Int 21 Fn 3F  - DOS 2+ - "read" - Read From File Or Device                 [D]

   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

See Also: AH=27h,AH=40h,AH=93h,INT 2F/AX=1108h,INT 2F/AX=1229h