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INT 1A - WORD PERFECT 5.0 Third Party Interface - INSTALLATION CHECK
        AX = 3601h
Return: DS:SI = routine to monitor keyboard input, immediately preceded by the
                ASCIZ string "WPCORP\0"
Notes:  WordPerfect 5.0 will call this interrupt at start up to determine if a
          third party product wants to interface with it.  The third party
          product must intercept this interrupt and return the address of a
          keyboard monitor routine.
        Before checking for keyboard input, and after every key entered by the
          user, Word Perfect will call the routine whose address was provided
          in DS:SI with the following parameters:
                Entry:  AX = key code or 0
                        BX = WordPerfect state flag
                Exit:   AX = 0 or key code
                        BX = 0 or segment address of buffer with key codes
        See the "WordPerfect 5.0 Developer's Toolkit" for further information.