I want to echo other's sentiments, that this seems like a complex design for your first build, particularly the video circuitry. Far be it from me to try to dissuade your enthusiasm, but you'd probably be better off getting the "computer" part working and talking over serial before worrying about video outputs.
I haven't had a chance to do a thorough inspection of your schematic, but a couple of things that immediately jumped out on a glance:
1. You've got the MAX3100 (UART)'s s IRQ output and the W65C22S (VIA)'s IRQ output tied together. Normally you can tie IRQ pins together on devices where the IRQ pin is "open-drain", as it is on the MAX3100. However the W65C22S's IRQ output is totem-pole and therefore you cannot do that in this case. As it is, what will happen is when the MAX3100 wants to indicate an interrupt whilst the VIA is not, you'll create a short between power and ground through the VIA to the MAX3100. This is not good. Best case the interrupt won't work, worst case you'll cook some chips! Additionally, you need a pull-up on the IRQ line for open-drain pins, because open-drain outputs only pull low: When they're inactive you need a pull-up otherwise it'll just float.
The classic solution here would be to put a pull-up (3.3K ohm would be a reasonable value) on the UART's IRQ output, and then run the VIA and the UART's IRQ signals separately into an AND gate, the output of which is now your IRQ input to your CPU.
Another solution that doesn't involving adding another gate, would be to cascade the UART's IRQ into the CB2 or CA2 inputs on the VIA, as these can be configured as interrupt inputs. Then when the UART triggers its IRQ, that cascades into the VIA triggering the VIA to interrupt too. Obviously you'll have to check the VIA's IFR register to see which device triggered the interrupt, but you'd have to do that anyway with the other configuration.
2. 7805 is absolutely fine for your power regulator, but as others have mentioned it is extremely inefficient and, at higher currents, will dissipate a lot of heat. You probably won't get to the point where it matters, but if you did you'd need a big heatsink and, again, its very inefficient so if you're on e.g. battery power, you're wasting a lot of your charge. Switching regulators are much better, but harder to implement, however RECOM make
drop-in 7805-replacement switching regulator modules, which I've used before and highly recommend for hobbyist use.
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