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

I’ve presented here, but the general outlines (and even a decent number of the specifics) are the same.
    The Clock
    Steps 1 through 12 in the previous section don’t take an arbitrary amount of
    time to complete. Rather, they’re performed according to the pulse of the
    clock that governs every action the processor takes.
    This clock pulse, which is generated by a clock generator module on the
    motherboard and is fed into the processor from the outside, times the func-
    tioning of the processor so that, on the DLW-1 at least, all three steps of the
    fetch-execute loop are completed in exactly one beat of the clock. Thus, the
    The Mechanics of Program Execution
    29
    program in Figure 2-9, as I’ve traced its execution in the preceding section,
    takes exactly four clock beats to finish execution, because a new instruction is fetched on each beat of the clock.
    One obvious way to speed up the execution of programs on the DLW-1
    would be to speed up its clock generator so that each step takes less time to
    complete. This is generally true of all microprocessors, hence the race among
    microprocessor designers to build and market chips with ever-higher clock
    speeds. (We’ll talk more about the relationship between clock speed and
    performance in Chapter 3.)
    Branch Instructions
    As I’ve presented it so far, the processor moves through each line in a pro-
    gram in sequence until it reaches the end of the program, at which point the
    program’s output is available to the user.
    There are certain instructions in the instruction stream, however, that
    allow the processor to jump to a program line that is out of sequence. For
    instance, by inserting a branch instruction into line 5 of a program, we could cause the processor’s control unit to jump all the way down to line 20 and
    begin executing there (a forward branch ), or we could cause it to jump back up to line 1 (a backward branch ). Because a program is an ordered sequence of instructions, by including forward and backward branch instructions, we
    can arbitrarily move about in the program. This is a powerful ability, and
    branches are an essential part of computing.
    Rather than thinking about forward or backward branches, it’s more
    useful for our present purposes to categorize all branches as being one of the

following two types: conditional branches or unconditional branches.
    Unconditional Branch
    An unconditional branch instruction consists of two parts: the branch instruction and the target address.
    jump #target
    For an unconditional branch, #target can be either an immediate value,
    like #12, or an address stored in a register, like #D.
    Unconditional branches are fairly easy to execute, since all that the com-
    puter needs to do upon decoding such a branch in the instruction register is
    to have the control unit replace the address currently in the program counter
    with branch’s target address. Then the next time the processor goes to fetch
    the instruction at the address given by the program counter, it’ll fetch the
    address at the branch target instead.
    Conditional Branch
    Though it has the same basic instruction format as the unconditional
    branch (instruction #target), the conditional branch instruction is a
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    Chapter 2
    little more complicated, because it involves jumping to the target
    address only if a certain condition is met.
    For example, say we want to jump to a new line of the program only if
    the previous arithmetic instruction’s result is zero; if the result is nonzero,
    we want to continue executing normally. We would use a conditional
    branch instruction that first checks to see if the previously executed
    arithmetic instruction yielded a zero result, and then writes the branch
    target into the program counter if it did.
    Because of such conditional jumps, we need a special register or set
    of registers in which to store information about the results of arithmetic
    instructions—information such as whether the previous result was