Capability-based computer systems by Henry M Levy

By Henry M Levy

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To create an inferior sphere, a process executes a CREATE SPHERE meta-instruction. 2 Dennis and Van Horn's Supervisor Capability Process C-list Capability Computation «^ Capability Capability Segment ► Segment C-list Figure 3-1: Processes, Computations, and C-Lists process specifies an entry in its C-list, in which the supervisor places a capability for the inferior. This capability can then be used to control the inferior process. When a process executes a CREATE SPHERE meta-instruction, the supervisor creates the inferior with an empty C-list.

The Rice University Computer had several limitations, but they were often due to implementation decisions. For example, codewords contained the length of the block they defined, but the length was not used by hardware to validate an array index. Instead, a trap facility was provided to allow software to check Principal Codewords Codeword to Symbol Codeword to Symbol Table Codeword to main program Codeword to user stack Table Value Table ► "2D Array" Codeword Codeword "VECTOR" Codeword Codeword "VECTORLEN" 120 1 Vec tor Procedure Instructions Linkage to external variables, arrays, etc.

A 128-word stack. • A symbol table defining each named global object in the system. • A corresponding value table containing values for scalars and codewords for arrays named in the symbol table. 27 Early Descriptor Architectures 28 The remainder of memory is allocated dynamically to user programs and data, including those addressed through the value table. Figure 2-4 shows the structure of a Rice University Computer sample procedure. Procedure instructions can address variables within the procedure segment without reference to codewords (that is, relative to the program counter).

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