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87 lines
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87 lines
4.1 KiB
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<title>End notes - 6502bench SourceGen</title>
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<h1>6502bench SourceGen: End Notes</h1>
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<p><a href="index.html">Back to index</a></p>
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<h2><a name="origins">Origins</a></h2>
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<p>The inspiration for SourceGen goes a long way back. While in high
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school in the late 1980s, I read Don Lancaster's
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<i>Enhancing Your Apple II, Vol. 1</i> (available for download
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<a href="https://www.tinaja.com/ebksamp1.shtml">here</a>). This
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included a very detailed methodology for disassembling 6502 software
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(nicely reformatted
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<a href="https://www.tinaja.com/ebooks/tearing_rework.pdf">here</a>).
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I wanted to give it a try, so I generated a monitor listing of an
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operating system (called "RDOS") that SSI used on their games, and
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printed it out on my Epson RX-80 -- tractor feed paper was helpful for
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this sort of thing -- then set to work.</p>
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<p>Lancaster's methodology involved highlighting different types of
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instructions with different colors, making notes, and adding labels.
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All this being done with felt-tip and colored highlighter pens. The
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process worked remarkably well: by the time I was finished marking
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things up, I knew how everything in the code worked.</p>
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<p>I really wanted a better system though. The disassembler built into
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the Apple II could get out of sync when it walked through a data area,
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so sometimes you had to hand-write in the correct instruction. Applying
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a label to every place that referenced it was tedious. When you got to
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the end, you had a colorful print out, but you can't run that through
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an assembler.</p>
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<p>There were commercially-available disassemblers that generated source
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code and removed some of the tedium from the process, and for many tasks
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they solved the problem nicely. What I really wanted, though, looked more
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like a modern IDE, because I didn't just want it to translate machine code
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into readable form. I wanted it to help me with the process of
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understanding the code, by providing cross-reference tables and symbol
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lists and giving me a place to scribble notes to myself while I worked.
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I especially wanted the note-scribbling, because learning how something
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works is usually an iterative process, where the function of a chunk of
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code gradually reveals itself over time.</p>
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<p>In 2002, while writing the 6502/65816 disassembler for CiderPress, I
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ran into the same problems I had with the original Apple II monitor: it
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blundered through data sections and got lost briefly when a new code
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section started. You had to pick long or short registers for the entire
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diassembly, which made 65816 code something of a disaster. I
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jotted down some notes on what I thought the core features of a good
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6502 disassembler should be, then moved on to work on other features. It
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was another 15 years before I picked up the idea again.</p>
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<p>More recently, I disassembled some code by dumping it to a text
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file with CiderPress and then fiddling with it in a text editor. I could
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leave free-form notes, but when I found some code that I wanted to
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exercise a bit I realized that getting it into an assembler was going
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to take some effort. Raw addresses needed to be converted to labels,
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the address and byte dump in the left column needed to be stripped out --
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really just some basic text and string replace operations, but tedious
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to do by hand.</p>
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<p>The original design for SourceGen was substantially less feature-rich
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than the final result. I kept discovering opportunities for features
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that I wanted to have, or at least wanted to write. The result is
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something of a monument to creeping featurism. Hopefully the core features
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are solid enough to excuse the excesses.</p>
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<p>-- Andy McFadden, September 2018</p>
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