Last night I added an 74HC138 3 into 8 line decoder to my project. Ignore the fact that I’m not using resistors with the LED’s. It’s just for testing until I decide just how I’m going to handle them.
Essentially it takes a 3 bit binary value (3 address inputs) that represents 0 to 7, and enables one of 8 possible outputs. It has many uses, enabling one of several SPI devices, I’ve used it in the past to replace rotary switches to interface with analog devices. In this specific case, I’m going to use it to select what “slot” I want to look at to read/write an address bus, set registers, for my project. I have Apple II compatible card edge slot connectors that I can put onto a protoboard and interface with “Real” hardware. I’m not quite that far yet, as I’m just beginning to look at memory mapping, loading “Roms” from an SD card into the memory block on the RAM. One thing I did decide was using a configuration file for memory mapping. Since I can address the full 64K with the emulator I can map the memory however I want and load whatever I need into the map.
Another thought crossed my mind, if I were to take a 40 pin DIP connector and wire the outputs and inputs as appropriate address bus, data bus, and other outputs, if I could emulate the 6502 directly in an Apple II, using the apple’s clock pulse to trigger an interrupt that stepped the emulator once cycle. It’s not actually that outrageous an idea. By doing that I can inspect and step data on a more modern PC, or even modify or replace portions of memory.