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[nextpage title=”Introduction”]

In response to the release of the new quad-core CPUs by Intel, AMD launched its Quad FX platform. Contrary to what many people could think, Quad FX isn’t a new AMD quad-core processor, but a platform, as we will describe in this article. Check it out.

Intel released their first quad-core processor, Core 2 Extreme QX6700, in November 2006. At the beginning of this year Intel has already announced three more quad-core CPUs: Core 2 Quad Q6600, Xeon X3220 and Xeon X3210. So far AMD didn’t release any quad-core CPU.

The solution found by AMD to counter-attack Intel’s quad-core CPUs was to release Quad FX platform, formerly known as 4×4. This platform uses two dual-core Athlon 64 FX CPUs and several other “heavy” specs, as we will see in the next pages. Thus a Quad FX PC has two dual-core physical processors working in parallel, for a total of four CPU cores on the system. These two CPUs are interconnected using a dedicated coherent HyperTransport bus. In summary, Quad FX platform is essentially a SMP (Symmetrical Multiprocessing) system with two dual-core Athlon 64 FX CPUs.

It is important to note that from the software point of view dual-core and quad-core technologies are also symmetrical multiprocessing technologies.

On Intel quad-core CPUs the cores are organized in pairs. The cores of each pair can exchange information directly between them – the same way it happens on dual-core CPUs from AMD and on Core 2 Duo CPUs from Intel – but in order to exchange information with any one of the cores located on the other pair they need to access the CPU external bus – what is exactly what happens on Quad FX platform, where the CPUs talk to each other using an external bus, the coherent HyperTransport bus.

In order to clarify the difference between Quad FX platform and the architecture used by Intel quad-core processors, see Figures 1 and 2.

Intel Quad CoreFigure 1: Architecture currently used by Intel quad-core CPUs.

AMD Quad FXFigure 2: Architecture used by Quad FX platform from AMD.

As you can see comparing Figures 1 and 2, Quad FX platform has some advantage on memory access. On Intel quad-core CPUs the CPU external bus (a.k.a. FSB, Front Side Bus) is used for accessing the RAM memory, other devices present on the PC and for the communication between each pair of cores. The communication between each pair of cores can be done up to 8 GB/s.

On Quad FX platform the CPUs use a dedicated communications channel (the coherent HyperTransport bus), which transfers data up to 4 GB/s in each direction. What is important here is that the HyperTransport bus provides two communications channels, one in each direction. Also, since on AMD processors the memory controller is embedded on the CPU, the memory is accessed using a dedicated bus, separated from the channel used by the CPU to access the rest of the PC.

As Quad FX platform uses symmetric multiprocessing architecture, each CPU accesses its own RAM memory. Processors used on Quad FX platform can also access the memory that is controlled by the other CPU, as we will explain in the next page.

A detailed explanation about Intel’s quad-core architecture can be found on our article Intel Quad-Core CPU Overview and Roadmap.

Let’s see the details of this new platform.

[nextpage title=”Platform Details”]

 As we have already said, the two processors used on Quad FX platform are interconnected using a HyperTransport bus. Because of that, a CPU to be compatible with Quad FX platform would need at least two HyperTransport busses: one for interconnecting the two CPUs and another to connect the CPU to the chipset. As Athlon 64 FX based on socket AM2 have only one HyperTransport bus, AMD had to launch a new version of Athlon 64 FX processors for Quad FX platform. These new Athlon FX models start with the number 7 and three models were released so far: FX-70, FX-72 and FX-74. These new processors use the new socket F (1,207 pins) pinout, which was originally targeted to the new Opteron processors, and three HyperTransport busses. The main technical specs of these new processors are:

  • Dual-core technology
  • 128 KB L1 cache (64 KB for data + 64 KB for instructions) per core
  • 1 MB L2 cache per core
  • 90 nm manufacturing process
  • Socket F (1,207 pin)
  • DDR2 memory controller supporting DDR2-533/667/800 under dual channel mode
  • Three HyperTransport busses working at 1 GHz (4 GB/s) each. This clock can also be referred as “2.000 MHz”

In the table below you can see the details of the Athlon 64 FX-7x released so far.

OPN (Tray) OPN (Box) Model Clock TDP Max. Temp. (ºC) Voltage
ADAFX74GAA6DI ADAFX74DIBOX FX-74 3 GHz 125 W 56 1.35 V – 1.40V
ADAFX72GAA6DI ADAFX72DIBOX FX-72 2.8 GHz 125 W 63 1.35 V – 1.40V
ADAFX70GAA6DI ADAFX70DIBOX FX-70 2.6 GHz 125 W 63 1.35 V – 1.40V

You will have two buy two identical processors to use them on Quad FX platform. You cannot use an Athlon 64 FX-74 together with an Athlon 64 FX-70 on the same motherboard, for example.

Regarding the RAM memory, as we explained in the previous page, each processor controls its own RAM memory. Thus in order to configure dual channel mode correctly on the platform you will need at least four identical memory modules (two for each CPU) and not two as usual (in fact this is another reason this platform is called Quad FX or “4×4” as it was called before its official release). In theory the installation of more RAM memory would be done in multiples of four modules, but the reference motherboard for this platform has only four memory sockets.

Also, the memory controller found on Athlon 64 FX-7x CPUs support a technology called NUMA (Non-Unified Memory Architecture), which allows one processor to access the memory controlled by the other processor through the coherent HyperTransport bus that interconnects the two CPUs. The difference between a coherent HyperTransport bus and a regular HyperTransport bus is exactly the ability of carrying memory information by the former.

The rest of the features found on Quad FX platform isn’t set by the processors but by the chipset. Let’s talk about it.

[nextpage title=”Chipset Details”]

The chipset chosen to be used on Quad FX platform was nForce 680a SLI from NVIDIA. The features described below are from this chipset, not being set by the Athlon 64 FX-7x processors. We don’t know if AMD plans to use a different chipset with this platform.

We commented previously that each Athlon 64 FX from 7x series has three HyperTranspor busses. One was used for the communication between the two processors but instead of using only one HyperTransport to connect the CPU to the chipset – and from the chipset to the other peripherals found on the PC that we didn’t mention so far, like the video card and the hard disk drive – AMD and NVIDIA decided to use two busses, each one connected to a nForce 680a SLI chip, allowing an amazing number of features

On Figures 3 and 4 you can see the block diagram of Quad FX platform showing all features available.

AMD Quad FXFigure 3: Quad FX plaform block diagram (AMD’s slide).

AMD Quad FXFigure 4: Quad FX plaform block diagram (NVIDIA’s slide).

The main features from Quad FX platform using NVIDIA nForce 680a chipset are:

  • Two x16 PCI Express slots working at x16 and supporting SLI mode
  • Two x16 PCI Express slots working at x8 and not supporting SLI mode
  • One x1 PCI Express slot
  • One regular PCI slot
  • Four Gigabit Ethernet ports
  • 12 SATA-300 ports
  • Four ATA-133 ports
  • 20 USB 2.0 ports
  • RAID (0,1,0+1,5, JBOD)
  • 7.1 high-definition audio

Even though QuadFX motherboard has for x16 PCI Express slots two of them work at x8. Besides that, only the ones that truly run at x16 support SLI mode. With four video cards installed, you can have a total of eight displays connected to the PC, four of them connected to two video cards running under SLI mode.

The first Quad FX-compatible motherboard released was ASUS L1N64-SLI WS. You can see the details of this motherboard in Figure 5.

AMD Quad FXFigure 5: ASUS L1N64-SLI WS motherboard.

[nextpage title=”Conclusions”]

Quad FX platform is a provisory solution from AMD to compete with Intel quad-core CPUs, since AMD doesn’t have yet technology to build CPUs with more than two cores. Nowadays Quad FX platform supports two CPUs (four cores), but AMD has already announced their plans to release on the second half of this year an eight-core platform – probably using two four-core CPUs.

We haven’t the opportunity yet to benchmark AMD’s Quad FX platform in order to compare its performance to a system using Intel’s top quad-core CPU. This would be a very interesting review that would tell us what is the best buy, a Quad FX system or a quad-core Core 2 Extreme one.

Quad FX platform brings as advantage an amazing number of extra ports that today can’t be found on an out-of-the-box PC based on Intel’s quad-core CPU, like four PCI Express x16 slots, four Gigabit Ethernet ports, 20 USB 2.0 ports and 12 SATA-300 ports. Of course it is also possible to build an Intel-based PC with four Gigabit ports, 20 USB 2.0 ports and 12 SATA-300 ports, but you would need to install add-in cards to expand the number of ports available by your motherboard.

If Quad FX will be a winner or a flop isn’t certain, only time will tell.