Virtual machines have finally arrived. Dismissed for a number of years as merely academic curiosities, they are now seen as cost-effective techniques for organizing computer systems resources to provide extraordinary system flexibility and support for certain unique applications."
Virtual machine systems have been implemented on a limited number of third generation computer systems, for example CP-67 on the IBM 360/67. The value of virtual machine techniques to ease the development of operating systems, to aid in program transferability, and to allow the concurrent running of disparate operating systems, test and diagnostic programs has been well recognized. However, from previous empirical studies, it is known that many third generation computer systems, e.g. the DEC PDP-IO, cannot support a virtual machine system.In this paper, the hardware architectural requirements for virtual machine systems are discussed. First, a fairly specific definition of a virtual machine is presented which includes the aspects of efficiency, isolation, and identical behavior. A model of third generation-like computer systems is then developed. The model includes a processor with supervisor and user modes, memory that has a simnle protection mechanism, and a trap facility. In this context, instruction behavior is then carefully characterized.Formal techniques are then applied to derive precise necessary and sufficient conditions to test whether an architecture can support virtual machines. The major theorem of the paper states that a virtual machine system can be constructed if and only if certain properties regarding the real machine's instruction set hold. A constructive proof is presented, explicating several underlying ideas.The conditions obtained are fairly simple, and may easily be used to evaluate existing hardware or as a quide in the design of new machines.Using the same formal base, related notions such as hybrid virtual machines and recursive virtualization are also described and the requirements for these features specified.The paper closes by noting the simplifications contained in the model, possible extensions and applications, and the utility of the formal approach that was taken. 121
The enzyme complex nitrogenase, which reduces N2 to NH+4, involves two redox proteins, both irreversibly damaged by O2 (ref. 1). Enzyme activity therefore requires anaerobic conditions, a source of reductant and a large amount of ATP (approximately 16 ATPs per N2). In both aerobic and facultative anaerobic N2-fixing bacteria, nitrogenase synthesis is regulated by O2 and NH+4, but in the aerobes there are also processes to protect the enzyme from O2 damage. The mechanisms of repression by O2 and NH+4 seem to be independent in the organisms so far examined. In the facultative anaerobe, Klebsiella pneumoniae, O2 was shown to repress nitrogenase synthesis in an NH+4-constitutive strain. The fusion of the Escherichia coli lacZ gene into each transcriptional unit of the nitrogen fixation (nif) gene cluster in K. pneumoniae has facilitated studies with O2, because expression from the various nif promoters results in an O2-stable product (beta-galactosidase). Notably, the nifHDK operon (the nitrogenase structural genes) was more sensitive to O2 repression than the nifLA operon (regulatory genes). The characterization of mutants, reported here, indicates the involvement of a nif-regulatory gene product in the mechanism of O2 control of nitrogenase synthesis.
Mutations causing constitutive synthesis of glutamine synthetase (GlnCphenotype) were transferred from Klebsiella aerogenes into Klebsiella pneumoniae by P1-mediated transduction. Such GlnCstrains of K. pneumoniae have constitutive levels of glutamine synthetase. Two of three GlnC strains of K. pneumoniae studied, each containing independently isolated mutations that confer the GlnCphenotype, continue to synthesize nitrogenase in the presence of NH4+. One strain, KP5069, produces 30% as much nitrogenase when grown in the presence of 15 mM NH4+ as in its absence. The GlnCphenotype allows the synthesis of nitrogenase to continue under conditions that completely repress nitrogenase synthesis in the wild-type strain. Glutamine auxotrophs of K. pneumoniae, that do not produce catalytically active glutamine synthetase, are unable to synthesize nitrogenase during nitrogen limited growth. Complementation of K. pneumoniae Glnstrains by an Escherichia coli episome (F'133) simultaneously
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