Windows 3.11 Parallel Port Printing Driver

In Microsoft Windows 3.11, low-level parallel port printing operations were primarily managed by the 386 Enhanced Mode virtual device driver known as LPT.386 (working alongside the internal Virtual Printer Device, or VPD), while standard port communications were abstracted by COMM.DRV. This driver architecture allowed Windows to bypass slow MS-DOS and BIOS routines, handle parallel port contention between virtual machines, and dramatically increase data throughput to connected printers.

The Role of LPT.386 and VPD

In 16-bit Windows environments running in 386 Enhanced Mode, hardware ports could not simply be left to standard MS-DOS interrupts without causing stability issues. The LPT.386 virtual device driver (VxD) was responsible for virtualizing parallel printer ports (LPT1, LPT2, and LPT3).

When installed and declared in the SYSTEM.INI file under the [386Enh] section (often referenced as device=lpt.386 or handled by internal system modules like *vpd), it intercepted input/output operations directed at parallel port hardware addresses (such as 0x378 or 0x278). This virtualization prevented conflicts when both a background MS-DOS prompt and a native Windows application attempted to send data to the printer simultaneously.

"Fast Printing Direct to Port"

Windows 3.1 and Windows for Workgroups 3.11 included an option in the Control Panel under Printer Setup titled Fast Printing Direct to Port.

  • When enabled: Windows bypassed the standard MS-DOS Interrupt 21h and BIOS Interrupt 17h handlers. The LPT.386 driver took direct control of the parallel port hardware, streaming raw printer data straight to the port registers. This removed significant software overhead and substantially improved printing speeds.
  • When disabled: Windows routed print output through standard MS-DOS system calls, which was slower but provided maximum compatibility for finicky hardware or specialized software redirectors.

The Complete Printing Stack

LPT.386 operated at the lowest software layer near the physical hardware, but it worked in coordination with several other components:

  • COMM.DRV: The general communications driver that served as the standard interface between Windows applications and serial or parallel ports.
  • Print Manager (PRINTMAN.EXE): The spooler application that queued print jobs and managed background transmission so users could continue working while documents printed.
  • Printer Drivers: High-level drivers (such as UNIDRV.DLL or model-specific drivers like HPPCL.DRV) that translated application graphical calls (GDI) into printer-specific control languages like PostScript or PCL before handing the raw data to the port handlers.