The mechanics of receiving parameters through the Hook are covered in the "Custom Stream Handlers" section, and are not duplicated here. Refer to the EntryHandler() and ExitHandler() Autodocs for the contents of the registers upon entry. Once inside your handler, you can call nearly all functions in the iffparse.library, however, you should avoid calling ParseIFF() from within chunk handlers. Your handler runs in the same environment as whoever called ParseIFF(). The propagation sequence is: __________ ____________ ________________ | | | | | | | mainline |--calls-->| ParseIFF() |--calls-->| your_handler() | |__________| |____________| |________________| Thus, your handler runs on your mainline's stack, and can call any OS functions the mainline code can. (Your handler will have to set the global base pointer if your code uses base-relative addressing.) When leaving the handler, you must follow the standard register preservation conventions (D0/D1/A0/A1 may be trashed, all others must be preserved). D0 contains your return code, which will affect the parser in a number of ways: If you return zero (a normal, uneventful return), ParseIFF() will continue normally. If you return the value IFFERR_RETURN2CLIENT, ParseIFF() will stop and return the value zero to the mainline code. If you return any other value, ParseIFF() will stop and return that value to the mainline code. This is how you should return error conditions to the client code.