By Henry M Levy
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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.
However, there are some significant conceptual differences in the general way capability addressing is applied, in the lifetimes of capabilities and the objects addressed, and in the protected procedure mechanism that allows users to extend the functions of the operating system supervisor. Capabilities are protected addresses; that is, a process can create new capabilities in its C-list only by calling a supervisor meta-instruction. Once a process receives a capability, it cannot directly modify the bits in the capability.
A discussion of the general advantages of tagged architectures can be found in [Feustel 73]. 1 Introduction Although the Burroughs, Rice, and BLM systems included capability-like addressing structures, the word "capability" was not introduced until 1966, by Dennis and Van Horn of MIT [Dennis 66]. Dennis and Van Horn defined a hypothetical operating system supervisor for a multiprogramming system. Multiprogramming systems were already in use at that time; however, many difficult problems had yet to be solved.