Inside MP/M's Process Dispatcher

MP/M, or “Multi-Programming Monitor Control Program” is usually described as a multi-tasking and multi-user version of CP/M. I never had the chance to use it myself, but having learned to program on a Z80‑based microcomputer, I was astonished that something like MP/M could even exist on such limited hardware.At some point I decided to get a better understanding of MP/M’s internals - specifically the part that enables multitasking. The source code I examined is for MPM II and can be downloaded from the Digital Research Source Code web page. To make sense of the design, I also relied on the MP/M and MP/M II manuals from Tim Olmstead Memorial Digital Research CP/M Library.This post is a summary of my findings.Background and HistoryTom Rolander1 completed his master’s degree at the University of Washington in 1976 and soon after was recruited by Intel. There he joined a team that designed and developed RMX/80, a real‑time multitasking operating system for process‑control systems. After…