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all the specifications associated with it. By software, they generally mean the pro￾grams, whether operating systems like Android, ChromeOS, Linux, or Windows, or database systems like Access, MongoDB, Oracle, or DB-terrific, or applica￾tion programs like Facebook, Chrome, Excel, or Word. The implication is that the person knows a whole lot about one of these two things and precious little about the other. Usually, there is the further implication that it is OK to be an expert at one of these (hardware OR software) and clueless about the other. It is as if there were a big wall between the hardware (the computer and how it actu￾ally works) and the software (the programs that direct the computer to do their bidding), and that one should be content to remain on one side of that wall or the other. The power of abstraction allows us to “usually” operate at a level where we do not have to think about the underlying layers all the time. This is a good thing. It enables us to be more productive. But if we are clueless about the underlying layers, then we are not able to take advantage of the nuances of those underlying layers when it is very important to be able to. That is not to say that you must work at the lower level of abstraction and not take advantage of the productivity enhancements that the abstractions provide. On the contrary, you are encouraged to work at the highest level of abstraction available to you. But in doing so, if you are able to, at the same time, keep in mind the underlying levels, you will find yourself able to do a much better job. As you approach your study and practice of computing, we urge you to take the approach that hardware and software are names for components of two parts of a computing system that work best when they are designed by people who take into account the capabilities and limitations of both. Microprocessor designers who understand the needs of the programs that will execute on the microprocessor they are designing can design much more effective microprocessors than those who don’t. For example, Intel, AMD, ARM, and other major producers of microprocessors recognized a few years ago that a large fraction of future programs would contain video clips as part of e-mail, video games, and full-length movies. They recognized that it would be impor￾tant for such programs to execute efficiently. The result: most microprocessors today contain special hardware capability to process those video clips. Intel defined additional instructions, initially called their MMX instruction set, and developed special hardware for it. Motorola, IBM, and Apple did essentially the same thing, resulting in the AltiVec instruction set and special hardware to support it. A similar story can be told about software designers. The designer of a large computer program who understands the capabilities and limitations of the hard￾ware that will carry out the tasks of that program can design the program so it executes more efficiently than the designer who does not understand the nature of the hardware. One important task that almost all large software systems need to carry out is called sorting, where a number of items have to be arranged in some order. The words in a dictionary are arranged in alphabetical order. Students in a class are often graded based on a numerical order, according to their scores on the final exam. There is a large number of fundamentally different programs one can write to arrange a collection of items in order. Donald Knuth, one of the