[cdumler]

The catch about developing things back then is that you were working with duct tape and chicken wire. The 6502 CPU was developed literally by creating a hand drawn design. Each transistor laid out created additional complexity that humans had to understand. Therefore, making it as simple as possible by reducing the gates used to a minimum was important. This also meant that not all combinations inputs were valid (or event checked).

The 6502 has a few "undocumented" instructions due the fact that if you have certain on/off input pin set, you're actually crossing gates used for multiple instructions. These may crash the chip or do non-useful things, but a few do something useful in vary specific circumstances. The trick to developing on these old systems is to experiment with the chips to understand what the system does in various circumstances.

What I liked about that time was that it was possible to truly understand everything about a system because nothing is hidden from you by software drivers. The were good times.

[coldpie]

> The 6502 CPU was developed literally by creating a hand drawn design. Each transistor laid out created additional complexity that humans had to understand.

Want to understand a tiny chunk of that complexity yourself? Check out how the addition operation overflow flag is implemented in silicon on the 6502: http://www.righto.com/2013/01/a-small-part-of-6502-chip-expl...

[ubermonkey]

This reminder of undocumented behavior reminded me of The story of Mel, a Real Programmer.

See

http://www.pbm.com/~lindahl/mel.html

[soulofmischief]

I'd never seen that photo before. Mel looks a bit like Brad Pitt from far away.

[ubermonkey]

Until very recently, I assumed the story was mostly apocryphal, but I happened to look it up about a year ago and learned that the actual Mel had been identified, and that there was even that picture. Really cool, considering I first saw the story of Mel in probably 1989.