There are two things guaranteed to make me cry at least twice a year. Cars and computers. Speed is essential to both.
So how fast is fast? First we need to look at the speed of PC's to understand a little about computer marketing. Let us take a typical model number 486DX4 (experienced tech-heads can skip a paragraph or two). Let's break this down
486 - the type of CPU, largely equivalent in power to a 68040, e.g. a Quadra,
DX - it has an FPU (Floating Point Unit), that is, a second chip to do heavy maths
stuff. A model without an FPU is an SX
4 - a measurement in multiplication of the CPU speed.
However rather than just say '4' people usually precede it with the FPU designator, thus DX4.
So 4 times what? As it says above, the chip speed. There are many different speeds of many different things in your computer, but the two important ones are CPU speed and bus speed (but this bus won't jump bridges). The speed of the CPU is important because the CPU is the thing that does the actual processing of your commands. Whether that be playing with some huge image in Photoshop, beating the high score in Tetris or reading this Mac Savvy, is because the CPU will normally only do one thing at a time,
although this depends on other factors.
The bus is the thing that lets the computer deal with things outside of the CPU, for example memory, video, user input and things further outside such as printers, scanners or disks. The CPU is the strength and the bus is much of that strength you get at any one time. It's no good having a massive army if you send them all into battle through one small door. You want them to go as many abreast as possible, and as fast as possible. So if you wanted you could have a really fast CPU and a really slow bus and you would then have lots of computing power and no way to access it properly.
This takes us back to the DX4 which is just such a case. The CPU and the bus all go at 25MHz, but the timing of the CPU has been ramped up to 100MHz. Is this better? Well it would be if the bus was also made faster, but it isn't. A faster CPU is usually better than a slow one, but the advantages are negligible in this case. The overall speed will be about 30MHz. Which means that a DX4 at 100MHz is slower than a DX2 at 66MHz. A
DX2 has been doubled from 33MHz, which is faster than an overall speed of 30MHz in the DX4.
Confused? You will be.
So why do they do this? Simple marketing. By emphasising the speed of CPU in your slow computer you can make it sound faster than your competitors computer which actually is faster than yours.
By the way the architecture of 586's (Pentiums) was changed so that the bus and CPU operate as one. So a 100MHz Pentium really is 100MHz. And the 200MHz Pentiums really are 200MHz.
Now that you have this knowledge of speed and how it is measured we can move onto Macs. All the Macs up until the 68030 were measured by the identical CPU and bus speeds. Then about 2 years ago Apple decided to jump onto the same bandwagon as PC makers. They started to quote Mac speeds as a multiplication of the base level. However Macs don't use CPU's at doubled speeds. The fastest CPU as of this writing is in the Genesis model (Graphics houses are the only ones who can usually afford these - they are over 10,000 UKP, weigh 60lbs and have multiple CPU's) and some new desktop
machines which are 200MHz fast.
So what does the multiplied speed tag refer to? Well, inside the Mac CPU (both PowerMacs and 68040) there is a small section where the timing is referenced against itself. This is called a feedback loop. Simply put the timing signal is forked, once to its destination and once round in a loop to a small length back along the wire it has just come in on. Because the length of the wire is set you can then tell whether the timing signal is of the proper length by seeing whether the feedback signal sits cleanly on top
of the new one. This is what the adverts are referring to in their speed quotes of any Mac model. As you can see it is nothing to do with the bus (that lets you get to the power), and it is even unlike the term 'DX4' because the Mac CPU is not going at twice the speed**, nor is the timing function an integer multiplication of the CPU speed.
How do these speeds cash out? How much speed do you actually get for your money? Let us take a few random examples:
 
** although kits are available. See -
http://violet.berkeley.edu/~schrier/mhz.html
(this does not constitute an endorsement of the actions proposed on the above URL)
Thought For The Day.
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Remember kids, the computer you use need not dictate your choice of
