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Extension ROMs are ROMs fitted in addition to the main ROM set, which provide software modules which are automatically loaded by RISC OS on a reset. Note that RISC OS 2 does not support extension ROMs.
Extension ROMs are provided so that Acorn can add extra modules to RISC OS, or provide replacement modules for those already in RISC OS. You must not use them. It is the Expansion Card Manager's responsibility to recognise extension ROMs. For it to do so, extension ROMs need to have headers, which are detailed in the chapter Expansion Cards and Extension ROMs. That chapter also gives details of the software that RISC OS provides to manage and communicate with extension ROMs (and, of course, expansion cards). Expansion cards and extension ROMs are covered together because both use substantially the same layout of code and data, and the same SWIs.
It is the kernel's responsibility to load any relocatable modules from any extension ROMs - once the Expansion Card Manager has recognised them. For your information, the chapter entitled Modules gives details of how the modules are loaded on a reset.