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-After building [chippy](https://github.com/bradford-hamilton/chippy), I wanted finding something a little more challenging. I really enjoyed it so I'm going to give an Apple 1 computer a shot.
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+After building [chippy](https://github.com/bradford-hamilton/chippy), I wanted finding something a little more challenging. I really enjoyed it so I'm going to give an Apple 1 computer a shot.
+
+Current references:
+- [6502 instruction set](https://www.masswerk.at/6502/6502_instruction_set.html)
+- [6502 memory test](http://www.willegal.net/appleii/6502mem.htm)
+- [6502 instructions description with undocumented commands](http://www.zimmers.net/anonftp/pub/cbm/documents/chipdata/64doc)
+- [a rust implementation](https://github.com/alexander-akhmetov/mos6502)
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