Kernel Implements Processes The relationship between the abstract kernel and an individual task is pictured in Figure 4, and is formalized by the theorem AK-IMPLEMENTS-PARALLEL-TASKS. Intuitively, this theorem says that for a given good abstract kernel state AK and abstract kernel oracle ORACLE, the final state reached by task I can equivalently be achieved by running TASK-PROCESSOR on the initial task state, with an oracle constructed by the function CONTROL-ORACLE. The oracle constructed for TASK-PROCESSOR accounts for the precise sequence of delays to task I in the abstract kernel. Task project AK Figure 4: AK Implements Parallel Tasks THEOREM AK-IMPLEMENTS-PARALLEL-TASKS (IMPLIES (AND (GOOD-AK AK) (FINITE-NUMBERP I (LENGTH (AK-PSTATES AK)))) (EQUAL (PROJECT I (AK-PROCESSOR AK ORACLE)) (TASK-PROCESSOR (PROJECT I AK) I (CONTROL-ORACLE I AK ORACLE)))) 6. The Target Machine The target machine TM is a simple von Neumann computer. It is not based on an existing physical machine becaus...

We present a multitasking operating system kernel, called KIT, written in the machine language of a uni-processor von Neumann computer. The kernel is proved to implement, on this shared computer, a fixed number of conceptually distributed communicating processes. In addition to implementing processes, the kernel provides the following verified services: process scheduling, error handling, message passing, and an interface to asynchronous devices. The problem is stated in the Boyer-Moore logic, and the proof is mechanically checked with the Boyer-Moore theorem prover.

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This paper provides a retrospective view of the design of SRI's Provably Secure Operating System (PSOS), a formally specified tagged-capability hierarchical system architecture. It examines PSOS in the light of what has happened in computer system developments since 1980, and assesses the relevance of the PSOS concepts in that light.