An enhancement to Ben Eater's 6502 address decoding logic based somewhat on @krischik STEEMIT article at https://steemit.com/hive-175254/@krischik/ben-eater-6502-kit-day-16.

This circuit adds a Dual 4 input AND chip (74LS21 per krischik) to recover unusable RAM in Ben's original design and a 3 to 8 decoder chip (74LS138) to provide additional device selects such as the additional VIA chip.

Ben Eater's amazing 6502 build series: https://eater.net/6502