I'm having trouble with the backlight on my 14" iBook G4( PowerBook6,5 ).
Here is how iMic described the backlight control hack to the OS ROM:
Backlight Control
The iBook G4 now has full backlight controller support under Mac OS 9.2. The solution was found in the Open Firmware device tree, at the following branch:
/pci@f2000000/mac-io@17/backlight@f300
The backlight-control property carried the value 6d6e6361 00, which is the string mnca in hex. I compared this to an iBook G3 Clamshell, which has a backlight-control property with the value 61746900, which is the string ati in hex. Changing mnca in Open Firmware to ati restored the backlight functionality on the iBook G4.
However, Mac OS 9 already has support for backlight controllers of the type mnca in the Mac OS ROM, as evidenced by the presence of this data at data fork offset 0x20D4AD:
panel-family DFP,EDID ATY NVDA driver-ref backlight-control ati pmu mnca
All I did was reverse the order of ati and mnca in the ROM:
panel-family DFP,EDID ATY NVDA driver-ref backlight-control mnca pmu ati
Now, I can control the internal display brightness with the brightness keys and from within the Monitors control panel. This does require another revision of the Mac OS ROM for the iBook G4. An ideal scenario would be to somehow write the revised backlight driver into an extension (which may not be possible) that is loaded alongside the standard 10.2.1 Generic ROM, but this is still a reasonable solution.
The thing is the value for my backlight-control is the same 6d6e6361, but whenever the loading screen loads, it turns the backlight off. I have a 12" iBook with the R9550, and iMic's hack works just fine on that system.
Reverting the hack makes the backlight stay on on the 14", but I have no control of it.
Also, changing the value to ati( 61746900 ) in Open Firmware, with the unhacked ROM, makes the backlight go off.