Often, when looking at digital system signal plots over time there are these graphs with two parallel lines that often "swap" place. I am wondering what precisely they represent? I noticed they usually are used for buses, but for the rest I'm clueless. It's probably quite trivial.
This is an example of the graphs I mean, in this case from a discussion about how CPU's execute instructions:
I've tried googling, but I couldn't find an explanation.
Best Answer
The parallel lines indicate a multi-bit bus, and represent any possible combinations of low/high values. This bus could be two bits, or 64 bits, or whatever the design calls for. As shown here, the grey regions indicate that the data value is unknown or indeterminate. In this case, immediately after the positive clock transition, the address and data are unknown. Soon, after that, the address data becomes stable and known. Some time after that the data is known and can be read.
edit: As Peter says, the grey regions can also mean "don't care".