A500 512KB Chip VS A500 1MB Chip

[kirk_m]

I know this is an old thread, but, if you are still around, can you provide a picture of this setup?  I have already done the 1mb chip mod on my Rev6a board, but, want the ability to switch between 512k/512k configuration as you describe.  I have a 512K expansion in the trapdoor port.

Would I use a switch like that in the link below?

https://www.adafruit.com/product/3220?gclid=EAIaIQobChMIjNaop8-K1gIVkrXACh1VLwIaEAQYAiABEgKPgvD_BwE

To change a 512/512 KB machine to 1 MB chip you need
- 1 MB Agnus
- change the A19 jumper to map the 2nd 512 KB to $080000 (instead of $C00000)
- cut the _EXRAM trace or jumper to disable the $C00000 RAM

The 1 MB Agnus by itself doesn't cause any incompatibility issues.
Some (extremely few) programs get confused by 1 MB chip RAM. For those you can add a switch to the A19 line (JP2 jumper) to switch between both jumper positions. $C00000 pseudo fast RAM will only be present if _EXRAM is active (usually closed by the trapdoor board), so you need a 2nd switch or a dual contact one.
Just don't set Agnus A19 to CPU A19 and activate _EXRAM at the same time - the machine will crash on boot-up. Also note that if you switch the A19 line while powered up the system will crash violently.

[kirk_m]

Thanks!  Do you know if such a switch setup will work if I have a megachip installed on this a500?  Currently, I do, so there are really 2 megs of chip in there.