Modifications to UNIX to Allow Four Megabytes of Main Memory on an 11/40 Class Processor

This document details the necessary software and hardware modifications to allow a standard UNIX Version 7 system to address 4 Megabytes of main memory on a PDP-11/40 class processor. The authors explain the limitations of the original PDP-11 architecture regarding memory addressing and file system block mapping, leading to the adoption of an Able Computer ENABLE memory management board. The document provides technical implementation details, including modifications to the kernel code, adjustments to memory management unit initialization, and performance benchmarking results comparing the modified system against standard configurations.