I have a specific question about the DMACON/R registers found in Commodore Amiga range of computers. As you may know they are the DMA control registers that controls all the DMA channels used by the Amiga chipset - one for read and one for write. Now, does anyone have an insight in detail how those two registers works? I know that you use the bit #15 to set or clear the different bits in the write register (DMACON), but how does the DMA controller interpreter the different register bits? Also, you have two bits that are unused on those registers: #11, and #12; is it possible to take advantage of them and use those two bits if you can extend the existing chip register set? As far as I know the official hardware reference manual doesn't tell anything about it.
It would be very interesting to know because it "might" be possible to enhance the chipset with external blitter and copper processors.
First off: thanks to both hippietrail/lvd? and Brian H for your good answers.
To give you more 'flesh' (information) and background to my question: I've been toying with John "Chucky" Hertell's ReAmiga 1200 pcb files and were wondering if there is a possibility to enhance the chipset in anyway. The PB8 and PB9 connectors that were designed to give the A1200 additional 1mb of chipram before the Amiga developers decided to go for the full two megabyte, could be a possible gateway to enhance the chipset. Those two connectors contains full data- and memory address bus (plus other signals) to chipram and such. I know that you also need the Register Address Bus (RGA) which you can find present at TestPoint3 (TP3) right below the Alice chip. If one could build a piggyback board containing chips in form of CPLD or FPGA of some sort and those chips could contain enhanced 16-bit copper running at higher synchronus speed (i.e. faster than the original ~3.58MHz) and a faster blitter - maybe 32-bit as long as it doesn't need access to 16-bit chip registers. Imagine that this is possible; you would need to deactivate the original blitter and copper and then activate the 'new' blitter and copper by using the DMACON/R registers (bit #11, and #12). Maybe it is possible to just turn off the original blitter and copper and then call the new ones in software by reading and writing directly to the new registers that controls the new co-processors. If you study the Amiga Hardware Reference Manual and look at all the chip registers, you can see that there are several registers that is unused (17 of them in AGA chipset, if I remember correctly). Taking advantage of those unused registers you could enhance the functionallity of the Amiga chipset.
Well, those are my two cents about it.