All external buses and even some internal ones group wires in terms of function. There are three majors groups of wires based on function:

data wires -- wires that carry actual data, character or numerical (either integer or floating point) address wires -- wires carrying an address, always an unsigned binary number control wires -- wires that carry signals to turn on or off various components or to control the flow of data and addresses

A good example of the control wires are the two we used to communicate with memory in previous chapters: MA and RD. MA, memory active, tells whether or not memory is being summoned to do something, while RD, read, tells whether the memory is being read or written. We also explored in an exercise an alternate way to encode these operations using two different wires. (There were three situations: memory read, memory write, and memory quiescent.) Note that in earlier chapters we used WR (write) instead of RD (read), but since they are opposite and symmetric, it really doesn't matter which wire we use.