INT 50 - PC Thuis Organizer Shell - PROMPT YES/NO
        AX = 0005h
        ES:BX -> prompt string (ES:0000h if no prompt)
Return: AX = key pressed
            0000h "J" (Dutch "Ja" = "Yes")
            FFFFh "N" (Dutch "Nee" = "No")
Program: The PC Thuis Organizer Shell was written by John Vanderaart and
          published in the June/July 1990 issue of PC Thuis Power magazine