Mega Midget Racer in 68000 mode doesn't work with A590 SCSI controller?

[Azryl]

Most CPU socket accelerators fail due to DMA bus to memory problems. Once the onboard 680x0 chip forces itself dominant on the bus you get DMA to Fast RAM issues.

The problem is memory no longer accessible to the motherboard but now addressed to your socketed accelerator.

It breaks the DMA, ie Direct Memory Access.

PIO modes can sometimes work with socketed accelerators, PAMC and Adspeed etc

Az

[Azryl]

the MMR works fine when using the 68030 processor, and A590.

If I remember the a500 DMA channels correctly some are from gary, some from denise and the remainder from Agnus. Depends which DMA channels are used by the a590 to bring data from the drive into the motherboard then ram via the 86pin side expansion bus.

the A590 works fine when using a 68000/68010 processor plugged directly into the A500 motherboard.

Because this is the standard mode for the 86pin side expansion, all commodore branded expansions should work with this configuration of hardware.

the only issue seems to be when trying to use the MMR in 68000 mode, with the A590.

The 86pin side expansion is a near pass thru of the 68pin cpu socket, but now you will have some extra glue logic involved when you use the fallback mode of the MMR, something unexpected is getting in the way. You will need to research the MMR some more, or try to place the a590 into a different mode of data transfer.

As I said earlier, cpu accelerators that use PIO mode transfers from 86pin expansion bus work, Mtec i68020 + Mtec megabody etc.

If you remove the cpu memory adapter and try the onboard a590 2meg fast ram maybe the ram config will allow the MMR fallback mode.

Az