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Eric Kohlbrenner
Dana Morris
Brett Morris


The OSI Virtual Machine Model

The term "virtual machine" can conjure up a wide range of images for many people. The term "virtual" is commonly used by the popular media and news broadcasts in reference to new developments in computer simulations and computer gaming programs. In either case, "virtual" is used to imply that an object or device is observable without physically existing at that time and place. This use of the term virtual is a reasonable assumption concerning the development of an extended virtual machine. The creation and continued development of an extended virtual machine is a logical growth of computer operating systems in both ability and versatility.

..."virtual" is used to imply that an object or device is observable without physically existing at that time and place.

The need to understand how a modern computer is designed and manipulated by an operating system is key to comprehending the extended virtual machine model. Modern operating systems consist of sets of instructions combined into service routines. These service routines are combined with data into programs that are entered into the computer as application programs. The combining of instructions into service routines is analogous to combining the simple actions of an office clerk to perform a more complex task such as an inventory of office supplies. For example, the file clerk could have a limited set of basic instructions that he can perform from memory. These instructions might be as limited as counting something, record result of a previous operation, and file or store some information for later use. These instructions used individually are of limited value. However, the instructions could be combined in a sequence of instructions to direct the office clerk to perform and inventory of office supplies. The sequence of instructions could look something like: count note paper, record result, count pencils, record result, count paper clips, record result, file results for storage. This sequence could then be used over repeatedly to perform the inventory service when ever it was requested. The use of instructions such as "count note paper" does assume that the office clerk has certain "built-in" abilities, such as recognizing note paper and counting it properly. In a human office clerk this ability to recognize objects and count is a natural function of the clerks brain and eyes. In the simulated office clerk or computer these functions would be designed and built into the computer circuitry the at the hardware level. The need to implement some basic frequently used functions in circuitry instead of combining simpler instructions is apparent when examining the office clerks actions in detail. The office clerk has to physically move around the office, visually detect objects, use a note pad and pencil to record counts etc. These functions would be implemented as repeated sequences of hardware actions and software instructions. For example, the instruction "record result of last operation" would involve accessing a storage device such as a disk drive, locate space to record data, retrieve the data to be recorded from local registers, transferring the data, and writing it to the space on the hard disk. There has to be a basic level where all the instructions originate at the circuit level.

The most basic instruction is when a pattern of ones and zeros is presented to the computer and produces an action to move data, configure the system or set up a condition for needed for the next action. This level can be considered the machine instruction level from which the operating system level instructions are made. It should be clear now that any instruction set is simply a combination of lower level instruction sets. The office clerk is performing tasks that are really a series of sub tasks strung together to form a more complex sequence of actions. When the instruction "inventory" is given to the office clerk an observer perceives only the meaning of the word inventory and the associated action of the office clerk. The underlying sub sets of instructions are hidden from the observer. Each successive layer of instructions, machine circuitry, machine code, operating system instructions, and application program code, hides the underlying levels of instructions from the next user. This ability to combine instructions and construct more complicated actions is a key concept in understanding the extended virtual machine.

The eventual goal of the extended virtual machine is to build capabilities and simulated devices that do not actually exist physically in the computer. For example, most computers have a hard disk for storing data and program instructions. The hard disk may in fact be located on a different computer or consist of multiple drives attached to the same computer. An extended virtual machine approach to this situation could be to program sequences of instructions to present the user with the appearance that there is only one hard disk drive attached to the computer. The single simulated hard drive would have a storage capacity equal to all the attached hard drives. The user would see only one hard drive through the computer and simply use the combined instructions to store and retrieve data as if there was one large hard drive available. The instructions called by the user access sub routines that are combinations of lower level instructions that handle the details of tracking what information was stored on which hard disk.