Service Functions

The service functions let drivers and TSRs obtain the Windows version number, obtain the current virtual-machine (VM) identifier, set critical sections, and retrieve the addresses of the entry points for virtual-device service functions. There are the following service functions:

• Get Windows Installed State (Interrupt 2Fh Function 1600h)
• Get Windows Entry-Point Address (Interrupt 2Fh Function 1602h)
• Release Current VM Time-Slice (Interrupt 2Fh Function 1680h)
• Begin Critical Section (Interrupt 2Fh Function 1681h)
• End Critical Section (Interrupt 2Fh Function 1682h)
• Get Current Virtual Machine ID (Interrupt 2Fh Function 1683h)
• Get Device Entry Point Address (Interrupt 2Fh Function 1684h)
• Switch VMs and CallBack (Interrupt 2Fh Function 1685h)
• Detect Interrupt 31h Services (Interrupt 2Fh Function 1686h)

Windows issues Windows Initialization Notification (Interrupt 2Fh Function 1605h) and Windows Termination Notification (Interrupt 2Fh Function 1606h). These notifications gives MS-DOS drivers and TSR programs in either Windows environment the opportunity to free extended memory before Windows starts, and reallocate the memory when Windows stops.

Windows also support Detect Interrupt 31h Services. Supporting these services lets MS-DOS drivers and TSR programs check for and use these service functions.

If the user starts a TSR program after Windows starts, the TSR program can always use Get Windows Installed State (Interrupt 2Fh Function 1600h) to determine whether Windows is running.