited in its application to the emulation of a particular computer system architecture, particularly the Intel 80X86 architecture. Rather, the emulation technique disclosed herein is applicable any time it is desirable that a host operating system be transparently disconnected from the processor with respect to at least some of the processor settings of the processor.

0044Although the present invention has been described in detail, it should be understood that various changes, substitutions, and alterations can be made thereto without departing from the scope of the invention as defined by the appended claims.