256 Kb integrated Level 2 cache.

The main difference between the Athlon "Thunderbird" and the preceding K7/K75 Athlon processors is the level 2 cache arrangement found on these chips.

The Athlon "Thunderbird" features 256 Kb of full-speed level 2 cache directly integrated onto the processor core. This allows AMD to run the Level 2 cache at the same speed as the processor core, thus reducing the latency of a Level 2 cache read. Whilst the "Thundebird" may only have half the amount of Level 2 cache compared to previous Athlon processors, the drop in latency to 11 clock cycles from 12 clock cycles results in significant performance benefits. Only in a few server related functions (and maybe SETI operations) will the reduced size of the Level 2 cache result in a performance loss.

The Level 2 cache of the Athlon "Thunderbird" though goes beyond just integrating it onto the processor core. With the Athlon "Thunderbird" AMD implemented a new cache architechture in the form of "Exclusive" Level 2 cache as opposed to "Inclusive" Level 2 cache. The Athlon "Thunderbird's" Level 2 cache is a 16-Way Set-Associative cache.

Exclusive Level 2 cache sees no duplication of data between the 128 Kb Level 1 and 256 Kb Level 2 cache of the "Thunderbird". This uses the Level 2 cache essentially as an overflow from the processors Level 1 cache. For all intents and purposes we can view both the Level 1 and and Level 2 cache of the Athlon "Thunderbird" as 384 Kb of unified cache as this is how it is used (in simple terms). This can be compared to the Inclusive Level 2 cache of older Athlon processors, where the contents of the Level 1 cache would be directly mapped to the Level 2 cache thus using up 128 Kb of it. Although there would be a total of 640 Kb cache available, the system would only be able to use 512 KB due to the duplication of data in the Level 2 cache.

The Athlon "Thunderbird" though still only features the 64 bit data path to and from it's Level 2 cache as the K7/K75 Athlon. Compared to the 256 bit data patch to and from the Level 2 cache featured on Intel's "Coppermine" Pentium III it does seem that AMD may have missed a performance enhancing opportunity here.

Chipsets.

The issue of chipsets for the new Athlon "Thunderbird" has been particularly vexed as it emerged shortly before launch that the VIA Apollo KX133 will not be compatible with the new Athlon. Although the Athlon "Thunderbird" will be compatible with the AMD 750 chipset, it is unlikely that we will see it used outside of the limited production run of Slot A "Thunderbirds" intended for OEM use only.

Users with existing Slot A, VIA Apollo KX133 motherboards WILL NOT be able to run the Athlon "Thunderbird".

In order for the new Socket A Athlon process to run, it must be used with the new Socket A designed chipsets. These include:

• VIA KT133. This is the KX133 modified for Socket A architechture.
  
  
• VIA KM133. This is the KT133 but with integrated S3 Savage 4 graphics. It does though include an external AGP 4X connector.
  
  
• VIA KL133. Like the KM133 featuring Savage 4 graphics, but with no external AGP slot.
  
  
• SiS 730. This is an single chip solution which incorporates a SiS graphics accelerator & integrated sound. It also features an external AGP connector.

All chipsets feature the 200 MHz (100 MHz DDR) Alpha EV6 bus used by the Athlon processor to date.

Whilst not readily available at launch, most motherboard manufacturers plan to sell boards for the Socket A architechture so there will not be the problem of limited choice of boards which affected early adopters of the AMD Athlon platform.

Athlon Thunderbird - Page 1.

Athlon T'bird - Page 3.