add 1 MB onboard ram to A500 Revision 6A MB

[Zac67]

If you want to have 1 MB chip, the first thing you need is that your Agnus has to be able to handle it. Whether the 2nd 512K sits on an A501 or onboard does not matter a single bit.

With a 1 MB Agnus, you just change the A19 jumper and open the _EXRAM jumper, that's it.

[Zac67]

Without somewhat severe modifications to the board, esp. Gary's address decoding logic, you can't use more than 1 MB chip/'fast' total.
There have been lots of boards that used a Gary adapter to actually multiply the slow-fast RAM to up to 1.8 MB, but they'd have to be made to allow 1 MB chip, else you'd also have to heavily modify them.

You should note that the 2 MB Agnuses in A500+/A600 and A3000 are essentially different in pinout as the latter can provide a 32 bit data path to chip RAM.

[Zac67]

Zac67 wrote:
Without somewhat severe modifications to the board, esp. Gary's address decoding logic, you can't use more than 1 MB chip/'fast' total.

Third try to explain:
You can't use the 2nd onboard 512K and the trapdoor RAM simultaneously unless you rework the addressing logic. If you don't understand what this is about, you won't be able to do it.

[Zac67]

A500+ cards only fit an A500+.
You'll need a sidecar expansion or something that goes to the CPU socket for real fast RAM.