Agnus 8375

[Zac67]

If you relocate the slow RAM make sure you cut _EXRAM as well (otherwise the RAM will appear in two different locations and the system will crash). The 2 MB chip RAM expansion requires this mod as well.

2. The Gary chip lines are for providing the extra addressing required. The Agnus socket as is can only address 1 MB.
3. Agnus pinouts can be found in the service manuals. I've added a comparison chart to Wikipedia a while ago. The inner workings are not so easy, there are several somewhat complex state engines. You should take a look at the Hardware Reference Manuals.
4. Yes, Agnus is doing chip/slow RAM refresh as well.

[Zac67]

The _EXRAM signal is set by JP7A if I remember correctly. Just cut it open and all will be well. And JP2 need to be changed to the other position. This is what I have read about the 1MB CHIP RAM mod. Hope I haven't forgot anything.

Yes, that's it.

That meanst that Gary is having address line for up to 2MB CHIP RAM but these lines are unused?

Gary has access to all upper address pins.

I've seen this pinout table. Thanks! Unfortunately, it's not mentioned which specific chip is it. There are some incompatibilities, even within the same four number series what I've heard.

Some time ago, alexh made a pretty good survey on how to determine the Agnus version... Here it is: http://eab.abime.net/support-hardware/21961-help-update-amiga-hardware-agnus.html