INT 60 - MDEBUG - POP UP
        AH = 04h
        DS:SI -> password or a null byte
Return: AX = return code
            FFFFh call not allowed
            FFFEh password is invalid
            FFFDh display mode is invalid
           else successful
SeeAlso: AH=03h"MDEBUG",AH=07h"MDEBUG"

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