On the A1200 and A4000 motherboards there is a jumper which allows you to swap between 2Mb and 8Mb of Chip ram. Is there any way that this can be set to 8Mb?

No, the registers in Alice chip only address 2Mb of memory. Whilst it is possible with a modified Alice (redesigned in CMOS), you could have 8MB addressing, and a plug-in upgrade to 8Mb. This seems to be how the later versions of UAE address 8Mb Chip RAM. It is easier to redesign something in software than it is to design a whole new chip.

The story behind this is that in the process of designing the Amiga 4000 there was the possibility that the custom chipset could change to allow access to 8 megabytes of Chip RAM. Commodore decided that the time and money could be better spent elsewhere, so the new chipset remained able to deal with only 2 megabytes of chip RAM. At this point the A4000 motherboard had already been designed, so there exists a jumper on the motherboard marked CHIP RAM 2M/8M. Dave Haynie when asked on the possibility gave some now famous advice:

Look folks, if the Alice chip supported 4MB or 8MB of RAM, it would do that

right out of the box.  Honest.  There's no reason to change around any PCB

jumpers, it's only going to get you in trouble.  NEVER EVER mess with a PCB

jumper you don't understand, even if it's labelled "SEX AND FREE BEER".

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