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This will be a commented disassembly of the Atari 8-bit computer
Defender cartridge.

I found 2 versions of the ROM. "defender.rom" is from here:

http://www.atarimania.com/game-atari-400-800-xl-xe-defender_1561.html

defender.8humans.rom is found in both the Holmes Archive and the TOSEC
collection (Atari_8_bit_TOSEC_2012_04_23.zip).

These are almost identical. See "notes" for differences between the two
images. It turns out that the 8 humans version is a badly done crack,
that must have been converted from a cracked XEX back into a ROM!

$ md5sum *.rom
e8d52b4aa33da38f99312f9aa8b48885  defender.8humans.rom
8709351e76ec7c407e63c47ac53f8d19  defender.rom

Disassembly is done with da65 (from the cc65 suite), using the old
"edit the da65 .info file, disassemble, name things, edit again, etc"
cycle. At some point, I'll clean up the formatting to make it look more
human-readable, and use some conditional assembly (ifdefs) to allow
assembling either version, and maybe as either a .xex or a ROM.

At any point, you can check whether the disassembly really assembles
back into the original ROM image by running "make test".