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Sorting Out the New AMD CPUs

Sorting Out the New AMD CPUs

AMD's new Athlon and Duron CPUs provide very high performance at competitive pricing, but changes in form factor, chipsets, and memory requirements when compared to older AMD Athlon CPUs can be confusing. In this article, you'll learn how to correctly match up Athlon and Duron CPUs, chipsets, and memory.

Slot A Versus Socket A

If you're contemplating adding an AMD Athlon or Duron CPU to your system, the first issue to keep in mind is that Slot A, the original Athlon form factor, is obsolete. Just as Intel has returned to the socketed CPU with its late model Pentium III, Celeron and new Pentium 4, AMD has also returned to the CPU socket with its latest Athlon models, using a unique design called Socket A. Socket A is also called Socket 462 because it has 462 pins. Chips which install in these sockets are referred to as having a PGA (Pin Grid Array) form factor, due to the interface pins located on the bottom of the CPU.

Moving from a slot back to a socket was made possible because, like Intel, AMD also changed their processors to incorporate an on-die L2 cache. The only reason Intel and AMD had originally switched from socket (chip-based) processors to slot (cartridge based) processors was because they moved the L2 cache memory chips from the motherboard to the processor, and needed a board on which to mount the CPU along with the adjacent L2 cache chips. Now that the L2 cache is fully integrated into the processor die, there is no need for separate chips, and thus no need for a board/cartridge setup, hence the return to sockets.

There are several benefits to the new PGA (Socket A) Athlons. Not only are the Socket A versions cheaper to make than the Slot A (cartridge) versions, but they are significantly faster as well. This is because the on-die L2 cache runs at the full core speed of the processor, while the separate L2 chips previously used ran at either 1/2, 2/5 or 1/3 of the actual CPU speed. AMD originally gave the Athlon with on-die cache the codename, "Thunderbird," and it is often referred to by that codename today. All AMD Duron processors (codenamed, "Spitfire"), the low-cost member of the Athlon family, also make use of the Socket A form factor.

One crucial difference between Intel and AMD's revival of socketed CPUs is AMD's lack of support for Slot A to Socket A adapters. Basically, if you have a Slot A motherboard and want to upgrade to a newer Athlon or Duron that uses Socket A, you must replace your motherboard. If you own one of the early Slot A Athlon systems, now is the time to look at picking up the fastest Slot A Athlon CPU your system will support, because prices are down and, sooner or later, the supply will run out.

Thunderbird - Smaller but Faster

It is easy to show that a small amount of high-speed cache is faster than a larger (even significantly larger) amount of low-speed cache. Cache is one area where size doesn't matter, at least not nearly as much as speed. Accordingly, the newest Athlons have switched from 512KB of cache running at 1/2 (or 2/5 or 1/3) of the CPU speed to 256KB of on-die cache which runs at the full CPU speed. Known during pre-development as "Thunderbird," Athlons with on-die cache are officially called "Athlons with Performance-Enhancing Cache Memory". AMD did make Slot A versions of the Athlon with the Thunderbird core, these are the "Model 4" versions. If you are purchasing a Slot A (cartridge type) Athlon, make sure it is has Model 4/Thunderbird core with the full-speed on-die L2 cache. Fortunately, all Socket A Athlons (also referred to as PGA [Pin Grid Array] Athlons) use the Thunderbird core.

As you can see from Figure 1, your upgrade path for Slot A is limited to 1GHz. If you want to experience greater than 1GHz performance, you must use the Socket A design.

Figure 1: AMD Athlon and Duron speed and form factor comparison.

Because of voltage differences between Versions 1, 2, and 4 of the Slot A Athlon, not all Athlon motherboards can use all speeds. Check with your motherboard vendor to see if you can switch to a faster Model 4 Slot A Athlon.

Athlon or Duron?

Both the newest AMD Athlon versions and the AMD Duron are designed for Socket A. What are the differences in these two CPUs?

  • Both the Athlon and Duron have 128KB Level 1 cache, but the Duron has only 64KB of on-die Level 2 cache, compared to the Athlon's 256KB of on-die Level 2 cache.

  • The Athlon is currently available in clock speeds up to 1.33GHz, while the Duron currently tops out at 900MHz. Of course, both will be available at higher speeds in the future.

If you want the fastest AMD processor, get the Athlon, but if you want to save some money now but keep your upgrade options open for future processors, get the Duron. According to a wide variety of business, gaming, and high-end benchmarks run by The Tech Report, the Duron is between 6 and 20% slower than the Athlon at equivalent clock speeds, with most test results coming it at less than a 10% difference. This bears out the "size doesn't matter much" statement made earlier about cache, as that is really the only difference between them.

Comparing Chipsets

The last major piece of the puzzle is the chipset used by your Athlon or Duron motherboard. The AMD 750 chipset supports conventional PC100 SDRAM; the VIA KT133 chipset supports PC133 SDRAM (now available at prices comparable to PC100), but neither of these fully unleashes the performance potential of your new AMD CPU..

The newest high-speed memory technology on the market is DDR SDRAM; the DDR stands for "Double Data Rate", and it works by performing two memory transfers per cycle instead of only one per cycle as with ordinary SDRAM. DDR SDRAM is available in two varieties at present:

  • PC2100 DDR memory provides 2.1GB/second throughput of data and supports the 266MHz front-side bus (processor bus) speed of the fastest Athlon CPUs; this speed of SDRAM is supported by Athlons running at 1.2GHz down to 1GHz clock speeds.

  • PC1600 DDR memory provides 1.6GB/ second throughput of data and supports the 200MHz front-side bus originally used by the Athlon CPU; this speed of SDRAM is supported by Athlons running at 1.2GHz down to 850MHz clock speeds.

AMD's new 761 chipset was the first system controller chip to support both varieties of DDR SDRAM. Recent benchmark tests performed at Tom's Hardware reveal the 761 system controller is the superior choice over Acer Labs' new AliMagiK 1 chipset, which also supports both speeds of DDR SDRAM as well as PC133/PC100/PC66 SDRAM (although motherboards using the AliMagiK 1 chipset have sockets for either SDRAM or DDR SDRAM, but not both). Keep in mind that DDR DIMMs use a 184-pin socket, instead of the 168-pin socket used by standard PC100 and PC133 SDRAM DIMMs; in other words you can't interchange conventional and DDR SDRAM in the same memory sockets.

Pay Your Money, Take Your Choice

For the best performance, get a 1.33 GHz Socket A Athlon CPU with the AMD 761 chipset and PC2100 DDR SDRAM.

Want to save money on RAM and still have good performance? Combine the 1.33GHz Athlons with motherboards that support PC133 SDRAM. Even though the DDR memory is twice as fast as the previous SDRAM memory, your processor normally retrieves data from the L1/L2 caches and only uses the main memory directly about 1% of the time. Thus even a doubling of main memory performance doesn't have a very large impact on overall system performance. The interesting thing is that currently DDR SDRAM is priced, depending on the source, almost the same as standard SDRAM, and soon it may even be cheaper! Let the price be your guide, if you can get PC1600 or PC2100 for the same price as PC100 or PC133, then go with the faster DDR memory. If you find a large difference in price, then going with the slower PC100 or PC133 really won't hurt performance all that much.

You can save even more by substituting the Duron for the Athlon. As stated earlier, the smaller L2 cache on the Duron (as compared to the Athlon) really doesn't hurt performance all that much either. A little cache goes a long way, a lot more only helps a little. Of course, if you are willing to spend the extra money, then go with the larger cached Athlon.

If you're looking for a medium-performance stopgap solution for an older PC and don't mind a motherboard change down the road, snap up the many bargains currently available on 700MHz or faster Slot A Athlons and motherboards. Unfortunately, both the slower pre-Thunderbird models as well as the faster Thunderbird models were available in cartridge versions at speeds up to 1GHz so be careful which you buy. The best rule of thumb is to insure that you get only a Model 4 version, as the older pre-Thunderbird Model 1 and 2 designs have the slower L2 cache.

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