Capability-Based Computer Systems by Henry M Levy

By Henry M Levy

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The Basic Language Machine made several important advances over the Rice University Computer. First, it extended the design to encompass multiprogramming, using a separate Process Base for each process. Second, it provided a more general addressing structure to give users flexibility in performing address arithmetic and saving results. Third, it used a relatively efficient typing mechanism to reduce the number of operators in the instruction set. However, despite the advantages of its structure, the experimental BLM was dismantled in 1970 and no product evolved from the research effort.

The CAL-TSS operating system is a layered design in which each layer provides a virtual machine to the next higher layer. Each layer is specified as a set of objects and operations on those objects. This section examines the innermost layer of the supervisor which handles capabilities and object addressing. The basic unit of protection in the CAL-TSS system is a domain, an environment containing hardware registers, primary memory, and a C-list. ) Access to objects outside the domain can occur only through invocation of a C-list capability; the possessor of a capability invokes an operation on the object it addresses by specifying the capability, the operation to be performed, and other optional parameters.

If the codeword type field specifies a mixed-type segment, the seg- 31 Early Descriptor Architectures ment can contain elements of any type. However, in mixedtype segments, each element must contain its own tag. A tag is a field contained within the information unit indicating its interpretation. All elements in a mixed-type segment are 64 bits long and contain a 3-bit tag. The four tags defined are: • 32-bit binary word, • escape (an attempt to use such an element as an operand causes a trap to software), • 45-bit address (stored in 64 bits), and • 61-bitfloatingnumeric element.

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