The virtual line printer device driver VLPT.SYS supports access to three virtual parallel port controller devices from DOS applications running in virtual DOS machines.
VLPT hooks INT 17h and processes requests for INT 17h services itself, rather than allowing these requests to be handled by CBIOS. INT 17h support includes support for function 02h (Read Status).
VLPT does not support virtual hardware interrupts. If a DOS application attempts to enable interrupts (that is, it attempts to set control port bit 4, "IRQ EN"), that I/O operation is ignored, and the application will not receive interrupts from the parallel port hardware.
VLPT buffers the print data which is subsequently directed to the OS/2 spooler using file system services provided by the Virtual DOS Machine Manager. The spooler may be configured for output on each printer device (LPTx) that will be accessed by DOS applications from a VDM. Figure "Virtual Printer Device Driver Operation" shows the various operations performed by the virtual printer device driver.
If VLPTDD.SYS is the only virtual printer device driver installed, no INT 17 will be issued when printing is done on numbered I/O devices (for example, LPT1, LPT2, etc.). However, if an application such as a TSR program must catch all INT 17 interrupts, the LPTDD.SYS device driver must be installed as well. You can find LPTDD.SYS in the subdirectory \os2\mdos.
When LPTDD.SYS is available, a request from the DOS file system issuing INT 21 is converted by LPTDD.SYS into INT 17. INT 17 is then forwarded to VLPT.SYS and from this point on the print request proceeds as described above in Figure "Virtual Printer Device Driver Operation". Note, however, that installing LPTDD.SYS in addition to VLPT.SYS will cause the printing from a VDM to slow down.
Spooling
In order to support spooled print output, the OS/2 spooler must be installed. A new IOCTL interface is defined in order to allow the spooler to identify itself to the physical printer device driver. The new IOCTL functions Set Spooler Status and Get Spooler Status are called by the spooler and the Virtual DOS Machine Manager.
The spooler invokes the Set Spooler Status function (Category 5, Function 4Ch) at spooler monitor registration time to inform the physical device driver that a particular port is actively configured as the output device for a particular spool queue. It also invokes this interface whenever the user manipulates the spooler queue setup by invoking the Print Manager's Setup Printers/Change Printers dialog.
The Virtual DOS Machine Manager invokes the Get Spooler Status function (Category 5, Function 6Ch) during an implicit open of a print device whenever VLPT processes the first print output (INT 17h) from a VDM. This allows the Virtual DOS Machine Manager to determine if the spooler is active so that it can return the spool queue file handle to VLPT to continue printing.
Print Screen
VLPT also supports the Print Screen function by hooking INT 05h so that it is notified before the CBIOS INT 05h handler. The CBIOS INT O5h handler invokes INT 17h functions, and the VLPT INT 17h emulation in turn sends this data to a spool file, if the spooler is active. When the CBIOS INT 05h handler returns, VLPT also receives control so that it may close the spool file.
File Transfer
To support DOS file transfer programs which use the parallel ports (such as the IBM Data Migration Facility), and which typically program the parallel port controller directly, VLPT provides a mode known as direct I/O access. In this mode, the physical printer device driver grants temporary exclusive ownership of the parallel port hardware to the VDM in which the application is running. This mode allows the application to have direct access to the parallel port's data, status and control registers.
If the port is not currently active (printing) under control of the physical printer device driver, the physical device driver will grant VLPT exclusive access to the port, and continue to service incoming file system I/O requests. Any incoming monitor requests from the spooler are blocked until exclusive access is released (no error or status monitor packets are sent to the monitor chain).
If the physical printer device driver is actively processing a hardware I/O operation, when VLPT makes a request for exclusive access, the physical device driver will return an error code to VLPT. VLPT will then display a pop-up message (via the VDHPopUp() helper service), allowing the user to select the most appropriate system action ("End program" or "Retry").
Note: Due to the multitasking nature of OS/2 V2.0, data communications using this type of application have an increased probability of error when multiple processes are concurrently active and/or when the virtual DOS machine is switched to the background.
PS/2 Register Virtualization
On PS/2 systems, the physical printer device driver ensures that all LPT ports which support extended mode (read/write 8-bit parallel I/O) will be enabled for extended mode at system initialization time. On any PS/2 models which do not enable this mode by default, the physical printer device driver enables extended mode via the system's Programmable Option Select (POS) function. This ensures that all PS/2 LPT ports will support manipulation of the control port Direction Control bit.
On all PS/2 models (but not on IBM PC/AT systems), VLPT will virtualize the adapter enable/setup register. All bits of this register are virtualized for read operations, but only bit 7 of the enable/setup register is virtualized for write operations. DOS applications may modify bit 7 of this register in order to gain access to the system board's POS Register 2, thereby enabling or disabling extended mode operation of the parallel ports. When bit 7 is set to 0 (the default state), the parallel port is configured as an 8-bit, parallel, bidirectional interface. When bit 7 is set to 1, the parallel port bidirectional mode is disabled. As described above, the physical printer device driver ensures that all PS/2 models have this bit set to 0 (extended mode enabled) during system initialization.
Note that only bit 7 is virtualized and may be manipulated; attempting to manipulate any other bits of this register will result in termination of the VDM. As the behavior is virtualized, the true state of the hardware register is not affected by any operations of a DOS application running in a virtual DOS machine.
Printer Close
VLPT exports the VDHPrintClose() service. This interface may be called by another virtual device driver such as VKBD to force any open printers in a VDM to close. This technique is used to handle a forced End-of-Job (Ctrl+Alt+PrintScreen) character, and is required because some DOS applications do not explicitly close or disable the printer when their print activity is completed.
VLPT may also close open print files whenever a VDM is terminated. VLPT registers an event hook with the Virtual DOS Machine Manager, and is therefore notified upon termination of a VDM. All open print files in the terminating VDM are closed, after any buffered data has been sent to the spooler.
When operating in direct I/O access mode, VLPT can detect application termination in one of three ways:
When this termination event is detected, direct I/O access mode is cancelled and VLPT relinquishes the VDM's exclusive control of the parallel port hardware. The physical printer device driver then reclaims ownership of the port and resumes normal I/O operations.
Note that VLPT will not always close an open print file when the DOS application terminates. Depending on the DOS application's behavior, the VDM may remain active when the program ends, and the spooler print file may therefore remain open. If so, the user can cause the open print file(s) to close by using the Ctrl-Alt-PrintScreen control key sequence. Alternatively, the user can leave it to the system by setting the PRINT_TIMEOUT value in the DOS settings to the time in seconds that the operating system should wait before forcing the print job to the printer. Consequently there is no need to exit these DOS programs to have the print job released from the print spooler. For more information on PRINT_TIMEOUT see DOS Settings.