The Execute Instruction

This document from 1970 discusses the technical implementation of the Execute (XCT) instruction for the PDP-11/40 computer. It covers the rationale behind the instruction, which is used to facilitate reentrant code and execute selected instructions. The memorandum establishes that the XCT sequence must be non-interruptable due to hardware complexity and provides a design requirement for two program counters: a standard Program Counter (PC) and a dummy Program Counter (DPC) for address computation during the XCT sequence. It also details the addressing modes available for the D field of the XCT instruction and provides guidance on how branching or transfer of control instructions are handled when executed via XCT.