Why do hardware and software always have to work together?

This is actually a very complex question, and would probably be difficult tofully answer with even with an entire textbook. But I think the most crutial element serving asa "bridge" between software and hardware would probably be the "register." Commands or services requested by the software are finally encoded intobinary valuesof "0"s and "1"s and are written into a "register". For example, some register may hold a value of "01101100". The above encoding has a total of 8 "bits" (one byte), where each value of either "0" or "1" is called a "bit" (and 8 bits are collectively called a "byte"). At the hardware level, the computer hardware does not see these "0"s and "1"s as numbers, but as voltage levels, for example, a high voltage of +3 volts may represent a "1" and a low voltage of near 0 volts may represent a "0" (or vice versa, depending on how the circuitry is designed). These high and low voltages are fed into a series of "logic gates" which in turn, through the correct logic design, will go about with producing the results specified by the particular software command. For example, if the software is requesting a caculation, the hardware logic gates may add the first two bits of the encoding together, produce a result "1" ("0" + "1" = "1"), and then write this result into another register somewhere for the software to subsequently go fetch and read. Or, if the encoding is a request for some kind of service, the logic gates may in turn access or write into some other registers which would in turn trigger other logic gates to inititiate this service. But again, this is a very complex process with indeed many variations depending on what kind of technologies the designers of the system choose to implement anduse.