Compared to the typical 1MHz and 2MHz clock rates used in the first desktop computers, the 33MHz and 40MHz clock rates in use today pose some interesting problems for designers and users. The difficulty stems from the fact that current clock rates are well into the radio frequency range. For example, some FM radio stations transmit on frequencies lower than 80MHz and stations in the long wave band use frequencies as low as 0.1MHz! The reason why this becomes more of a problem as clock rates increase is that the efficiency of an aerial increases with frequency. This means that the copper tracks connecting the chips that make up a computer radiate significant amounts of radio frequency noise as the clock rate increases. If you have tried listening to a radio near to a computer you will have heard the problem.
However, the problem isn’t to do with radio interference, this can be reduced by using a specially treated metal case, the problem is that the radio emissions cause distortion of the pulses that the copper tracks carry and lead to interference or ‘crosstalk’ between tracks. You can think of it as the copper tracks becoming increasingly leaky as the clock rate goes up. It is possible to do something about this by careful layout of the printed circuit board but many early machines used printed circuit boards that were designed for slower speeds and were often unreliable. Today there is no real problem with clock rates up to 40MHz but there is some doubt about the possibility of pushing the rate any higher without unreasonable increases in cost. (This is one reason for the use of clock doubling circuits to increase a 25MHz external clock to 50MHz.)
Another common practice that can cause problems is using chips above their recommended clock rate. The frequency at which a chip is guaranteed to work is usually a very conservative estimate of actual maximum operating frequency. Before true 40MHz processors were available it was a common practice for manufacturers to select the higher quality components from a batch of 33MHz parts and push them to 50MHz. This sometimes resulted in a machine that would refused to work if the room temperature was too high !
doesn’t result in the program taking half the time. Even so clock speed is a valuable measure of potential performance. The point is that it determines the rate at which instructions are obeyed. Doubling a machine’s clock speed halves the time a program takes to run in the best possible case.
If you are comparing identical processors then clock speed is a good measure but it isn’t as meaningful between processors. Even so clock speed is still quoted as a way of showing how machines based on the Intel family have increased in power. The original PC had a clock speed of 4.77MHz and the first AT 6MHz (quickly increased to 8MHz). The fastest 286 machines used clock speeds of 10MHz, 12MHz, 16MHz and even on occasion 20MHz. 386 systems have been available in clock speeds of 16MHz, 20MHz, 25MHz, 33MHz and even higher. The 486 has been manufactured in 25MHz, 33MHz and 40MHz versions.
At this point you might be wondering why every processor, especially those in the 386 family, isn’t available at every clock speed. The answer is that manufacturers have an obvious desire to rationalise the range that they offer and this goes for chip and machine manufacturers. This means that processors
are produced in a range of clock speeds that tries to eliminate overlap in processing power. For example, the 20MHz 386DX died out when the 25MHz 386SX became available. In this case the 386SX version was as powerful and cheaper than the DX version. Thus while a complete listing of all the clock speeds that have ever been produced makes interesting reading it isn’t likely to correspond to the range available or desirable at any given time.
To get some idea of how clock speeds and MIPS compare and how the different processor types offer different ranges of power then see Figure 2.1.
Clock speed is the simplest measure of the speed of a processor and it works very well as long as you confine your attention to a single type of processor. However it is prone to misrepresent the speed of a processor if the number of Instructions that are obeyed per clock pulse is modified. For example, you could modify a 386 chip to execute its Instructions in half the number of clock pulses. This means that you could have two 386 chips with the same clock speed but one would have twice the MIPS of the other! If you think that this is entirely theoretical it is worth mentioning that the
486 is in many senses nothing more than a 386 that executes an important subset of instructions in fewer clock cycles and the 486DX2 doubles the external clock speed so in effect obeying all instructions in half the time!
To compensate for the possibility of there being different numbers of instructions per clock pulse it is useful to use the Landmark speed. This is simply the speed that an original 286AT would have to be run at to give the same performance. The Landmark speed is good in the sense that it does provide a reasonably realistic measure of performance but it can also be used to mislead if a manufacturer quotes a clock speed that is in fact a Landmark speed. A typical set of Landmark speeds quoted for a range of machines from one manufacturer can be seen in Figure 2.2. Notice that this graph is also influenced by other design aspects than the type of processor. For example in the case of the 486 machines a cache (see Chapter 4) was used to produce an even greater speed increase over the 386SX and 386DX machines.
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