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"Hello, I'd like to buy an Athlon".

Sounds so simple, doesn't it? Getting your hands on one of AMD's flagship processors used to be a fairly simple affair - decide the price/speed point you're comfortable with, pay your money, take your chip and go home. Nowadays, you get asked all kinds of silly questions like "Classic or Thunderbird? Model A or C? SlotA or SocketA? .25 micron or .18 micron?" Arrgh! What's going on? Well.. what's going on is innovation. When you're competing with the world's biggest CPU manufacturer, you have to move fast to stay ahead of the game. AMD have been refining and improving their Athlon processors almost from day one, and the end result is you have to be a little more careful when choosing your chip.

First up, the most fundamental difference between the various models is the mounting package. Either you have a SlotA motherboard, or a SocketA one. Right now there are no converters in either direction (unlike "Slockets" for Intel chips) so make sure you pick up the right kind for your motherboard. AMD is moving towards a pure SocketA product line and this will mark the end of the SlotA era. With that in mind, I thought it might be useful to summarise the 3 types of SlotA processors, identifying the differences between them. So, let's get all this jargon straightened out..

This article turned into something of a monster with lots of info, images and graphs, and is therefore spread over 5 pages. You can quickly jump to a page you're interested in with the "Jump To:" box above, or click a link below:

Table of Contents:
Page 1 - Physical Differences (case, product numbers)
Page 2 - Positive ID (L2 cache location, WCPUID)
Page 3 - Performance comparison of the 3 types at 700MHz
Page 4 - Artificial comparison showing the difference L2 cache speed makes
Page 5 - KX133 Thunderbird overclocking (good results!) and Conclusions

Physical Differences (or, how to spot a TBird)
While the processor cartridges for SlotA "classic" Athlons and Tbirds are pretty similar, there are a few ways you can make sure you're getting what you're after. Actually, firstly let me clarify something - from now on, when I say "Athlon", I mean "classic pre-Thunderbird Athlon". I will call the Thunderbirds "Thunderbirds", even though they are, of course, just another type of Athlon.

Ok. First, look at the heat plate on the back of the processor. This is where you'd mount a heatsink/fan unit. On the Tbirds, this plate is silver. On Athlons, this plate is black. You can see this clearly in the rightmost photo above, there is a silvery aluminium plate under the heatsink. They're actually aluminium in both cases, and they both have little indentations on the back for the L2 cache chips, but the older Athlons are black. Usually. I say this because I've heard reports of newer non-Tbird Athlons having silver backing plates. So maybe this isn't the best way to identify them..

Another way is to read the writing on the top of the CPU cartridge. This is a good way to identify the 3 different types of SlotA cpu's if you can remember a few simple rules. Here's some examples:

Here, the top string reads AMD-K7700MTR51B C. The "C" denotes that this is the older, .25 micron process Athlon. The K7700 means 700MHz K7. So this is an original, non-Thunderbird 700MHz Athlon. This horrible fuzzy photo was taken with my old camera, too. :)

Ahh, much sharper! On this one, the string reads K7750MTR52B A. Obviously, K7750 means a 750MHz K7. The difference here is the final letter, A. This signifies this Athlon was made with the newer .18 micron process.. but it's not a Thunderbird! It's still a classic Athlon, just a more recent one.

In this example, the string reads AMD-A0700MPR24B A. The "A" signifies the .18 micron process, as above, but the K7xxx has been replaced by A0xxx. This means it's a Thunderbird - in this case, a 700MHz one.

So, to summarise: The last character denotes the silicon process, either C for .25 (old) or A for .18 (new). The first 2 digits after the "AMD-" denote the family, be it K7 (classic Athlon) or A0 (Thunderbird). The next 3 (or 4) digits denote the speed (500, 750, 1000, for example). Of course, you should NEVER see a 'C' chip with AO - there's no such thing as a .25 micron Thunderbird.

What does "micron process" mean? Well, it refers to the size of the tracks in the silicon core of the chip. A smaller process means less voltage, in theory less heat, lower in-core latency so higher core speeds, and higher yield for the manufacturer, which means lower price. So, the newer and hence smaller the process, generally speaking, the faster and (once market demand is met) cheaper the chip.

Now, this is all well and good.. but remember those dastardly rebadging people. These unscrupulous folks take chips that are sold at a lower speed, overclock them and re-sell them as higher-speed chips, making a hefty profit. Some of these are even clever enough to have made false cases made up, with incorrect core speeds listed on them. Although I've not seen it, I wouldn't be surprised if they bought old classic Athlons and tried to pass them off as Thunderbirds.. so how can you be ABSOLUTELY CERTAIN you've got the right one?

Well, there's something that you can't change without destroying the chip, and that's the location of the L2 cache.. so follow me, and I'll tell you all about that.