You're pretty much on the right track now - the main point is: there are three different versions/flavors/pinouts of (PLCC) Agnus:
- the one from A500/2000/CDTV - controls 1 MB RAM, older revisions can only use 512 K of that as chipmem, the other 512 K are pseudo fastmem
- the one from A3000 - controls 2 MB chip RAM, A3000s had 1 MB chipmem out of the box, but are easily upgradable
- the one from A500+/A600 - controls 2 MB chip RAM, machines again had 1 MB out of the box but are easily upgradable
That leaves the A500/2000 owners with the wish to upgrade to 2 MB chip. Here come the adapter boards (Mini Megi et al), allowing to fit an 2 MB-Agnus (A3k mostly) on a otherwise not suited mainboard. Which of the 2 MB chips they require depends on their design - the A3k Agnus was first, so it was favoured.
PS: what seriously confuses me is that BBoAH talks about the A500+ as if it has the same chipmem as the A500, even the A501+ is stated to be a 512K expansion. The printing on the board also says so even though it clearly carries 8x 514256 chips = 1 MB! Can someone enlighten me? Is there an A500+ and another A500 Plus and I've been mixing them up all along???
