Expects: AX    4100H
Returns: AH    EMM status (0=successful)
BX    segment of EMS frame
Info: Obtains the segment at which the EMS frame is mapped.

The EMS frame is a 64K area, often starting at d000:0.  Depending
upon switches used when the driver is installed, the frame may
actually start at any segment address from 8000H to 9000H (seen
rarely) and c000H through e000H in increments of 400H.

When you read or write to EMS memory, you will use INT 67H 4000H
to map part of the EMS into this frame, then read or write to the
64K starting at the segment returned in BX.

- -

INT 67H 4100H: Get Frame Segment Address