INT 21 - ELRES v1.1 - INSTALLATION CHECK
        AH = 2Bh
        CX = 454Ch ('EL')
        DX = 5253h ('RS')
Return: ES:BX -> ELRES history structure (see below)
        DX = DABEh (signature, DAve BEnnett)
Note:   ELRES is an MSDOS return code (errorlevel) recorder by David H. Bennett
          which stores recent errorlevel values, allows them to be retrieved
          for use in batch files, and can place them in an environment variable
SeeAlso: AH=4Dh

Format of ELRES history structure:
Offset  Size    Description
 00h    WORD    number of return codes which can be stored by following buffer
 02h    WORD    current position in buffer (treated as a ring)
 04h  N BYTEs   ELRES buffer