A (very) basic 6502 disassembler for Abacuc

Photo by Vipul Jha on Unsplash

As I wrote in my last post, I am currently spending some time putting together a 6502 breadboard computer. I got the system up and running in a very basic configuration.

I have some debugging output from my Python code. Every time there is a transition of the clock from high to low, I output a debug line that contains the following information:

  • Status of the RESB, SYNC, RWB, BE, SOB, IRQB, RDY, VPB, and NMIB microprocessor pins.
  • A binary and hex dump of the addressbus address
  • A binary and hex dump of the databus
  • An indication if the microprocessor is trying to read or write based on the status of the RWB pin. (Redundant with what I do but I wanted a clear visual indication)

This gives a good idea of what is happening while executing code. Since I can lower the operating frequency to whatever value I want and single-step, I can look closely at what the microprocessor is doing. (Thank you again for the full static design of the W65C02)

If you read the W65C02 datasheet, you will learn that:

(SYNC) The OpCode fetch cycle of the microprocessor instruction is indicated with SYNC high. The SYNC output is provided to identify those cycles during which the microprocessor is fetching an OpCode. The SYNC line goes high during the clock cycle of an OpCode fetch and stays high for the entire cycle. If the RDY line is pulled low during the clock cycle in which SYNC went high, the processor will stop in its current state and will remain in the state until the RDY line goes high. In this manner, the SYNC signal can be used to control RDY to cause single instruction execution.

This is cool. I can check if the microprocessor fetches an OpCode and disassemble it on the fly, writing the disassembler output to my console.

A few days ago, I stumbled upon a GitHub repository from Bill Zissimopoulos, basically doing the same thing I was trying to do. The only difference was that he used an Arduino Mega instead of a Raspberry Pi. Looking at the Arduino code he wrote, I noticed he had the same idea about using the SYNC pin. The disassembler code was written in C, and I ported it to Python, spending an hour or so in the process.

It worked like a charm! Thank you, Bill, for your code.

What I wrote is quick and dirty, but it does the job.

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