24 lines
2.6 KiB
Markdown
24 lines
2.6 KiB
Markdown
Disassembly and analysis of SubLOGIC's Flight Simulator II (FS2) for the Apple II platform, circa 1984.
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Two halves:
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1. **Disassembly** of the original 6502 binary under `src/` (chunks `chunk2.s` .. `chunk5.s`) plus the scenery-file code overlays (`ovl25.s` edit mode, `ovl2d.s` WW1 Ace, `ovl42.s` Ctrl+E, `ovl2b.s` boot page). `make validate` enforces byte-identity against the original FS2 chunks and the overlay sections.
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2. **C port** in `port/` (SDL2). A byte-faithful C translation of the whole program (no 6502 emulation in the game) running over a 64K RAM image; it is verified tick by tick against a 6502 oracle run of the real binary (`port/tools/verify/regress.sh`). The edit-mode editor, the WW1 mode and the Ctrl+E page, which the original loads as 6502 overlays from the scenery file, are disassembled in `src/ovl*.s` and translated too. See [`port/PORT_STATUS.md`](port/PORT_STATUS.md) and [`ARCHITECTURE.md`](ARCHITECTURE.md).
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# Disassembly
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The target is @qkumba's ProDOS port of FS2, which has a custom loader which decompressess chunks of code/data into memory. The project primarily focuses on understanding those chunks, which represent the memory-resident code of the original FS2.
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The [cc65](http://cc65.github.io/cc65/) tool chain is used; source files target the ca65 macro assembler; `ca65` and `ld65` must be available. Build via `make` and use `make validate` to verify that the built chunks (`out/?_????-????`) are byte-identical to the original chunks (`orig/?_????-????`), and that the overlay sources `src/ovl*.s` rebuild the FS2.1 file sections in `orig/ovl_*.bin`.
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PRs welcome. Comments are strongly encouraged. Macro use for common patterns is also encouraged, where it helps understanding the original code.
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# Loader
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The source for @qkumba's ProDOS loader is included. To build the binary, use `make binary`. The output will be `out/fs2#062499`. This requires the following addtional tools to be available on the command line.
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* `apultra` - https://github.com/emmanuel-marty/apultra (compression)
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* `acme` - https://sourceforge.net/projects/acme-crossass/ (cross-assembler)
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Note that the binary on its own is insufficient to actually run FS2; an adjacent file representing a full image of the 140k FS2 disk is required, which includes the dynamically loaded scenery database. This file (and more) are present on the `orig/flight simulator 2 with scenery PRODOS (san inc pack).po`. If you do build the binary, you can transfer it to a copy of that disk image and `BRUN` it.
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Additionally, a `out/loader.system#ff2000` file will also be produced. This can be transferred to a disk as a SYS file and executed to load/run the `FS2` binary.
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