INT 2F - European MSDOS 4.0 POPUP - "CheckPu" - INSTALLATION CHECK
        AX = 1400h
Return: AX = FFFFh if installed
            BX = maximum memory required to save screen and keyboard info
        CF clear if successful
        CF set on error
            AX = error code
                0002h invalid function
                0004h unknown error
Note:   the POPUP interface is used by background programs (see INT 21/AH=80h)
          to communicate with the user
SeeAlso: AX=1402h"POPUP",AX=1403h"POPUP"