A little over a week ago, AMD dropped some surprising news on industry watchers and enthusiasts, AMD would not be releasing its 64-bit desktop processor, Athlon 64, until September 2003. This chip was expected to make its debut in early Q2 of this year, although AMD always said it would be a release with limited initial availability. Instead, the company announced that it was pinning its immediate future on its upcoming “Barton” Athlon XP core. AMD would debut Barton with its Athlon XP 3000+ on February 10, today, with a 3200+ model to follow in mid-2003.
If you recall AMD’s earlier roadmaps, Barton was originally scheduled for release in the second half of 2002. Setbacks pushed the chip into 2003, and AMD dropped its plans to incorporate silicon-on-insulator manufacturing technology. Silicon-on-insulator allows the transistors within the processor to operate faster while at the same time reducing power consumption, but this technology will now have to wait for Athlon 64 later this year.
So what does the new Barton core bring to the table? The big addition is the new L2 cache, which has doubled in size from 256K to 512K.
If you recall CPU micro-architecture, today’s latest desktop processors contain two forms of cache memory Level One cache (L1) and Level Two cache (L2). L1 cache is the first place the processor looks for the data it needs. All Athlon XP processors (including Barton) contain a 128K L1 cache, 64K for data, and 64K for instructions.
If the data can’t be found in the L1 cache, the processor next turns to the L2 cache. This is the second, and final line of defense for the processor. This is the last form of high-speed memory that the processor has access to, after the L2 cache the processor next turns to system memory, and following that, the worst case scenario: the system hard drive. Each additional stage introduces more latency, and thus, reduces performance. Therefore, its important that the processor’s memory subsystem is capable of keeping it well fed with data (especially as clock speed rises).
As we mentioned previously, AMD has implemented a 512K L2 cache that is 16-way set associative (the more associative the cache is the greater the hit rate i.e. the probability of the processor finding the data it needs), to accomplish this. Traditionally, doubling the L2 cache has brought a performance improvement of roughly one processor speed grade. In the case of Intel’s “Northwood” Pentium 4 processor core, we witnessed a 9% performance gain at 2GHz. If AMD can get similar results out of Barton, the new core could be perfect for holding off the competition from Intel. Barton essentially bridges the gap between the Athlon XP family and Athlon 64. Lets delve a little further into the new core.