DEC-20-OMUGA-A-D Monitor Calls Users Guide May76

This document, the DECsystem-20 Monitor Calls User's Guide (DEC-20-0MUGA-A-D), published in May 1976, serves as an introductory manual for assembly language programmers unfamiliar with the DECsystem-20 operating system. It teaches users how to implement basic monitor calls (system functions) within their assembly language programs.

The guide is structured to progress from simple to more advanced concepts, covering a range of functionalities:

• Introduction to Monitor Calls: Explains what monitor calls are, their calling sequences, and how they return control or error information. It also introduces the program environment within the TOPS-20 operating system, including job and process structures.
• Terminal Input/Output: Details basic operations for interacting with the user terminal, such as printing strings (PSOUT), reading/writing numbers (NIN, NOUT), and reading/writing single bytes (PBIN, PBOUT), including terminal editing features (RDTTY).
• File Management: Describes how to use files, covering the association of files with Job File Numbers (JFNs), opening and closing files (GTJFN, OPENF, CLOSF), and various methods for transferring data (sequential bytes, strings, nonsequential bytes, and page mapping).
• Software Interrupt System: Explains interrupt conditions, channels, priority levels, setting up interrupt tables, enabling/disabling interrupts, and processing interrupt routines.
• Process Structure: Covers the creation, starting, termination, and status of processes, as well as inter-process communication and memory sharing.
• Enqueue/Dequeue Facility: Introduces mechanisms for resource sharing and dynamic resource allocation among cooperating processes, including requesting and releasing resources, understanding resource ownership (exclusive/shared), and strategies for avoiding "deadly embraces."
• Inter-Process Communication Facility (IPCF): Details sending and receiving informational messages between processes using "packets," including packet structure and Process Identifiers (PIDs).
• Error Codes and Message Strings: An appendix listing various error codes and their corresponding messages.

The guide emphasizes its role as an introductory resource, recommending the DECsystem-20 Monitor Calls Reference Manual for comprehensive details. It also assumes user familiarity with MACRO assembly language, DECsystem-20 machine instructions, and the TOPS-20 Command Language.