Compatibility: Win 3.x
Expects: AX    1686H
Returns: AX    0000h = INT 31H (DPMI) services are available
else  = not available
Info: TSRs and device drivers can use this to detect whether INT 31H
(DPMI) protected-mode services are available.

Protected mode services are used to allocate and manage memory
while the CPU is in protected mode.

See DOS Protected Mode Interface for related information.

Note: The DPMI spec describes this fn differently.  It says INT 2fH
AX=1686H is a way to determine the current CPU operating mode.
If AX=0000H on return, then you can assume protected mode.
Otherwise, the CPU is in either Real Mode or Virtual 86 Mode.

I'm not sure who wins the INT 2fH API war.

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INT 2fH 1686H: Detect INT 31h DPMI Services