INT 21 - DOS 2+ - "READ" - READ FROM FILE OR DEVICE
        AH = 3Fh
        BX = file handle
        CX = number of bytes to read
        DS:DX -> buffer for data
Return: CF clear if successful
            AX = number of bytes actually read (0 if at EOF before call)
        CF set on error
            AX = error code (05h,06h) (see AH=59h)
Notes:  data is read beginning at current file position, and the file position
          is updated after a successful read
        the returned AX may be smaller than the request in CX if a partial
          read occurred
        if reading from CON, read stops at first CR
        under the FlashTek X-32 DOS extender, the pointer is in DS:EDX
SeeAlso: AH=40h,AH=93h,INT 2F/AX=1108h,INT 2F/AX=1229h