UnibusPortDescription

This document, "Storage System Unibus Port Description AA-L621A-TK," provides detailed information for programmers tasked with writing a port driver for Digital Equipment Corporation's Unibus-based mass storage systems using the Mass Storage Control Protocol (MSCP).

It introduces the MSCP subsystem, highlighting how intelligent controllers offload detailed I/O handling from the host, presenting a "perfect media" view to higher-level software. The manual then delves into the Unibus Port Architecture, distinguishing between the lower-level "Transport Layer" (physical bus and adapter logic) and the higher-level "Logical Layer" (control message exchange, verification).

Key topics covered include:

• Communication Mechanisms: Detailed descriptions of the Unibus I/O page registers, command and response rings for message transmission (commands, responses, and interrupts), and polling methods.
• Initialization: A comprehensive four-step procedure for initializing the port and controller.
• Error Handling: Classification and management of transmission errors (Bus Master, Interrupt Master, Bus Data Timeout, Bus Parity) and self-detected fatal port/controller errors, including a list of error codes.
• Port Performance: Considerations for optimizing port performance, particularly concerning ring sizes and message credit schemes.
• Diagnostic Facilities: Information on Diagnostic Wrap Mode and maintenance commands (READ/WRITE) for host-based diagnostics.

Appendices provide tables for assigned connection IDs, error code ranges, and port type numbers. The document's scope is specifically limited to the port's details, independent of specific host processors, operating systems, or controller/device types.