IAS Guide to Writing a Device Handler Task

This manual is a comprehensive guide for system managers and programmers developing custom device handler tasks for non-standard peripheral I/O devices within the IAS operating system (Version 3.4). It details the fundamental architecture and operational requirements of device handlers, which are privileged tasks responsible for interpreting and executing I/O requests issued by user tasks.

The document outlines the five core sections of a handler task: system communication tables (such as the Unit Identification Table and Dispatch Table), initialization code (including declaring residency, connecting to interrupts, and setting up power failure ASTs), main code for processing I/O requests (dequeuing, validating, dispatching, and completing), interrupt service routines (ISRs), and power failure recovery. It explains how handlers interact with the IAS Executive and a suite of system subroutines for functions like I/O completion, task swapping, and error logging. Special considerations are also provided for handling Direct Memory Access (DMA) devices, particularly regarding UNIBUS Mapping Registers (UMRs) on PDP-11/44 and PDP-11/70 systems. The guide concludes with instructions on system generation and linking procedures required to integrate custom handlers into the IAS environment.