MSX complaints

You seem to be stuck on the idea that the "no disturb" is a good thing while in real life it is very bad thing... My point was: What good is there that each main slot has own subslot? Give me one example where this is more handy than having common sub slot select in I/O port? What are your "programming reasons", what is wrong with the "disturb" you talk about and how you see this would be any "simpler logic design". I actually think current solution needs more logic gates than common I/O port handling would ever need. You say that now it can be done. I say now it must be be done because of this bad design and I would rather just skip this useless step as it gives 0 benefit in any possible programming or HW handling situation I can think of.

Also it complicates the BIOS slot selection routines by a lot, slows them down significantly and restricts their capabilities. I don’t see value in preserving the subslot selection for each primary slot, I don’t think that was a reason for the current design.

I think the reason for the current design lies in detecting the presence of expanded slots. A seemingly minor task performed by the BIOS, however without it using slots would be problematic. If we would control subslots via I/O ports, since the slot system only applies to memory I/O, it would’ve been difficult if not impossible to reliably detect expanded slots. So one must use a memory I/O address to detect which primary slots are expanded.

And then, from a cost perspective, it makes sense to reuse that same decoding logic (the same address) for the control register, rather than an I/O port requiring separate decoding logic.

Not to say that I like the current design or that I don’t agree it’s awkward, and I’m sure there would’ve been a better alternative, but for me this seems the logical explanation and it helps me understand their reasons and how they would arrive at the current solution, especially when the subslot support was added later in the system design process after the primary slot system had already been designed.

If I could make one improvement, I would’ve made slot expanders ignore the slot select signal for writes to FFFFH, causing writes to that address to bypass the slotting system and affect all slotexpanders at once, basically eliminating all our troubles.

HAHAHA! I think that's the real reason!

Ok, you found one reason for this to exist, but I can think many better alternatives... ie. just defining 1 pin in cartridge slot connector for this purpose -> Would not need any logic, just jump wire inside slot expander. Inside computer there would need to be just one 4bit read gate that would handle ALL the slots expanded status.

Why I think it would be less trouble to implement I/O port decoding logic? I/O ports use only 8 bit address compared to memory address bus 16bits (less decoding logic needed) also cartridge would not need to handle reading logic as only one logic inside computer would need to be able handle also reads. Also if computer would have ie. both slot 0 & 3 expanded this same logic could be reused in both slots without doubling it... Way more simple.

I hope you didn't mix those $FFFF registers, Each $FFFF register value must be independent, one for each primary slot.

Yes, MSX standard complicated it a lot unnecesarily if they just setup one global $FFFF register from the beginning as required, but not.

Each $FFFF register is independent, and that is way the slotting routines must set the target's primary slot in page 3 aswell, just to adjust the related $FFFF register, and then to restore page 3 to its original... :(