Physical Architecture
As with the other Athlon XP processors, the Athlon XP 1900+ utilizes the improved Palomino core design, which is basically a refinement of the older Thunderbird core. Even at the 1900+ model number, the Athlon XP does not use the 0.13-micron core, as all Athlon XP processors are still built on the existing 0.18-micron die.
In terms of internal cache levels, the Athlon XP sports the exact same 128K of L1 cache and 256K of full-speed, on-die L2 cache as previous Athlon processors. This is not a negative by any means, especially since the Athlon and Athlon XP currently have a greater internal cache level than even the Pentium 4 (8K/256K). AMD also has an advantage with its exclusive cache design (ensuring data is not duplicated through the L1 and L2 entries) though Intel does have the edge with its full 256-bit data pathways, compared to the Athlon XP's 64-bit.
Where things do change a bit is in the area of system bandwidth, as the Athlon XP's enhanced features make the 266 MHz EV6 bus a virtual necessity. So no more 200/266 MHz variants for the Athlon XP line as the only available models utilize the 266 MHz bus. In addition to its need for a high bus speed, the Athlon XP is also not really a true upgrade processor, so transitioning to a new platform supporting the 266 MHz bus is regarded as the safest move anyway.
The 1.75V core voltage and Socket A design of the Athlon XP have also remained consistent, though the Athlon XP does use a bit different bus timings than the previous Athlon. We'll have a bit more on upgrades and viable Athlon XP platforms a bit later in the review. For now, here's a short chart outlining the various Athlon XP models, including their true core speeds and clock multipliers.
Palomino Core Enhancements
The AMD Athlon XP includes several core enhancements over the older Athlon design, with some dealing with performance gains and others relating to core refinements. Potentially the most important is the improved hardware prefetch found on the Athlon XP. This allows the processor to prefetch data that is predicted to be loaded next, thereby enhancing performance in many applications.
Along those same lines are some improvements to the processor's Translation Look-aside Buffers (TLB), which include increasing the data entries, adding exclusivity to both data and instruction TLBs, along with introducing speculative reloads. In plain English, the data prefetch and TLB enhancements are significant improvements over the internal architecture of the older Athlon design. These are also excellent features in that their use does not require any specific API support and will automatically improve performance across a wide spectrum of games and applications.
AMD has also added 52 new instructions to their SIMD set, which is now referred to as 3DNow! Professional. In terms of the Athlon XP, this allows it to be SSE-compatible, while not losing any of the enhanced 3DNow! capability, and puts AMD on equal SSE ground with competitive Intel processors. The only issues are finding out exactly how fast an SSE performer the Athlon XP really is, as well as recognizing that the Pentium 4 supports the newer SSE2 extensions.
The last enhancements relate to the physical core of the Palomino, as it can potentially use less voltage and have a lower power requirement than an Athlon of the same speed. We use "potentially" in this case, because as with the faster Duron models, the Athlon XP 1900+ actually uses the same core voltage as the Athlon, but its core does tend to produce less heat even at much higher clock speeds. AMD has also included an on-die thermal diode and PowerNow! functionality to the Athlon XP, but both of these features are highly dependent on motherboard manufacturers actually implementing them in their products.
Taking a more rudimentary examination of the new Athlon XP core does bring up a glaring omission. Even though Intel has moved their newest processor lines to a very safe and efficient "heat spreader" design, AMD has continued to expose the bare core to the perils of heatsink-fan (HSF) installation. The Athlon XP does feature a sturdier ceramic core, but we'd still feel a lot safer sliding on an Orb HSF without having to look the Palomino core right in the eye.
There is also the noted issue with motherboards not making proper use of the Athlon XP's on-die temperature diode, which can result in core damage if the HSF is improperly installed or happens to mysterious fall off later. This is not so much a critique of the Athlon XP as an observation of limitations with the current motherboard selection.
