Verdict: The most important chip since the 386
AMD hasn't launched a genuinely new microprocessor since the original Athlon way back in 1999 - if that doesn't sound long ago to you, remember that it was in the last millennium. To be fair, the Athlon design has served the market well, spanning several form factors and a huge range of clock speeds, from 500MHz up to 2.2GHz (Athlon XP 3200+). But things have now moved on.
The Athlon 64 is the first 64-bit desktop CPU ever, and is being hailed as the most important CPU since the 386DX - the first ever 32-bit desktop CPU, introduced in October 1985.
Unfortunately, just because it's the most revolutionary chip in nearly two decades doesn't mean the operating system companies are on track to take advantage of it. It took Microsoft nearly one and a half decades to come up with a fully 32-bit consumer OS in the shape of Windows XP.
Although it doesn't look like it'll take nearly as long this time, the infrastructure isn't quite there yet, so without Windows 64 (or 64-bit Linux if that tickles your fancy), 64-bit drivers and 64-bit applications, the Athlon 64 is forced to run in good old-fashioned 32-bit mode. And it's likely to have to do that for some time yet until 64-bit applications become commonplace.
Unlike Intel's approach of basing the Itanium processor family on the IA64 architecture, the AMD64 architecture on which the Athlon 64 is based has been developed to run both 64-bit and 32-bit code. So, while an Itanium has to emulate 32-bit code in software, which we found makes it run like a tortoise, an Athlon 64 is perfectly happy to run legacy code. With a 64-bit version of Windows XP not due to make a full appearance until at least 2004, solid 32-bit performance therefore is a must-have for the Athlon 64 family.
Specifications
AMD doesn't seem to have learned from the drubbing it got for its Athlon XP naming strategy, and is using an even more ludicrous nomenclature for the Athlon 64 family. The basic Athlon 64 chip will be called the Athlon 64 3200+ while the high-end chip is called the Athlon 64 FX-51.
Even though it's the only 64-bit desktop chip in mass production today, currently it's the architectural changes that are more important, as they determine its performance with existing 32-bit applications.
For starters, the Athlon 64 has a fundamentally different way of handling memory to any other x86 CPU. In other systems, the main memory communicates with the CPU via the Northbridge across the front side bus. In an Athlon 64 system, the memory controller is integrated inside the CPU and so there is no Northbridge. This has several dramatic effects, such as significantly reducing memory latency, boosting system performance and simplifying motherboard design (there's one less chip). The memory controller in an Athlon 64 FX-51 supports up to PC3200 DDR running in a dual-channel configuration through a 128-bit bus, while the Athlon 64 3200+ has a 64-bit memory controller.
With no Northbridge there's no FSB, so instead the Athlon 64 uses a variant of the much-touted (but little-seen) HyperTransport bus to talk to the Southbridge and AGP bus. As HyperTransport is a multi-platform bus (it can be used for networking, for example) there are several different versions available. In this initial release the Athlon 64 architecture has two uni-directional 16-bit point-to-point links (like two roads of two-way traffic) providing a heady 6.4GB/sec of bandwidth when clocked at 800MHz (200MHz x 4). The CPU frequency is determined from the HyperTransport speed (a standard PC uses the Northbridge frequency - the FSB) and the CPU multiplier. So by increasing the HyperTransport frequency from 200MHz it's possible to overclock an Athlon 64, just as increasing the FSB overclocks existing processors.
Both chips have 128KB of Level 1 cache, like the Athlon XP, but double the Level 2 cache - now a full 1MB. Also added for the first time in an AMD chip is full support for SSE2. This has been a major Achilles heel for AMD as many multimedia applications are optimised for SSE2, giving Pentium 4 a big advantage compared with Athlon XP. Clock speeds are still lower than Pentium 4, though, and AMD is trying to hide them again with its daft naming system. However, we can tell you the FX-51 runs at 2.2GHz and the 64 3200+ at 2GHz.
Physically, both chips are very different too. The FX-51 has 940 pins while the 64 3200+ is packaged in 754 pins, meaning they use different (incompatible) motherboards. AMD has also fitted an integrated heatspreader for the first time, which covers the CPU core, protecting it from harm.
Performance
But is it any faster? To find out we built two PCs - one based around an Athlon 64 FX-51 and one around a 3.2GHz Pentium 4. We used the same hard disk (Western Digital 1200JB), graphics card (GeForce FX 5900 Ultra) and optical drive (Samsung SM-348) - the only difference being the motherboard and RAM. For the Athlon 64 we used an Asus SK8N motherboard based on the nForce3 chipset and 1GB of Legacy Electronics PC3200 registered DDR; for the Pentium 4 we used an Asus P4P800 motherboard based on the Intel 875P chipset and 1GB of Corsair PC3500 DDR running at PC3200 speeds.
We won't keep you hanging on - the FX-51 stormed past the Pentium 4 in almost every benchmark we ran. A significant factor was the fast encoding performance, no doubt due principally to the addition of SSE2 to the instruction set. The VirtualDub DivX encoding test took 349 seconds on the Pentium 4, while the FX-51 completed the benchmark in just 242 seconds. If you were to extrapolate these figures you'd see that an FX-51 would take approximately an hour less to encode a full feature-length movie to DivX than a 3.2GHz Pentium 4.
Interestingly, the only benchmark where Pentium 4 was faster was the Cinebench CPU test, taking just 69.4 seconds compared to a lengthy 85 seconds on the FX-51. This is because Cinebench has been fully optimised for an SMP system, and therefore runs faster because of the Pentium 4's Hyperthreading technology. We reran the test with Hyperthreading disabled, which upped the time to 82.7 seconds - a similar time to the FX-51.
Conclusion
Unfortunately, at the time of going to press we couldn't get a price for the Athlon 64 FX-51, but we can compare the 1,000 unit trade prices from AMD and Intel, which gives an indication of the price difference between the two chips. The 3.2GHz Pentium 4 sells for around £400 while the FX-51 will sell for approximately £461. It's strange to see an AMD processor priced higher than an Intel one, but given the huge performance differences in multimedia applications it's more than justifiable.
The only real downside to the Athlon 64 FX-51 is that it doesn't work with standard PC3200 DDR memory, requiring more expensive registered modules that are also harder to get hold of. It's also worth noting that AMD will change the socket design from 940 pins to 939 pins in the first half of 2004, and also drop the requirement for registered DIMMs, so the upgrade path for an FX-51 motherboard could well be extremely limited.
Ultimately, it's really good to see a processor company coming up with something genuinely new and original. How well it will fare against the forthcoming Intel 'Prescott' CPU when it's released in the next few months remains to be seen though. Until 64-bit apps come along it's difficult to predict what the major benefits will be, but if its 32-bit performance is anything to go by, the Athlon 64 FX-51 will be a bit of a screamer.
