Fundamentals of Computer Design

Fundamentals of Computer Design

Computer technology has made incredible progress in the roughly from last 55 years. This rapid rate of improvement has come both from advances in the technology used to build computers and from innovation in computer design. During the first 25 years of electronic computers, both forces made a major contribution; but beginning in about 1970, computer designers became largely dependent upon integrated circuit technology.

During the 1970s, performance continued to improve at about 25% to 30% per year for the mainframes and minicomputers that dominated the industry.

The late 1970s after invention of microprocessor the growth roughly increased 35% per year in performance. This growth rate, combined with the cost advantages of a mass-produced microprocessor, led to an increasing fraction of the computer business. In addition, two significant changes are observed in computer industry.

•   First, the virtual elimination of assembly language programming reduced the need for object-code compatibility.

•  Second, the creation of standardized, vendor-independent operating systems, such as UNIX and its clone, Linux, lowered the cost and risk of bringing out a new architecture.

These changes made it possible to successfully develop a new set of architectures, called RISC (Reduced Instruction Set Computer) architectures. In the early 1980s. The RISC-based machines focused the attention of designers on two critical performance techniques, the exploitation of instruction-level parallelism and the use of caches. The combination of architectural and organizational enhancements has led to 20 years of sustained growth in performance at an annual rate of over 50%. Figure 1.1 shows the effect of this difference in performance growth rates.

The effect of this dramatic growth rate has been twofold.

•   First, it has significantly enhanced the capability available to computer users. For many applications, the highest performance microprocessors of today outperform the supercomputer of less than 10 years ago.

• Second, this dramatic rate of improvement has led to the dominance of microprocessor-based computers across the entire range of the computer design.

1 Technology Trends

The changes in the computer applications space over the last decade have dramatically changed the metrics. Desktop computers remain focused on optimizing cost-performance as measured by a single user, servers focus on availability, scalability, and throughput cost-performance, and embedded computers are driven by price and often power issues.

If an instruction set architecture is to be successful, it must be designed to survive rapid changes in computer technology. An architect must plan for technology changes that can increase the lifetime of a computer.

The following Four implementation technologies changed the computer industry:

Integrated circuit logic technology

Transistor density increases by about 35% per year, and die size increases 10% to 20% per year. The combined effect is a growth rate in transistor count on a chip of about 55% per year.

Semiconductor DRAM:

Density increases by between 40% and 60% per year and Cycle time has improved very slowly, decreasing by about one-third in 10 years. Bandwidth per chip increases about twice as fast as latency decreases. In addition, changes to the DRAM interface have also improved the bandwidth.

Magnetic disk technology:

it is improving more than 100% per year. Prior to 1990, density increased by about 30% per year, doubling in three years. It appears that disk technology will continue the faster density growth rate for some time to come. Access time has improved by one-third in 10 years.

Network technology:

Network performance depends both on the performance of switches and on the performance of the transmission system, both latency and bandwidth can be improved, though recently bandwidth has been the primary focus. For many years, networking technology appeared to improve slowly: for example, it took about 10 years for Ethernet technology to move from 10 Mb to 100 Mb. The increased importance of networking has led to a faster rate of progress with 1 Gb Ethernet becoming available about five years after 100 Mb.

These rapidly changing technologies impact the design of a microprocessor that may, with speed and technology enhancements, have a lifetime of five or more years.

Scaling of Transistor Performance, Wires, and Power in Integrated Circuits

Integrated circuit processes are characterized by the feature size, which is decreased from 10 microns in 1971 to 0.18 microns in 2001. Since a transistor is a 2-dimensional object, the density of transistors increases quadratically with a linear decrease in feature size. The increase in transistor performance, this combination of scaling factors leads to a complex interrelationship between transistor performance and process feature size.

First approximation, transistor performance improves linearly with decreasing feature size. In the early days of microprocessors, the higher rate of improvement in density was used to quickly move from 4-bit, to 8bit, to 16-bit, to 32-bit microprocessors. More recently, density improvements have supported the introduction of 64-bit microprocessors as well as many of the innovations in pipelining and caches.

The signal delay for a wire increases in proportion to the product of its resistance and capacitance. As feature size shrinks wires get shorter, but the resistance and capacitance per unit length gets worse. Since both resistance and capacitance depend on detailed aspects of the process, the geometry of a wire, the loading on a wire, and even the adjacency to other structures. In the past few years, wire delay has become a major design limitation for large integrated circuits and is often more critical than transistor switching delay. Larger and larger fractions of the clock cycle have been consumed by the propagation delay of signals on wires. In 2001, the Pentium 4 broke new ground by allocating two stages of its 20+ stage pipeline just for propagating signals across the chip.

Power also provides challenges as devices are scaled. For modern CMOS microprocessors, the dominant energy consumption is in switching transistors. The energy required per transistor is proportional to the product of the load capacitance of the transistor, the frequency of switching, and the square of the voltage. As we move from one process to the next, the increase in the number of transistors switching and the frequency with which they switch, dominates the decrease in load capacitance and voltage, leading to an overall growth in power consumption.

2 Cost, Price and their Trends

In the past 15 years, the use of technology improvements to achieve lower cost, as well as increased performance, has been a major theme in the computer industry.

• Price is what you sell a finished good for,

• Cost is the amount spent to produce it, including overhead.

The Impact of Time, Volume, Commodification, and Packaging

The cost of a manufactured computer component decreases over time even without major improvements in the basic implementation technology. The underlying principle that drives costs down is the learning curve manufacturing costs decrease over time. As an example of the learning curve in action, the price per megabyte of DRAM drops over the long term by 40% per year.

The Microprocessor prices also drop over time, but because they are less standardized than DRAMs, the relationship between price and cost is more complex. In a period of significant competition, price tends to track cost closely

The Volume is a second key factor in determining cost. Increasing volumes affect cost in several ways.

•    First, they decrease the time needed to get down the learning curve, which is partly proportional to the number of systems (or chips) manufactured.

• Second, volume decreases cost, since it increases purchasing and manufacturing efficiency.

As a rule of thumb, some designers have estimated that cost decreases about 10% for each doubling of volume.

The Commodities are products that are sold by multiple vendors in large volumes and are essentially identical. Virtually all the products sold on the shelves of grocery stores are commodities, as are standard DRAMs, disks, monitors, and keyboards. In the past 10 years, much of the low end of the computer business has become a commodity business focused on building IBM-compatible PCs. There are a variety of vendors that ship virtually identical products and are highly competitive. Of course, this competition decreases the gap between cost and selling price, but it also decreases cost.

Cost of an Integrated Circuit:

The cost of packaged integrated circuit is

Cost of die + Cost of testing die + Costof packaging and final testCost of integrated circuit=Final test yield

The number of good chips per wafer requires first learning how many dies fit on a wafer and then learning how to predict the percentage of those that will work. From there it is simple to predict cost:

Cost of waferCost of die=Dies per wafer × Die yield

The number of dies per wafer is basically the area of the wafer divided by the area of the die. It can be more accurately estimated by

2×(Wafer diameter/2) × Wafer diameterDies per wafer=Die area2 xDieAreaππ−

2

The first term is the ratio of wafer area (πr ) to die area. The second compensates for the

“square peg in a round hole” problem rectangular dies near the periphery of round wafers. Dividing the circumference (πd) by the diagonal of a square die is approximately the number of dies along the edge. For example, a wafer 30 cm (≈ 12 inch) in diameter produces π× 225 – (π ×

30 ⁄ 1.41) = 640 1-cm dies.

Cost Versus Price—Why They Differ and By How Much

Cost goes through a number of changes before it becomes price, and the computer designer should understand how a design decision will affect the potential selling price. For example, changing cost by $1000 may change price by $3000 to $4000.

The relationship between price and volume can increase the impact of changes in cost, especially at the low end of the market. Typically, fewer computers are sold as the price increases. Furthermore, as volume decreases, costs rise, leading to further increases in price.

Direct costs refer to the costs directly related to making a product. These include labor costs, purchasing components, scrap (the leftover from yield), and warranty. Direct cost typically adds 10% to 30% to component cost.

The next addition is called the gross margin , the company’s overhead that cannot be billed directly to one product. This can be thought of as indirect cost. It includes the company’s

research and development (R&D), marketing, sales, manufacturing equipment maintenance, building rental, cost of financing, pretax profits, and taxes. When the component costs are added to the direct cost and gross margin,

Average selling price is the money that comes directly to the company for each product sold. The gross margin is typically 10% to 45% of the average selling price, depending on the uniqueness of the product. Manufacturers of low-end PCs have lower gross margins for several reasons. First, their R&D expenses are lower. Second, their cost of sales is lower, since they use indirect distribution by mail, the Internet, phone order, or retail store) rather than salespeople. Third, because their products are less unique, competition is more intense, thus forcing lower prices and often lower profits, which in turn lead to a lower gross margin.

List price and average selling price are not the same. One reason for this is that companies offer volume discounts, lowering the average selling price. As personal computers became commodity products, the retail mark-ups have dropped significantly, so list price and average selling price have closed.