6502-hacking

A project to work with Ben Eater's 6502 computer kit, replacing the clock module and EEPROM, amongst other things.

> HACKING <

This is very much work in progress but does manage to demonstrate several things. It's made up of two parts. 6502-contoller is an Arduino firmware for interacting with the 6502 and 6502-loader is a python script to interact with the Arduino.

6502-contoller

This Arduino firmware started off fairly similarly to Ben's monitor firmware but I needed a couple additional things. As I didn't have the clock module it also provided a clock signal from the start. It can also be tied to the 6502 RSTb line and control the reset signal. I also opted to not buy the EEPROM reader for several reasons. One, I don't have the money and additionally I just hate the idea of having to pull a chip to program it, place it back in, blah, blah. I spend 20 years in web development damn it! CTRL-R is my compiler! So the firmware can load the entire ROM into memory, read the ADDRESS lines coming off the 6502 and provided the addressed byte on the DATA lines.

This second feature, while being generally great and speeding up development does have one major hiccup. It's requires not only an Arduino with many IO, like a Mega, it also requires a device with over about 60kb of memory... unlike a Mega. For this project I'm using an inexpressive STMicro Nucleo F401RE devboard. While inexpressive these are still not as accessible as a Mega for some people. The originally I had wanted to request each byte over serial as the address lines change but could not manage to make this work. Now that the entire project is working better and my understanding has improved I do think it's possible and might be worth revisiting.

6502-loader.py

This part interacts with the Arduino firmware. In it's basic flow it handles several things.

• Stopping the clock
• Putting the 6502 into reset
• Loading the ROM into the Arduinos memory
• Restarting the clock
• Bringing the 6502 out of reset
• Runs monitor mode

At this point the script can be exited and the 6502 will continue to run. A normal terminal can be opened to stop the clock and use single stepping from the devboard.