DTJ506PF

This document details the role and functionality of Microsoft Windows Network Virtual Device Drivers (VxDs) within Digital's PATHWORKS for DOS version 4.1.

Core Purpose: The PATHWORKS for DOS software includes two key network VxDs: VDNET.386 for DECnet socket calls and VNETBIOS.386 for NetBIOS calls. These drivers enable Windows applications (operating in protected processor mode) and standard DOS applications (running in virtual machines) to concurrently access network services designed for real-mode DOS environments, specifically within the Microsoft Windows enhanced mode.

Key Capabilities and Mechanisms:

1. Mode Bridging: VxDs act as a bridge, allowing protected-mode Windows applications to communicate with real-mode DOS/TSR (Terminate and Stay Resident) software. They map memory addresses between these different processor modes, ensuring data is accessed correctly.
2. Asynchronous Operations: They support asynchronous network operations by buffering data in shared memory pools and using callback routines to notify the requesting virtual machine upon task completion. This prevents blocking calls from monopolizing the system.
3. Hardware Virtualization: VxDs virtualize hardware ports and interrupts, emulating multiple 8086-based machines. This prevents conflicts when several DOS applications in separate virtual machines attempt to access hardware simultaneously.
4. Direct Interrupt Handling: By handling hardware/software interrupts directly, VxDs can replace real-mode resident software, improving performance by reducing context and mode switching overhead.

Operational Flow: The PATHWORKS VxDs work by intercepting network API calls (software interrupts), mapping the caller's control blocks and buffers into global memory (accessible by all virtual machines), and then invoking the real-mode network software. They can convert blocking calls to asynchronous ones to maintain system responsiveness. During virtual machine termination, the drivers ensure all outstanding network operations are canceled.

Environment: These VxDs operate exclusively in Microsoft Windows enhanced mode, which fully utilizes advanced processor features like virtual memory and multiple virtual machines (supporting both protected and virtual processor modes). The drivers are developed in assembly language using specific Microsoft tools for development and debugging.