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Hacking the Yamaha DX9 to Turn It into a DX7 (2023)
A brief account of hacking the Yamaha DX9's firmware ROM to make its functionality more closely match the DX7.
From here you can identify a broad cross-section of internal functionality —such as note on/off handlers, parameter changes, and patch functions— by tracing how the software handles the associated MIDI status messages. Yamaha employed an interesting programming technique in the DX9 firmware to preserve ROM space: The LCD string copy routine treats any byte it encounters with a value of '128' or above to be an index into a table of commonly repeated 'string fragments'. My personal theory about the DX9's design is that developing the FM sound chips incurred such a high up-front cost that Yamaha needed to get them moving off shelves by any means necessary.
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