AMD Athlon "Thunderbird" 1 GHz / 800 MHz

As we mentioned earlier, by placing the L2 cache on the die of the Athlon itself, there was no longer a need for the Athlon's processor card, which paves the way for the introduction of the socketed Athlon, more specifically the Socket-462 interface.

The Socket-462 interface has the same physical dimensions as a Socket-7 or a Socket-370 socket, which allows it to use the same cooling fans and clips that are used on Socket-7 and Socket-370 coolers.  

One thing you'll notice about the Socket-462 interface is that there are some missing pins in the otherwise full layout.  The reason for this is to prevent the accidental insertion of a Socket-370 processor into the socket since the missing pin holes line up with physical pins on all Socket-370 CPU.

The Thunderbird will be available primarily in a Socket-462 package since AMD is very intent on making the move to the cheaper socketed packaging on all of their processors. However, in order to support the OEMs that already have Slot-A system designs, AMD will be shipping limited quantities of the Thunderbird in a Slot-A package, which is basically identical to the older Athlons with the exception that it has no L2 cache chips on the processor card itself. 

We pointed out not too long ago that, although the Thunderbird will be available in a Slot-A package, for most end users it will be much easier finding a Socket-462 Thunderbird than it will be finding a Slot-A part.  The Slot-A parts are supposed to be allocated to OEMs alone and only in limited quantities, so while you may see them pop up on various vendors' sites online, don't expect to see them as readily available as the Socket-462 parts.

The situation gets even more complex when you take into account that the only Slot-A chipset that officially supports the Thunderbird is the AMD 750.  It was discovered early on that there was the potential for motherboards based on the VIA KX133 chipset to have compatibility problems with the limited number of Slot-A Thunderbirds in production. 

The only possible explanation for the ability of the Thunderbird to be guaranteed to work with the AMD 750 chipset but not with the KX133 is because the KX133 reference design features timing specs that are out of the Thunderbird's range.  This could also explain why certain motherboard manufacturers are claiming that their KX133 settings work fine with the Slot-A Thunderbirds; basically, if a KX133 motherboard manufacturer is claiming their board will work fine with a Slot-A Thunderbird, they're probably just increasing the tolerance the board has for out of spec signals or boosting the signal strength going to the processor itself.