Processors based on AMD64 architecture – such as Athlon 64, Athlon 64 X2, Athlon 64 FX, Opteron, Sempron and Phenom – have two external busses. One is used on the communication between the CPU and the memory, and it is simply called “memory bus”, and the other is used on the communication between the CPU and all other PC components thru the motherboard chipset and is called HyperTransport – being an I/O (Input/Output) bus. In this tutorial we will be explaining how the HyperTransport bus works and clarifying common mistakes people assume about this bus.

On all other processors – including AMD processors not based on AMD64 architecture, like the original Athlon, Athlon XP and Sempron socket 462 processors – the CPU has only one external bus, also known as front side bus (FSB). On this approach the external bus carries both memory and I/O communications.

Theoretically the architecture used on AMD64 processors is better, since in theory they can communicate with the memory and with other PC components (like the video card) at the same time, something impossible on other processors, as there is only one datapath out of the processor.

On Figure 1 you can see how an AMD64 processor communicates to the external world. The “bridge” chip is the motherboard chipset. Depending on the chipset you can have one or two chips. On two-chip solutions all peripherals (such as hard disk drives, add-on cards, sound cards, etc) are connected to the second chip (this second chip is called south bridge, not shown on Figure 1), while on single-chip solutions everything is connected to this single chip.

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Figure 1: Location of the HyperTransport bus on AMD64 processors.

AMD CPUs targeted to servers – i.e. Opteron processors – can have one, two or three HyperTransport busses, depending on the model. These extra busses are used to interconnect several CPUs allowing them to talk to each other, i.e. used on servers with more than one CPU on the motherboard. Since desktop and notebook CPUs does not support this kind of configuration there is only one HyperTransport bus on them.

For a more in-depth explanation of AMD64 architecture, please read our Inside AMD64 Architecture tutorial.

Besides providing AMD64 processors with separated datapaths for memory and I/O, HyperTransport brings another advantage: it provides separated links for the CPU input and output operations, allowing the CPU to transmit (“write”) and receive (“read”) I/O data at the same time (i.e. in parallel). On the traditional architecture using a single external bus since the external bus is used for both input and output operations reads and writes cannot be done at the same time.

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Figure 2: The HyperTransport bus provides separated input and output datapaths.