Compatibility: Win 3.x
Expect: AX    1605H
CX    0000H
DX    flags: bit 0=1 if standard mode
bit 0=0 if enhanced 368 mode
DI    version number (major in hi byte; minor is low byte)
DS:SI 0000:0000
Return: CX    0000H if OK to continue starting Windows
else  Windows will not start
DS:SI addr of an Enable/Disable Virtual Mode handler
ES:BX addr of a Win386_Startup_Info_Structure
Info: TSRs and DOS-level device drivers may intercept this function as
a way to be notified when Windows (standard mode or enhanced 386
mode) is about to start up.  It lets you free up extended memory
for use by Windows; for instance, DOS's SmartDrive disk cache
intercepts this call for that purpose.

By returning CX != 0, you can prevent Windows from running.  In
that case, you really ought to display a message to ell the user
what's going on.

If the CPU is in Virtual 8086 Mode, you must set it back to Real
Mode and there are several other important steps.  Please check
the Windows Device Driver Kit (DDK) for full information.

Your handler will later receive an INT 2fH 1608H notification to
indicate that windows is actually starting up or an INT 2fH 1609H
if Windows finds that it can't start.

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INT 2fH 1605H: Windows Is About to Startup