Read Inside the Machine: An Illustrated Introduction to Microprocessors and Computer Architecture for Free Online Page A

computer ,1 all computing devices, from the abacus to the earliest electronic computing machines, had to be manipulated by an operator or group of
    operators who manually entered a particular sequence of commands each
    time they wanted to make a particular calculation. In contrast, modern com-
    puters store and reuse such command sequences, and as such they have a
    level of flexibility and usefulness that sets them apart from everything that
    has come before. In the rest of this chapter, you’ll get a first-hand look at the many ways that the stored-program concept affects the design and capabilities of the modern computer.
    Refining the File-Clerk Model
    Let’s take a closer look at the relationship between the code, data, and
    results streams by means of a quick example. In this example, the code
    stream consists of a single instruction, an add, which tells the ALU to add
    two numbers together.
    1 In 1944 J. Presper Eckert, John Mauchly, and John von Neumann proposed the first stored-program computer, the EDVAC (Electronic Discrete Variable Automatic Computer), and in 1949 such a machine, the EDSAC, was built by Maurice Wilkes of Cambridge University.
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    Chapter 1
    The add instruction travels from code storage to the ALU. For now, let’s
    not concern ourselves with how the instruction gets from code storage to
    the ALU; let’s just assume that it shows up at the ALU’s code input port
    announcing that there is an addition to be carried out immediately. The
    ALU goes through the following sequence of steps:
    1.
    Obtain the two numbers to be added (the input operands) from data
    storage.
    2.
    Add the numbers.
    3.
    Place the results back into data storage.
    The preceding example probably sounds simple, but it conveys the basic
    manner in which computers— all computers—operate. Computers are fed
    a sequence of instructions one by one, and in order to execute them, the
    computer must first obtain the necessary data, then perform the calculation
    specified by the instruction, and finally write the result into a place where the end user can find it. Those three steps are carried out billions of times per
    second on a modern CPU, again and again and again. It’s only because the
    computer executes these steps so rapidly that it’s able to present the illusion
    that something much more conceptually complex is going on.
    To return to our file-clerk analogy, a computer is like a file clerk who
    sits at his desk all day waiting for messages from his boss. Eventually, the
    boss sends him a message telling him to perform a calculation on a pair of
    numbers. The message tells him which calculation to perform, and where in
    his personal filing cabinet the necessary numbers are located. So the clerk
    first retrieves the numbers from his filing cabinet, then performs the calcula-
    tion, and finally places the results back into the filing cabinet. It’s a boring, mindless, repetitive task that’s repeated endlessly, day in and day out, which
    is precisely why we’ve invented a machine that can do it efficiently and not
    complain.
    The Register File
    Since numbers must first be fetched from storage before they can be added,
    we want our data storage space to be as fast as possible so that the operation
    can be carried out quickly. Since the ALU is the part of the processor that
    does the actual addition, we’d like to place the data storage as close as
    possible to the ALU so it can read the operands almost instantaneously.
    However, practical considerations, such as a CPU’s limited surface area,
    constrain the size of the storage area that we can stick next to the ALU. This
    means that in real life, most computers have a relatively small number of very
    fast data storage locations attached to the ALU. These storage locations are
    called registers , and the first x 86 computers only had eight of them to work with. These registers, which are arrayed in a storage structure called a register file , store only a small subset of