Inside the Machine: An Illustrated Introduction to Microprocessors and Computer Architecture by jon stokes Page A

you’ve been around computers for any length of time, you’ve heard the
    terms reboot or boot up used in connection with either resetting the computer to its initial state or powering it on initially. The term boot is a shortened
    version of the term bootstrap, which is itself a reference to the seemingly
    impossible task a computer must perform on start-up, namely, “pulling itself
    up by its own bootstraps.”
    I say “seemingly impossible,” because when a computer is first powered
    on there is no program in memory, but programs contain the instructions
    that make the computer run. If the processor has no program running when
    it’s first powered on, then how does it know where to fetch the first instruc-
    tion from?
    The solution to this dilemma is that the microprocessor, in its power-on
    default state, is hard-wired to fetch that first instruction from a predetermined address in memory. This first instruction, which is loaded into the processor’s
    instruction register, is the first line of a program called the BIOS that lives in a special set of storage locations—a small read-only memory (ROM) module
    attached to the computer’s motherboard. It’s the job of the BIOS to perform
    basic tests of the RAM and peripherals in order to verify that everything is
    working properly. Then the boot process can continue.
    At the end of the BIOS program lies a jump instruction, the target of
    which is the location of a bootloader program. By using a jump, the BIOS
    hands off control of the system to this second program, whose job it is to
    search for and load the computer’s operating system from the hard disk.
    The operating system (OS) loads and unloads all of the other programs
    that run on the computer, so once the OS is up and running the computer
    is ready to interact with the user.
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    Chapter 2
    P I P E L I N E D E X E C U T I O N
    All of the processor architectures that you’ve looked at
    so far are relatively simple, and they reflect the earliest
    stages of computer evolution. This chapter will bring
    you closer to the modern computing era by introducing
    one of the key innovations that underlies the rapid
    performance increases that have characterized the past
    few decades of microprocessor development: pipelined
    execution .
    Pipelined execution is a technique that enables microprocessor designers
    to increase the speed at which a processor operates, thereby decreasing the
    amount of time that the processor takes to execute a program. This chapter
    will first introduce the concept of pipelining by means of a factory analogy,
    and it will then apply the analogy to microprocessors. You’ll then learn how
    to evaluate the benefits of pipelining, before I conclude with a discussion of
    the technique’s limitations and costs.
    NOTE
    This chapter’s discussion of pipelined execution focuses solely on the execution of arithmetic instructions. Memory instructions and branch instructions are pipelined using the same fundamental principles as arithmetic instructions, and later chapters will cover the peculiarities of the actual execution process of each of these two types of instruction.
    The Lifecycle of an Instruction
    In the previous chapter, you learned that a computer repeats three basic
    steps over and over again in order to execute a program:
    1.
    Fetch the next instruction from the address stored in the program
    counter and load that instruction into the instruction register.
    Increment the program counter.
    2.
    Decode the instruction in the instruction register.
    3.
    Execute the instruction in the instruction register.
    You should also recall that step 3, the execute step, itself can consist of
    multiple sub-steps, depending on the type of instruction being executed
    (arithmetic, memory access, or branch). In the case of the arithmetic
    instruction add A, B, C, the example we used last time, the three sub-steps
    are as follows:
    1.
    Read the contents of registers A and B.
    2.
    Add the contents of A and