ULTRIX SCSI/CAM Architecture Guide to Writing Device Drivers for the ULTRIX SCSI/CAM Architecture Interfaces

This document, "Guide to Writing Device Drivers for the ULTRIX SCSI/CAM Architecture Interfaces" (January 1992, for ULTRIX RISC 4.2), is a comprehensive manual for systems engineers developing device drivers for Digital's ULTRIX SCSI/CAM (Small Computer System Interface/Common Access Method) architecture.

The guide details the layered, hardware-independent software model of the ULTRIX SCSI CAM Architecture (USCA). Key components and concepts described include:

• CAM User Agent Modules: How user processes can interact with the SCSI/CAM subsystem, handling Control Block (CCB) requests, I/O, and error management.
• SCSI/CAM Peripheral Device Drivers: Explanation of common, generic, disk, tape, and CD-ROM/audio device driver modules, including their data structures, routines for I/O, queue manipulation, and CCB management.
• SCSI/CAM Special I/O Interface: A mechanism for processing specialized SCSI I/O control commands, facilitating the porting of existing drivers.
• CAM Configuration Driver: Responsible for initializing the CAM subsystem, scanning the SCSI bus for devices, and maintaining device information.
• CAM Transport Layer (XPT): The layer responsible for routing CAM requests to the appropriate SIM (SCSI Interface Module) and managing CCB allocation and deallocation.
• SCSI Interface Module (SIM): The lowest layer that interfaces directly with SCSI bus protocols and hardware-specific operations, further divided into SIM XPT, Scheduler, HBA, and DME sublayers.

The document provides extensive information on data structures (e.g., Control Blocks, I/O, Control, and Configuration CCBs), common and generic routines, error handling macros and structures, and debugging facilities. It aims to enable programmers to define and implement their own SCSI/CAM device drivers effectively, offering detailed examples and command reference information.