MSCP BasicDiscFnsV1.2 Apr82

This document, the "MSCP Basic Disk Functions Manual," details the Mass Storage Control Protocol (MSCP), a standard developed by Digital Equipment Corporation for managing disk storage. Published in April 1982, its primary purpose is to provide the comprehensive information necessary for programmers to write host class drivers that interact with MSCP-compliant disk storage subsystems.

The manual describes MSCP as a protocol where intelligent controllers (MSCP Servers) handle the intricate details of I/O operations (like device type, media geometry, and error recovery), allowing the host system (via a Class Driver and Port Driver) to send high-level command messages. This design ensures the host perceives a "perfect media" by abstracting underlying complexities.

Key aspects covered include:

• Architecture: The two-tiered software layers on both the host (Class Driver, Port Driver) and controller (MSCP Server, Port Driver) sides, and their physical vs. logical communication.
• Communication Services: Three types are defined: sequential (for control messages, guaranteed delivery), datagram (for error logs, high probability of delivery), and block data (for actual data transfers, reliable/efficient). Flow control mechanisms, using a credit system, are critical to prevent congestion.
• States and Identifiers: Defines states for controllers (Offline, Available, Online) and individual storage units (Unit-Offline, Unit-Available, Unit-Online), along with the conditions for transitions between these states. It also outlines requirements for unique unit and controller identifiers, and media type identifiers.
• Command Structure and Execution: Categorizes MSCP commands into Immediate, Non-Sequential, Sequential, and Special, detailing their execution order constraints and message formats (including command reference numbers, unit numbers, opcodes, modifiers, and flags).
• Data Management: Explains disk geometry in terms of logical blocks, the host area, and the Replacement and Caching Table (RCT) used for bad block replacement. It also covers hardware and software write protection mechanisms.
• Error Handling and Timeouts: Describes how MSCP servers detect and recover from errors, reporting significant ones via error log messages and command end messages. It also specifies host access timeouts (to release resources from failing hosts) and command timeouts (to detect controller failures).
• Minimal Subset: Provides detailed specifications for a core set of basic disk functions and their associated attention messages, including ABORT, ACCESS, ONLINE, READ, WRITE, REPLACE, status inquiries, and commands for setting characteristics.

Overall, the document serves as a foundational guide for understanding and implementing host-side software interactions with MSCP disk controllers, focusing on the protocol's rules, message structures, and operational algorithms.