INT 50 - PC Thuis Organizer Shell - READ FILE
        AX = 0004h
        ES:BX -> buffer for data
        CX = number of bytes to read or 0000h for entire file
        DL = file type
            01h setting shell
            02h setting sterm
            03h INT21 file
        DS:SI -> filename
Return: AX = status
            0000h successful
            FFFFh error
Note:   file type numbers are maintained by John Vanderaart; if a new file type
          is needed, a type number should be requested from him through the
          magazine:
                PC Thuis BV
                Spaarne 55
                2011 CE HAARLEM
                The Netherlands