People seem to have divergent ideas about what should go into a re-designed mainboard and what not, here's my 2 cents:
srowe wrote:Banking RAM could have been done using IO2 or 3.
This is a somewhat unfortunate suggestion, as any internal expansion using registers in I/O2 or I/O3 is likely going to clash with cartridges.
And not necessary either. There is enough room (and prior art!) in I/O0, i.e. $9000..$93FF to put new registers there with a finer decoding. Especially $92xx and $93xx could be used for that purpose, leaving $90xx for the VIC and $91xx for the two "stock" VIAs. A VIC-20 with extra registers in $92xx or $93xx ought remain compatible to >99.9% of all programs, possible exceptions being programs that use non-standard mirrors of the VIA registers (and thus are, IMO, broken by design), or specialist programs that use the "holes" in I/O2 or I/O3 to probe unconnected address space. For the latter, they could be rewritten to use $9100 instead.
JowBrawn wrote:All I've seen in this thread is the request to be able to disable the 3K RAM area.
I also made the suggestion in this thread here to incorporate the VIC-2020 MINIMON cartridge design. With its cartridge expander, it acts more like an expansion of the motherboard, less so as cartridge, even if it uses logic to de-multiplex between I/O2, I/O3 on the slave cartridge or its own memory chip (which hosts MINIMON as firmware).
Other than that, being able to have RAM under BASIC, KERNAL and character ROM (the latter with the option to have this choice separated between CPU and VIC access,
details see here) would be another nice feature. Naturally, chip registers in $92xx or $93xx could take care of the mapping.
Finally, there's always the option to incorporate my VFLI mod, with $0400..$0FFF accessible by VIC and a 16Kx4 SRAM as colour RAM, which I also mentioned here.
The last two features above could not be done with an external cartridge at all, so those would be first candidates for additional features of the VIC-20 motherboard. I'd much more concerned about any extra RAM/ROM that might clash with an external cartridge without a proper cartridge de-multiplexer.
That being said, beamrider's complaints regarding the memory configuration 'issues' are IMO a luxury problem. Tape and user port should be kept, at least as break-out ports, but as long as a pure software solution can revert to the original unexpanded RAM config, that should be in order. Not everything needs switches.
P.S. I still take priority on the VIC-2020 designator.