A Church–Turing machine is a digital computer that encapsulates the symbols in a thread of computation as a chain of protected abstractions by enforcing the dynamic binding laws of
Alonzo Church's
lambda calculus Other capability based computers, which include CHERI and
CAP computers, are hybrids. They retain default instructions that can access every word of accessible physical or logical (paged) memory. It is an unavoidable characteristic of the
von Neumann architecture that is founded on shared random access memory and trust in the sharing default access rights. For example, every word in every page managed by the virtual memory manager in an operating system using a
memory management unit (MMU) must be trusted. Using a default privilege among many compiled programs allows corruption to grow without any method of error detection. However, the range of virtual addresses given to the MMU or the range of physical addresses produced by the MMU is shared undetected corruption flows across the shared memory space from one software function to another. PP250 removed not only
virtual memory Instructions that access memory have an
opcode, a field specifying a data register operand, a field specifying a data register used as an
index register containing an offset into a segment, a field specifying a capability register referring to the segment containing the memory location, and a field containing a base offset into the segment. The offset into the segment is the sum of the base offset and the contents of the index register. The software was modular based on the universal model of computation and the
lambda calculus. Six Church instructions hide the details of a named
function application using capability keys for the typed concepts of variables, functions, abstractions, applications and a namespace. Instead of binding instructions to static linear memory as a default shared privilege used by malware and hackers, instructions are bound to typed and protected, private digital objects using capability keys in a
capability-based security system of immutable mathematical symbols. The result achieved many decades of trusted software reliability. ==See also==