Important Notice

The pages on this site contain documentation for very old MS-DOS software, purely for historical purposes. If you're looking for up-to-date documentation, particularly for programming, you should not rely on the information found here, as it will be woefully out of date.

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The Interrupt 2Fh services are a set of functions that MS-DOS drivers and
TSRs call to direct Windows to carry out specific actions. The
notifications are a set of functions that Windows calls to notify drivers
and programs of Windows-related events. Although these functions were
initially designed for use by MS-DOS drivers and TSRs, some Windows device
drivers, such as display drivers and grabbers, also use them to manage the
multitasking features of Windows.
 
A driver can call the service functions using the MS-DOS multiplex
interrupt, Interrupt 2Fh. The driver sets one or more registers to specified
values, sets the AX register to the desired function number, and issues the
interrupt using the int instruction.
 
A driver can receive notifications by installing its own interrupt-handling
routine in the Interrupt 2Fh interrupt chain. Once the interrupt handler is
installed, the driver checks the AX register on each interrupt for function
numbers that match the notifications. When the driver matches a
notification, it can carry out any related actions.
 
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 Enhanced-Mode Windows Installed State (Interrupt 2Fh Function 1600h)
 
♦  Get Enhanced-Mode 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)
 
The 386 enhanced-mode Windows display grabber uses the Get Device Entry
Point Address (Interrupt 2Fh Function 1684h) to retrieve the entry point
address for the virtual-display device (VDD). The grabber uses this address
to call VDD service functions to carry out capturing display contexts and
updating tasks.
 
Both standard- and 386 enhanced-mode 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.
 
Standard- and 386 enhanced-mode 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 Enhanced-Mode Windows Installed State (Interrupt 2Fh Function
1600h) to determine whether Windows is running.
 
Notification Functions
 
Windows broadcasts the notification functions by setting registers and
issuing Interrupt 2Fh. Drivers and TSRs use the notifications to load 386
enhanced-mode Windows installable devices, free extended memory, and enable
or disable various device services or features. There are the following
notification functions:
 
♦  Windows Initialization Notification (Interrupt 2Fh Function 1605)
 
♦  Windows Termination Notification (Interrupt 2Fh Function 1606h)
 
♦  Device Call Out (Interrupt 2Fh Function 1607h)
 
♦  Windows Initialization Complete Notification (Interrupt 2Fh Function
   1608h)
 
♦  Windows Begin Exit (Interrupt 2Fh Function 1609h)
 
MS-DOS device drivers, such as network drivers, use Windows Initialization
Notification to direct 386 enhanced-mode Windows to load a protected-mode
installable device that provides 32-bit support for the real-mode driver
while 386 enhanced-mode Windows runs. Other MS-DOS drivers (for example,
disk-cache drivers) use the same notification to free any extended memory
before Windows starts. The TSRs also use Windows Termination Notification to
reclaim the extended memory when Windows stops.
 
If an MS-DOS driver or TSR programs installs a corresponding virtual device,
that device can send notifications of its own to the driver or TSR using
Device Call Out.
 
Critical Section Handling
 
Occasionally, an MS-DOS driver or TSR program may need to run for a period
of time that may exceed its regular time slice. In such cases, the driver
can create a critical section that prevents Windows from switching the CPU
away from the driver or program.
 
A driver starts a critical section by using Begin Critical Section
(Interrupt 2Fh Function 1681h). While the critical section is in effect,
only device interrupts can divert execution from the driver. A driver ends
the critical section by using End Critical Section (Interrupt 2Fh Function
1682h). In general, a driver should end the critical section as soon as
possible to ensure that all drivers and programs in the system receive CPU
time. If a driver starts a critical section n times, it must end the
critical section n times before the critical section is actually released.
 
Ordinarily, Windows prevents rescheduling of the current virtual machine if
the one-byte, MS-DOS InDOS flag is nonzero. One exception is when a driver
or program issues the MS-DOS Idle interrupt (Interrupt 28h). In such cases,
Windows may reschedule regardless of the value of the InDOS flag. The only
way to prevent this rescheduling is to start a critical section using Begin
Critical Section. While in a critical section, Windows disregards Interrupt
28h.
 
In previous versions of Windows (for example, Windows/386(TM) version 2.x),
service functions to support critical sections were not supplied. Drivers
and programs achieved a similar effect by incrementing and decrementing the
MS-DOS InDOS flag. Although this method was acceptable for previous versions
of Windows, drivers should not use this method in versions of Windows that
supply Begin Critical Section and End Critical Section (Interrupt 2Fh
Function 1682h).
 
Drivers and programs that use the InDOS flag method retrieve the address of
the InDOS flag using Get InDOS Flag Address (Interrupt 21h Function 34h).
The function returns the address in the ES:BX register pair. When using this
method, drivers and programs check the flag value before decrementing
because some error conditions (such as when the user types CTRL+C) set the
InDOS flag to zero regardless of its current value. Decrementing the InDOS
flag to a number less than zero is a serious error.
 
Releasing the Time Slice
 
MS-DOS applications can also use the Interrupt 2Fh service functions. In
particular, applications can use Release Current VM Time Slice (Interrupt
2Fh Function 1680h) to release the current virtual machine's time slice. An
MS-DOS application typically uses this function when waiting for user input.
This function helps 386 enhanced-mode Windows multitask more efficiently by
letting it reschedule the CPU for other work immediately, rather than
waiting for the idle application to spend its entire time slice.
 
Virtual-Display Device Services and Notifications
 
The virtual display device for 386 enhanced-mode Windows also provides
Interrupt 2Fh service and notification functions. Windows display drivers
use these functions to check for and manage screen switching. There are the
following functions:
 
♦  Enable VM-Assisted Save/Restore (Interrupt 2Fh Function 4000h)
 
♦  Notify Background Switch (Interrupt 2Fh Function 4001h)
 
♦  Notify Foreground Switch (Interrupt 2Fh Function 4002h)
 
♦  Enter Critical Section (Interrupt 2Fh Function 4003h)
 
♦  Exit Critical Section (Interrupt 2Fh Function 4004h)
 
♦  Save Video Register State (Interrupt 2Fh Function 4005h)
 
♦  Restore Video Register State (Interrupt 2Fh Function 4006h)
 
♦  Disable VM-Assisted Save/Restore (Interrupt 2Fh Function 4007h)
 
For more information about using these functions, see Chapter 2, "Display
Drivers."
 
 
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