INT 15 - later PS/2s - GET PHYSICAL FIXED DISK DRIVE NUMBER (SELECTABLE BOOT)
        AH = D4h
        DL = logical fixed disk drive number
Return: AH = return code
            00h success
            01h specified logical drive number is invalid
            86h function not supported
            80h function not supported (on PCjr and PC)
        CF set on error
        CF clear on success
            AL = physical fixed disk drive number

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