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MSCDEX appears to use the Multiplexer Interrupt (2Fh),Functions 440Dh, and their subfunctions.
The function to eject removable media is:
MOV AX, 440DhMOV BX, DriveNumMOV CH, 8h ;Device catagoryMOV CL, 49h ;Eject Removable mediaINT 2Fh
Where DriveNum is 1 for A, 2 for B, etc.