Did you just enter codes randomly or did you have some way to read addresses and find possible memory locations before ROMs and emulation made their way to PCs?
You could enter codes from a code book and sometimes magazines published new codes. However, you could also do some "live editing" where you could watch for a value to change based on something (lose a life, value changes) then you could try to drill down to that piece where it changed and modify it to do things like give yourself unlimited lives.
Most kids would probably have hated it but I always thought it was kind of cool that you could "make your own cheats".
I knew about the code books (subscribed and had them all delivered as soon as they were printed) and whatever ones could be found in magazines at the time.
Making my own codes: the suggested way was to write down codes that you randomly generated, enter them, play the game and look for things that had changed. How were you monitoring the memory address or whatever you were doing to be able find a code?
I am probably mixing up versions of the hardware. They made these for the Sega later on too and I had one of those too. You could hit a key combo to bring up a menu while the game was running then you could search for specific values then "tag" them to watch for changes. At least that's how I remember it working.
I still used the one with the NES but right now I can't remember if it could do the same thing or if that was just the sega one later on.
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