Hi gang!
I have a couple of specific Cube related questions I haven't found answers to in the manuals/literature yet. So, here goes!
Memory addressing for invalid cards:-
If the CPU accesses a memory location in a position that relates to an expansion slot, and there's no device reported in that slot through the 16MB reporting mechanism, what happens? Does the cycle continue? Is there a bus error?
I ask specifically because I am looking to make a 1GB DRAM card. It would sit logically across four slots that were never allocated. Officially, I'm supposed to apply for a manufacturer ID and have an interrupt and read/write mask at FxFFFFFy. If it 'just works' for read/write accesses to addresses it makes the address decoder far simpler to ignore the need for a ROM. In practice, two of these cards could co-exist in a system. They'd have read/write access. Downside is, they would not be "memory" as far as the OS was concerned, in terms of expanding the system memory allocated by the OS.
If anyone has any thoughts on this, and if it is beneficial, well.....
Secondly, is there any list of NeXT issued expansion card codes/capabilities? Is there a standard mechanism for adding new codes to the OS? How does that work if done by the book, with NeXTCorp no longer a thing? Did anyone ever make a memory card that was slot-based?
Finally, it looks like the full address being accessed is placed on the AD bus. This allows a mechanism to respond to accesses even if they're not in "your slot" by using the top 4 bits to simply decode the actual slot being accessed. From my reading of the literature, all cards can always observe the full address/data being transacted on the NeXTBus. They are normally only selected when the address top 4 bits matches the slot ID. Can I have confidence that is also true for slot 0? Are CPU card transactions always expressed on the NeXTBus? Or is NeXTBus only active when an address in that range is being cycled?
Any knowledgeable answers and enlightenment appreciated.
Unfortunately that only tells me about internal memory connected directly to the memory controller on the main board. All of that memory is associated with slot 0.
It does carry a bright note, though. The next station with the 32MB limit should easily be expandable to 64MB with a little imagination.