Identification
- Part 2:
The major difference between classic Athlons and Thunderbirds
is that the Thunderbird has on-die L2 cache. The Level 2 cache
is a chunk of high-speed memory that the CPU uses to store data
and instructions it is going to need soon. Think of it as a scribble
pad for the CPU. The CPU is almost constantly pulling things out
of and writing things into the L2 cache. Therefore, the speed
of the cache has a big effect on the overall speed of the processor.
It's no good having a CPU capable of doing hundreds of millions
of instructions per second, doing nothing while it waits to get
the data it needs from cache.
The classic Athlons have 512KB of L2 cache. The Thunderbirds only have 256KB.. but this cache is actually located inside the processor die, instead of sitting on the CPU board. In both types of chips, inside the black plastic casing there is a green printed circuit board (PBC) with a gold edge-connector. The processor is attached to this board - the board takes the signals from the pins of the CPU and carries them (via some support circuitry) to the gold edge connector, and hence to your motherboard. It is on this green PCB that the L2 cache of the classic Athlon is located, whereas the Thunderbird has it inside the actual CPU core itself.
This allows the Thunderbird's cache to run at the full speed of the processor, which (in theory) gives a tremendous boost to performance. For example, on the Classic 1GHz Athlon, the L2 cache is running at 1/3 the CPU speed, or 333MHz. On the Thunderbird 1GHz, the L2 cache is running at the full 1GHz. Now, the ratio between the Classic Athlon's L2 speed and core speed varies depending on the core speed - the RAM chips used for L2 cache seem to have a limit of about 350 to 400MHz, so depending on the core speed, the ratio might be 1/2, 2/5, 2/3 or 1/3. Some motherboards, including the Asus K7V (reviewed here) and Abit's excellent KA7 (reviewed here) let you set this ratio in the BIOS, so you can overclock your L2 cache and experiment with performance and stability. Run it too fast, and you'll soon find burst errors creeping into the data stream and your machine will hang, crash, not boot or otherwise Fail To Cope. The point is, with the Thunderbird you don't need to worry about this stuff. The ratio is always 1/1, for optimum performance.
So how does this help you when you're standing in the shop, fondling black plastic shapes and wondering which is which? Well, as the L2 cache on the classic Athlons is mounted on the PCB, you can SEE it, if you flip the processor over and look up it's skirt (as it were). This first photo shows where you should be looking:
These next shots show you what you should be looking for. I've only marked one area, but there is an identical chip (or absence thereof) on the other end, too. In the middle you can see the pins connecting the Athlon core to the PCB.
Both "A" and "C" K7xxx Athlons should look like this. Above you can (hopefully!) see the lump on the heat plate pressing up against the L2 chip - you can see some thermal paste between them, and there's actually a very small air gap in this instance. It's not that uncommon to see a small air gap between the heat plate and the cache chips - there's been some debate about this, but cache chips really don't make that much heat and if anything, having them in contact with the heatplate may be a bad thing, as they will absorb heat from the processor via the heat plate. Anyway..
Here, you can see the heat plate lump, but it's sticking out into thin air - because the Thunderbird doesn't have the L2 cache here, it's inside the core of the CPU. If your chip looks like this, you have a Thunderbird which should be labelled "A0xxx" and "A".
Finally, to confirm all this, when you eventually get your CPU home and power it up in your machine, you can check the CPU family by using H-Oda's excellent WCPUID program. Get it here - it's free, it's tiny and it tells you more than you could possibly want to know about your CPU. In these sample snaps below, you can check the Model ID - "C" Athlons being 1, "A" Athlons being 2 and Thunderbirds being 4. Jai at insanehardware.com points out that the Duron (available in SocketA only) is Model 3. However, the simplest way is to look about half-way down the window, at the L2 cache size and speed. Classic Athlons are 512K at a variety of ratios depending on their core speed, Thunderbirds are always 256K at 1/1.
Ok. So, after all that, does it really mean anything if you have a classic Athlon or a Thunderbird? Well, let's look at some performance figures. On the next page I compare all 3 types of SlotA Athlons at the same core speed: