All of the above is done by the CPU of course. There is nobody else who can do it. After a reset IIRC the ROM is mapped to address 0 and the 68k reads address 0 in order to find the address where to continue. The address then points into ROM code.
The very first action of the code is to switch the power LED on (or bright on those models where the LED is always on). It does this by setting the respective bit in one of the CIA's I/O ports. It's the same bit which is responsible for the audio filter.
Screen colors are changed by writing a color code into $dff180. This register is part of the custom chips which are active all the time.
IMHO checking chip ram is one of the first actions of the ROM. I think the first few actions are: 1. switch power LED on, 2. change screen color to black (or white on 1.3 and below), 3. check ROM checksum, switch color to red if it failed, 4. check chip ram, switch color to green if it failed.
The failure then jumps to the guru meditation code which blinks with the power LED ten times and then resets the computer, so that everything starts from the beginning again in an endless loop.
If you see the power LED becomming brighter, then you can be sure that the CPU, the ROM and the CIA are ok. If the screen becomes green, then it's something memory related. Somehow the chip ram test failed before it found 128kb of it. This could mean no or defective RAM, but it can also be a problem with the memory interface.
At least in an A4000 the clock battery is quite near the RAM chips and a leaking battery often causes a green screen which is very difficult to repair.
You could try to contact Botfixer at the
http://www.a1k.org/ forum. He is very successful in repairing battery leakage problems. But he is over-busy right now, so that he can probably not take new rapir orders.
Bye,
Thomas
