6502bench SourceGen: End Notes
Origins
The inspiration for SourceGen goes a long way back. While in high school in the late 1980s, I read Don Lancaster's Enhancing Your Apple II, Vol. 1 (available for download here). This included a very detailed methodology for disassembling 6502 software. I wanted to give it a try, so I generated a monitor listing of an operating system (called "RDOS") that SSI used on their games, and printed it out on my Epson RX-80 -- tractor feed paper was helpful for this sort of thing -- then set to work.
Lancaster's methodology involved highlighting different types of instructions with different colors, making notes, and adding labels. All this being done with felt-tip and colored highlighter pens. The process worked remarkably well: by the time I was finished marking things up, I knew how everything in the code worked.
I really wanted a better system though. The disassembler built into the Apple II could get out of sync when it walked through a data area, so sometimes you had to hand-write in the correct instruction. Applying a label to every place that referenced it was tedious. When you got to the end, you had a colorful print out, but you can't run that through an assembler.
There were commercially-available disassemblers that generated source code and removed some of the tedium from the process, and for many tasks they solved the problem nicely. What I really wanted, though, looked more like a modern IDE, because I didn't just want it to translate machine code into readable form. I wanted it to help me with the process of understanding the code, by providing cross-reference tables and symbol lists and giving me a place to scribble notes to myself while I worked. I especially wanted the note-scribbling, because learning how something works is usually an iterative process, where the function of a chunk of code gradually reveals itself over time.
In 2002, while writing the 6502/65816 disassembler for CiderPress, I ran into the same problems I had with the original Apple II monitor: it blundered through data sections and got lost briefly when a new code section started. You had to pick long or short registers for the entire diassembly, which made 65816 code something of a disaster. I jotted down some notes on what I thought the core features of a good 6502 disassembler should be, then moved on to work on other features. It was another 15 years before I picked up the idea again.
More recently, I disassembled some code by dumping it to a text file with CiderPress and then fiddling with it in a text editor. I could leave free-form notes, but when I found some code that I wanted to exercise a bit I realized that getting it into an assembler was going to take some effort. Raw addresses needed to be converted to labels, the address and byte dump in the left column needed to be stripped out -- really just some basic text and string replace operations, but tedious to do by hand.
The original design for SourceGen was substantially less feature-rich than the final result. I kept discovering opportunities for features that I wanted to have, or at least wanted to write. The result is something of a monument to creeping featurism. Hopefully the core features are solid enough to excuse the excesses.
-- Andy McFadden, September 2018