How Windows Prints to a Network

• Microsoft Windows operating system versions 3.1, 3.11

SUMMARY

Microsoft Windows version 3.1 uses a network device driver to manage printing to network servers by integrating many network printing utilities into Windows. Printing enhancements that are not supported by the network driver and that rely on hooking MS-DOS calls or ROM BIOS calls may not work correctly when Windows prints directly to the port. Using LPT1.DOS (or LPT1.OS2) passes printer output through MS-DOS to these enhancements; however, being terminate-and-stay-resident (TSR), these enhancements may create problems in Windows unless the program takes the usual precautions necessary to make a TSR program compatible with Windows.

MORE INFORMATION

The most common MS-DOS enhancement for printing is the network printer redirection, which allows a network user to print documents on a printer attached to a server. Examples of printer redirection utilities include the NET USE command in Microsoft LAN Manager and the CAPTURE command in Novell NetWare.

Windows handles network redirections using a Windows network driver, a program that Windows uses to manage the network. These drivers make printing over the network transparent to the user and also allow the user to connect and disconnect to network printers.

If you have a network for which a network driver is not installed by Windows Setup, contact your network vendor to determine if a Windows driver is available. Windows 3.1 installs a driver for all major networks, including Novell Netware, Microsoft LAN Manager, Microsoft MS-Net, Banyan Vines, 3Com 3+Share and 3+Open, Artisoft LANtastic, and IBM DOS LAN Requester. In some cases, the driver ships with the network software and not Windows itself.

When you print a document on a network, Windows calls the network driver to determine if the port in use is connected to a network printer. If so, the network driver is expected to open a file handle to the network device. If the network driver reports that the device is not connected to the network, Windows prints to the port using the Windows COMM driver. The COMM driver does direct port input/output (I/O); it does not call MS-DOS or the BIOS. Therefore, a printing enhancement not supported by the network driver is bypassed by Windows. Printing enhancements work by hooking either INT 21h writes or INT 17h, the BIOS functions to control the printer. The COMM driver cannot use INT 17h because of performance considerations; calling INT 17h from Windows would require multiple protected-to-real mode transitions for each byte of output.

Remote spooling enhancements such as Novell's RPRINTER do not require any special support on the machine that creates the printout; the network driver supports connections to network printers whether they are on a server or on another workstation. There are known problems with earlier versions of RPRINTER that were corrected by Novell. If you encounter problems with RPRINTER in Windows 3.1, contact Novell for the latest version.

The only caution for using RPRINTER or LANSpool is not to print to the same port as a local port through the Windows COMM driver. This causes the output to be mixed randomly with the output from RPRINTER. If the workstation running RPRINTER needs to print to the same printer, connect the port over the network. Printing is not as fast in this case, but the output is correctly sequenced.

There is no explicit support in Windows for non-network printing enhancements that hook MS-DOS or ROM BIOS function calls. In general, the way to use these enhancements is to cause Windows to use MS-DOS calls instead of the Windows COMM driver. This is the reason for LPT1.DOS and LPT1.OS2 (new Windows 3.1 users see the former). Connecting to these ports causes Windows to print using MS-DOS calls instead of the Windows COMM driver. Alternately, you can turn off the Fast Printing Direct To Port check box in the Connect dialog box. (To access this dialog box, open Printers icon in Control Panel.) In general, printing performance and error condition handling are poorer; however, printing enhancements see the output to the printer when they are passed through MS-DOS.

Printing enhancements are TSR programs. Like all TSR programs, they require special precautions to avoid problems in Windows. If a printing enhancement fails in Windows, by crashing or causing incorrect output, it may be incompatible with Windows because it does not handle multiple MS-DOS Virtual Machines (VMs) or critical sections. Some enhancements may work if no MS-DOS VMs are open; try printing from a Windows application without any MS-DOS applications or MS-DOS prompts running.