xor bx, bx
mov es, bx ; es:bx contains 0000h:0000h
mov cx, 0000h
mov dx, [Flags] ; bit 0 clear if Windows 95
mov di, [Ver_Num] ; major/minor version numbers
; (in high/low bytes)
xor si, si
mov ds, si ; ds:si contains 0000h:0000h
mov ax, 1605h ; Windows Initialization Notification
int 2fh ; multiplex interrupt
cmp cx, 0
jne no_initialize ; if nonzero, don't continue initialization
Windows Initialization Notification (Interrupt 2Fh Function 1605h) notifies MS-DOS device drivers and TSRs that Windows is starting. Windows calls this function as it starts allowing MS-DOS device drivers and TSRs that monitor Interrupt 2Fh the opportunity to prepare for running in the Windows environment.
Flags
Specifies whether Windows 95 is initializing. Bit 0 set to 0 for Windows 95; bit 0 set to 1 for other, previous versions of Windows. Only bit 0 is used; all other bits reserved and undefined.
Ver_Num
Specifies the version number of Windows. The major version number is in the high-order byte, the minor version number in low-order byte.
Any MS-DOS device driver or TSR that either cannot run in the Windows environment, or must adapt its operation when in the Windows environment should add itself to the Interrupt 2Fh chain and watch for this function.
If the device driver or TSR cannot run in the Windows environment, it should set the CX register to a nonzero value, display a message informing the user of its incompatibility with Windows, and return. Windows does not print a message of its own. Instead, it calls Windows Termination Notification (Interrupt 2Fh Function 1606h) and returns to MS-DOS.
If the device driver or TSR can run in the Windows environment, it should do the following:
1. Call the next device driver or TSR in the Interrupt 2Fh chain to allow all device drivers and TSRs in the chain an opportunity to respond to this function.
2 Upon return from the interrupt chain, carry out the following actions:
3 Return (using the iret instruction) but without changing the CX register.
For more information about these procedures, see the following "Comments" section.
The device driver or TSR must preserve all registers and restore the original values before returning. The only exceptions to this rule are changes made to the BX, CX, DS, ES, and SI registers as a result of following the previous procedure.
Enable/Disable Virtual 8086 Mode CallBack Function
Some device drivers and TSRs, such as expanded memory emulators, switch the processor to virtual 8086 mode. Because Windows cannot start successfully while the processor is in this mode, any device driver or TSR that switches to virtual 8086 mode must either switch back to real mode or supply the address of a callback function that can switch between real and virtual 8086 modes.
Windows uses the callback function to disable virtual 8086 mode before Windows itself enters protected mode. Windows calls the callback function again to enable virtual 8086 mode after Windows exits protected mode. Windows calls the callback function using a far call instruction, and it specifies which action to take by setting the AX register to 0 or 1.
To disable virtual 8086 mode, Windows sets the AX register to 0, disables interrupts, and calls the callback function. The function should switch the processor to real mode, clear the carry flag to indicate success, and return. If an error occurs, the function sets the carry flag and returns. Windows checks the carry flag and terminates if it is set.
To enable virtual 8086 mode, Windows set the AX register to 1, disables interrupts, and calls the callback function. The function should switch the processor to virtual 8086 mode, clear the carry flag, and return. If an error occurs, the function sets the carry flag and returns. However, Windows ignores the carry flag, so if an error occurs no action is taken and the processor is left in real mode.
Whether an error occurs when enabling or disabling virtual 8086 mode, it is up to the callback function to display any error message to the user. Also, the callback function must not enable interrupts unless an error occurs, and the function sets the carry flag.
A device driver or TSR supplies a callback function by copying the address of the function to the DS:SI register pair when it processes the Windows Initialization Notification (Interrupt 2Fh Function 1605h). Windows permits only one callback function, so the device driver or TSR should first check to make sure that the DS and SI registers are both zero. If they are nonzero, the device driver or TSR should set the CX register to a nonzero value and return, directing Windows to terminate without starting.
Initializing a Win386_Startup_Info_Struc Structure
An MS-DOS device driver or TSR initializes a Win386_Startup_Info_Struc structure to direct Windows to load the virtual device and to reserve the instance data the device driver or TSR needs to operate in the Windows environment. The device driver or TSR is also responsible for establishing a chain of startup structures by copying the contents of the ES:BX register pair to the Next_Dev_Ptr member. It is assumed that any other device driver or TSR in the Interrupt 2Fh chain will have set the ES:BX register pair to the address of its own startup structure prior to returning.
Any device driver or TSR can use a Windows virtual device to help support its operation in the Windows environment. To specify a virtual device, the device driver or TSR sets the SIS_Virt_Dev_File_Ptr member to the address of the virtual device's filename. The device file is assumed to be in the Windows SYSTEM directory. The device driver or TSR can also set the SIS_Reference_Data member to specify additional data to be passed to the virtual device when loaded.
Any device driver or TSR can reserve instance data for itself. Instance data is one or more blocks of memory used by the device or TSR, and managed by Windows. For device drivers or TSRs loaded before Windows starts, reserving instance data allows the device driver or TSR to keep separate data for each virtual machine. Whenever Windows switches virtual machines, it saves the previous VMs instance data and loads the current VMs instance data. If a device driver or TSR does not specify instance data, the same data is used for all virtual machines.
A device driver or TSR reserves instance data by appending an array of Instance_Item_Struc structures to the Win386_Startup_Info_Struc structure. The last structure in the array must be set to zero. Each Instance_Item_Struc structure specifies the starting address and size (in bytes) of an instance data block.
The device driver or TSR must copy the address of its startup structure to the ES:BX register pair before returning.
See also Interrupt 2Fh Function 1606h Windows Termination Notification