[ Team LiB ] Previous Section Next Section


Every modern microprocessor starts with the basics—clocked-logic digital circuitry. The chip has millions of separate gates combined into three basic function blocks: the input/output unit (or I/O unit), the control unit, and the arithmetic/logic unit (ALU). The last two are sometimes jointly called the central processingunit (CPU), although the same term often is used as a synonym for the entire microprocessor. Some chipmakers further subdivide these units, give them other names, or include more than one of each in a particular microprocessor. In any case, the functions of these three units are an inherent part of any chip. The differences are mostly a matter of nomenclature, because you can understand the entire operation of any microprocessor as a product of these three functions.

All three parts of the microprocessor interact together. In all but the simplest microprocessor designs, the I/O unit is under the control of the control unit, and the operation of the control unit may be determined by the results of calculations of the arithmetic/logic unit CPU. The combination of the three parts determines the power and performance of the microprocessor.

Each part of the microprocessor also has its own effect on the processing speed of the system. The control unit operates the microprocessor's internal clock, which determines the rate at which the chip operates. The I/O unit determines the bus width of the microprocessor, which influences how quickly data and instructions can be moved in and out of the microprocessor. And the registers in the arithmetic/control unit determine how much data the microprocessor can operate on at one time.

Input/Output Unit

The input/output unit links the microprocessor to the rest of the circuitry of the computer, passing along program instructions and data to the registers of the control unit and arithmetic/logic unit. The I/O unit matches the signal levels and timing of the microprocessor's internal solid-state circuitry to the requirements of the other components inside the computer. The internal circuits of a microprocessor, for example, are designed to be stingy with electricity so that they can operate faster and cooler. These delicate internal circuits cannot handle the higher currents needed to link to external components. Consequently, each signal leaving the microprocessor goes through a signal buffer in the I/O unit that boosts its current capacity.

The input/output unit can be as simple as a few buffers, or it may involve many complex functions. In the latest Intel microprocessors used in some of the most powerful computers, the I/O unit includes cache memory and clock-doubling or -tripling logic to match the high operating speed of the microprocessor to slower external memory.

The microprocessors used in computers have two kinds of external connections to their input/output units: those connections that indicate the address of memory locations to or from which the microprocessor will send or receive data or instructions, and those connections that convey the meaning of the data or instructions. The former is called the address bus of the microprocessor; the latter, the data bus.

The number of bits in the data bus of a microprocessor directly influences how quickly it can move information. The more bits that a chip can use at a time, the faster it is. The first microprocessors had data buses only four bits wide. Pentium chips use a 32-bit data bus, as do the related Athlon, Celeron, and Duron chips. Itanium and Opteron chips have 64-bit data buses.

The number of bits available on the address bus influences how much memory a microprocessor can address. A microprocessor with 16 address lines, for example, can directly work with 216 addresses; that's 65,536 (or 64K) different memory locations. The different microprocessors used in various computers span a range of address bus widths from 32 to 64 or more bits.

The range of bit addresses used by a microprocessor and the physical number of address lines of the chip no longer correspond. That's because people and microprocessors look at memory differently. Although people tend to think of memory in terms of bytes, each comprising eight bits, microprocessors now deal in larger chunks of data, corresponding to the number of bits in their data buses. For example, a Pentium chip chews into data 32 bits at a time, so it doesn't need to look to individual bytes. It swallows them four at a time. Chipmakers consequently omit the address lines needed to distinguish chunks of memory smaller than their data buses. This bit of frugality saves the number of connections the chip needs to make with the computer's circuitry, an issue that becomes important once you see (as you will later) that the modern microprocessor requires several hundred external connections—each prone to failure.

Control Unit

The control unit of a microprocessor is a clocked logic circuit that, as its name implies, controls the operation of the entire chip. Unlike more common integrated circuits, whose function is fixed by hardware design, the control unit is more flexible. The control unit follows the instructions contained in an external program and tells the arithmetic/logic unit what to do. The control unit receives instructions from the I/O unit, translates them into a form that can be understood by the arithmetic/logic unit, and keeps track of which step of the program is being executed.

With the increasing complexity of microprocessors, the control unit has become more sophisticated. In the basic Pentium, for example, the control unit must decide how to route signals between what amounts to two separate processing units called pipelines. In other advanced microprocessors, the function of the control unit is split among other functional blocks, such as those that specialize in evaluating and handling branches in the stream of instructions.

Arithmetic/Logic Unit

The arithmetic/logic unit handles all the decision-making operations (the mathematical computations and logic functions) performed by the microprocessor. The unit takes the instructions decoded by the control unit and either carries them out directly or executes the appropriate microcode (see the section titled "Microcode" later in this chapter) to modify the data contained in its registers. The results are passed back out of the microprocessor through the I/O unit.

The first microprocessors had but one ALU. Modern chips may have several, which commonly are classed into two types. The basic form is the integer unit, one that carries out only the simplest mathematical operations. More powerful microprocessors also include one or more floating-point units, which handle advanced math operations (such as trigonometric and transcendental functions), typically at greater precision.

Floating-Point Unit

Although functionally a floating-point unit is part of the arithmetic/logic unit, engineers often discuss it separately because the floating-point unit is designed to process only floating-point numbers and not to take care of ordinary math or logic operations.

Floating-point describes a way of expressing values, not a mathematically defined type of number such as an integer, rational, or real number. The essence of a floating-point number is that its decimal point "floats" between a predefined number of significant digits rather than being fixed in place the way dollar values always have two decimal places.

Mathematically speaking, a floating-point number has three parts: a sign, which indicates whether the number is greater or less than zero; a significant (sometimes called a mantissa), which comprises all the digits that are mathematically meaningful; and an exponent, which determines the order of magnitude of the significant (essentially the location to which the decimal point floats). Think of a floating-point number as being like those represented by scientific notation. But whereas scientists are apt to deal in base-10 (the exponents in scientific notation are powers of 10), floating-point units think of numbers digitally in base-2 (all ones and zeros in powers of two).

As a practical matter, the form of floating-point numbers used in computer calculations follows standards laid down by the Institute of Electrical and Electronic Engineers (IEEE). The IEEE formats take values that can be represented in binary form using 80 bits. Although 80 bits seems somewhat arbitrary in a computer world that's based on powers of two and a steady doubling of register size from 8 to 16 to 32 to 64 bits, it's exactly the right size to accommodate 64 bits of the significant, with 15 bits leftover to hold an exponent value and an extra bit for the sign of the number held in the register. Although the IEEE standard allows for 32-bit and 64-bit floating-point values, most floating-point units are designed to accommodate the full 80-bit values. The floating-point unit (FPU) carries out all its calculations using the full 80 bits of the chip's registers, unlike the integer unit, which can independently manipulate its registers in byte-wide pieces.

The floating-point units of Intel-architecture processors have eight of these 80-bit registers in which to perform their calculations. Instructions in your programs tell the microprocessor whether to use its ordinary integer ALU or its floating-point unit to carry out a mathematical operation. The different instructions are important because the eight 80-bit registers in Intel floating-point units also differ from integer units in the way they are addressed. Commands for integer unit registers are directly routed to the appropriate register as if sent by a switchboard. Floating-point unit registers are arranged in a stack, sort of an elevator system. Values are pushed onto the stack, and with each new number the old one goes down one level. Stack machines are generally regarded as lean and mean computers. Their design is austere and streamlined, which helps them run more quickly. The same holds true for stack-oriented floating-point units.

Until the advent of the Pentium, a floating-point unit was not a guaranteed part of a microprocessor. Some 486 and all previous chips omitted floating-point circuitry. The floating-point circuitry simply added too much to the complexity of the chip, at least for the state of fabrication technology at that time. To cut costs, chipmakers simply left the floating-point unit as an option.

When it was necessary to accelerate numeric operations, the earliest microprocessors used in computers allowed you to add an additional, optional chip to your computer to accelerate the calculation of floating-point values. These external floating-point units were termed math coprocessors.

The floating-point units of modern microprocessors have evolved beyond mere number-crunching. They have been optimized to reflect the applications for which computers most often crunch floating-point numbers—graphics and multimedia (calculating dots, shapes, colors, depth, and action on your screen display).

Instruction Sets

Instructions are the basic units for telling a microprocessor what to do. Internally, the circuitry of the microprocessor has to carry out hundreds, thousands, or even millions of logic operations to carry out one instruction. The instruction, in effect, triggers a cascade of logical operations. How this cascade is controlled marks the great divide in microprocessor and computer design.

The first electronic computers used a hard-wired design. An instruction simply activated the circuits appropriate for carrying out all the steps required. This design has its advantages. It optimizes the speed of the system because the direct hard-wire connection adds nothing to slow down the system. Simplicity means speed, and the hard-wired approach is the simplest. Moreover, the hard-wired design was the practical and obvious choice. After all, computers were so new that no one had thought up any alternative.

However, the hard-wired computer design has a significant drawback. It ties the hardware and software together into a single unit. Any change in the hardware must be reflected in the software. A modification to the computer means that programs have to be modified. A new computer design may require that programs be entirely rewritten from the ground up.


The inspiration for breaking away from the hard-wired approach was the need for flexibility in instruction sets. Throughout most of the history of computing, determining exactly what instructions should make up a machine's instruction set was more an art than a science. IBM's first commercial computers, the 701 and 702, were designed more from intuition than from any study of which instructions programmers would need to use. Each machine was tailored to a specific application. The 701 ran instructions thought to serve scientific users; the 702 had instructions aimed at business and commercial applications.

When IBM tried to unite its many application-specific computers into a single, more general-purpose line, these instruction sets were combined so that one machine could satisfy all needs. The result was, of course, a wide, varied, and complex set of instructions. The new machine, the IBM 360 (introduced in 1964), was unlike previous computers in that it was created not as hardware but as an architecture. IBM developed specifications and rules for how the machine would operate but enabled the actual machine to be created from any hardware implementation designers found most expedient. In other words, IBM defined the instructions that the 360 would use but not the circuitry that would carry them out. Previous computers used instructions that directly controlled the underlying hardware. To adapt the instructions defined by the architecture to the actual hardware that made up the machine, IBM adopted an idea called microcode, originally conceived by Maurice Wilkes at Cambridge University.

In the microcode design, an instruction causes a computer to execute a small program to carry out the logic instructions required by the instruction. The collection of small programs for all the instructions the computer understands is its microcode.

Although the additional layer of microcode made machines more complex, it added a great deal of design flexibility. Engineers could incorporate whatever new technologies they wanted inside the computer, yet still run the same software with the same instructions originally written for older designs. In other words, microcode enabled new hardware designs and computer systems to have backward compatibility with earlier machines.

After the introduction of the IBM 360, nearly all mainframe computers used microcode. When the microprocessors came along, they followed the same design philosophy, using microcode to match instructions to hardware. Using this design, a microprocessor actually has a smaller microprocessor inside it, which is sometimes called a nanoprocessor, running the microcode.

This microcode-and-nanoprocessor approach makes creating a complex microprocessor easier. The powerful data-processing circuitry of the chip can be designed independently of the instructions it must carry out. The manner in which the chip handles its complex instructions can be fine-tuned even after the architecture of the main circuits are laid into place. Bugs in the design can be fixed relatively quickly by altering the microcode, which is an easy operation compared to the alternative of developing a new design for the whole chip (a task that's not trivial when millions of transistors are involved). The rich instruction set fostered by microcode also makes writing software for the microprocessor (and computers built from it) easier, thus reducing the number of instructions needed for each operation.

Microcode has a big disadvantage, however. It makes computers and microprocessors more complicated. In a microprocessor, the nanoprocessor must go through several of its own microcode instructions to carry out every instruction you send to the microprocessor. More steps means more processing time taken for each instruction. Extra processing time means slower operation. Engineers found that microcode had its own way to compensate for its performance penalty—complex instructions.

Using microcode, computer designers could easily give an architecture a rich repertoire of instructions that carry out elaborate functions. A single, complex instruction might do the job of half a dozen or more simpler instructions. Although each instruction would take longer to execute because of the microcode, programs would need fewer instructions overall. Moreover, adding more instructions could boost speed. One result of this micro code "more is merrier" instruction approach is that typical computer microprocessors have seven different subtraction commands.


Although long the mainstay of computer and microprocessor design, microcode is not necessary. While system architects were staying up nights concocting ever more powerful and obscure instructions, a counter force was gathering. Starting in the 1970s, the micro code approach came under attack by researchers who claimed it takes a greater toll on performance than its benefits justify.

By eliminating microcode, this design camp believed, simpler instructions could be executed at speeds so much higher that no degree of instruction complexity could compensate. By necessity, such hard-wired machines would offer only a few instructions because the complexity of their hard-wired circuitry would increase dramatically with every additional instruction added. Practical designs are best made with small instruction sets.

John Cocke at IBM's Yorktown Research Laboratory analyzed the usage of instructions by computers and discovered that most of the work done by computers involves relatively few instructions. Given a computer with a set of 200 instructions, for example, two-thirds of its processing involves using as few as 10 of the total instructions. Cocke went on to design a computer that was based on a few instructions that could be executed quickly. He is credited with inventing the Reduced Instruction Set Computer (RISC) in 1974. The term RISC itself is credited to David Peterson, who used it in a course in microprocessor design at the University of California at Berkeley in 1980.

The first chip to bear the label and to take advantage of Cocke's discoveries was RISC-I, a laboratory design that was completed in 1982. To distinguish this new design approach from traditional microprocessors, microcode-based systems with large instruction sets have come to be known as Complex Instruction Set Computers (CISC).

Cocke's research showed that most of the computing was done by basic instructions, not by the more powerful, complex, and specialized instructions. Further research at Berkeley and Stanford Universities demonstrated that there were even instances in which a sequence of simple instructions could perform a complex task faster than a single complex instruction could. The result of this research is often summarized as the 80/20 Rule, meaning that about 20 percent of a computer's instructions do about 80 percent of the work. The aim of the RISC design is to optimize a computer's performance for that 20 percent of instructions, speeding up their execution as much as possible. The remaining 80 percent of the commands could be duplicated, when necessary, by combinations of the quick 20 percent. Analysis and practical experience has shown that the 20 percent could be made so much faster that the overhead required to emulate the remaining 80 percent was no handicap at all.

To enable a microprocessor to carry out all the required functions with a handful of instructions requires a rethinking of the programming process. Instead of simply translating human instructions into machine-readable form, the compilers used by RISC processors attempt to find the optimum instructions to use. The compiler takes a more in-depth look at the requested operations and finds the best way to handle them. The result was the creation of optimizing compilers discussed in Chapter 3, "Software."

If effect, the RISC design shifts a lot of the processing from the microprocessor to the compiler—a lot of the work in running a program gets taken care of before the program actually runs. Of course, the compiler does more work and takes longer to run, but that's a fair tradeoff—a program needs to be compiled only once but runs many, many times when the streamlined execution really pays off.

RISC microprocessors have several distinguishing characteristics. Most instructions execute in a single clock cycle—or even faster with advanced microprocessor designs with several execution pathways. All the instructions are the same length with similar syntax. The processor itself does not use microcode; instead, the small repertory of instructions is hard-wired into the chip. RISC instructions operate only on data in the registers of the chip, not in memory, making what is called a load-store design. The design of the chip itself is relatively simple, with comparatively few logic gates that are themselves constructed from simple, almost cookie-cutter designs. And most of the hard work is shifted from the microprocessor itself to the compiler.


Both CISC and RISC have a compelling design rationale and performance, desirable enough that engineers working on one kind of chip often looked over the shoulders of those working in the other camp. As a result, they developed hybrid chips embodying elements of both the CISC and RISC design. All the latest processors—from the Pentium Pro to the Pentium 4, Athlon, and Duron as well—have RISC cores mated with complex instruction sets.

The basic technique involves converting the classic Intel instructions into RISC-style instructions to be processed by the internal chip circuitry. Intel calls the internal RISC-like instructions micro-ops. The term is often abbreviated as uops (strictly speaking, the initial u should be the Greek letter mu, which is an abbreviation for micro) and pronounced you-ops. Other companies use slightly different terminology.

By design, the micro-ops sidestep the primary shortcomings of the Intel instruction set by making the encoding of all commands more uniform, converting all instructions to the same length for processing, and eliminating arithmetic operations that directly change memory by loading memory data into registers before processing.

The translation to RISC-like instructions allows the microprocessor to function internally as a RISC engine. The code conversion occurs in hardware, completely invisible to your applications and out of the control of programmers. In other words, it shifts back from the RISC shift to doing the work in the compiler. There's a good reason for this backward shift: It lets the RISC code deal with existing programs—those compiled before the RISC designs were created.

Single Instruction, Multiple Data

In a quest to improve the performance of Intel microprocessors on common multimedia tasks, Intel's hardware and software engineers analyzed the operations multimedia programs most often required. They then sought the most efficient way to enable their chips to carry out these operations. They essentially worked to enhance the signal-processing capabilities of their general-purpose microprocessors so that they would be competitive with dedicated processors, such as digital signal processor (DSP) chips. They called the technology they developed Single Instruction, Multiple Data (SIMD). In effect a new class of microprocessor instructions, SIMD is the enabling element of Intel's MultiMedia Extensions (MMX) to its microprocessor command set. Intel further developed this technology to add its Streaming SIMD Extensions (SSE, once known as the Katmai New Instructions) to its Pentium III microprocessors to enhance their 3D processing power. The Pentium 4 further enhances SSE with more multimedia instructions to create what Intel calls SSE2.

As the name implies, SIMD allows one microprocessor instruction to operate across several bytes or words (or even larger blocks of data). In the MMX scheme of things, the SIMD instructions are matched to the 64-bit data buses of Intel's Pentium and newer microprocessors. All data, whether it originates as bytes, words, or 16-bit double-words, gets packed into 64-bit form. Eight bytes, four words, or two double-words get packed into a single 64-bit package that, in turn, gets loaded into a 64-bit register in the microprocessor. One microprocessor instruction then manipulates the entire 64-bit block.

Although the approach at first appears counterintuitive, it improves the handling of common graphic and audio data. In video processor applications, for example, it can trim the number of microprocessor clock cycles for some operations by 50 percent or more.

Very Long Instruction Words

Just as RISC started flowing into the product mainstream, a new idea started designers thinking in the opposite direction. Very long instruction word (VLIW) technology at first appears to run against the RISC stream by using long, complex instructions. In reality, VLIW is a refinement of RISC meant to better take advantage of superscalar microprocessors. Each very long instruction word is made from several RISC instructions. In a typical implementation, eight 32-bit RISC instructions combine to make one instruction word.

Ordinarily, combining RISC instructions would add little to overall speed. As with RISC, the secret of VLIW technology is in the software—the compiler that produces the final program code. The instructions in the long word are chosen so that they execute at the same time (or as close to it as possible) in parallel processing units in the superscalar microprocessor. The compiler chooses and arranges instructions to match the needs of the superscalar processor as best as possible, essentially taking the optimizing compiler one step further. In essence, the VLIW system takes advantage of preprocessing in the compiler to make the final code and microprocessor more efficient.

VLIW technology also takes advantage of the wider bus connections of the latest generation of microprocessors. Existing chips link to their support circuitry with 64-bit buses. Many have 128-bit internal buses. The 256-bit very long instruction words push a little further yet enable a microprocessor to load several cycles of work in a single memory cycle. Transmeta's Crusoe processor uses VLIW technology.

    [ Team LiB ] Previous Section Next Section