Hyper Transport

When AMD designed the Athlon 64, it was a big step in a new direction that really set it apart from its predecessors. The 64-bit instruction set was certainly its most unique feature. However, there are numerous other features, which are relevant to overclocking that we'll discuss. New acronyms like LDT, IMC and HTT have scared many 'old school' overclockers. But, once you read a bit about these new features, you'll realize that there is nothing daunting about these new platforms and you'll be ready to get down to business.

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AMD's A64 platforms have abolished the 'Front Side Bus'. The 'Front Side Bus' was essentially a data bus that carried data to and from system components and the CPU (usually by connecting the CPU to the north and southbridge chipsets). These chipsets provide connections to other buses, such as the AGP and PCI bus, and many other system components. All current Intel platforms, and pre-A64 AMD chips follow this basic model.

AMD decided to do things a little differently with the A64, and adopted 'Hyper Transport Technology'. Hyper Transport is a high-bandwith, low latency computer bus that replaces the aging FSB. The Hyper Transport bus does essentially the same thing as the FSB, only much faster. Many people find themselves still calling it FSB, but for the sake of correctness, we'll call it the HTT bus.

There is a common misconception that Hyper Transport Technology is a proprietary AMD technology. HTT was developed by the 'HyperTransport Technology Consortium'. Hyper Transport, sometimes called LDT (Lightning Data Transport) has been used by many vendors, including nVidia and Cisco Systems. You may recall that nVidia used HTT to provide high bandwidth, low latency communication between the north and southbridge chipsets in their older socket A 'nforce' platforms.

The Hyper Transport bus operates using a multiplier system to derive its overall speed. The 'base' or lowest HTT frequency that the HTT Consortium defined is 200MHz. The overall operating clock speed that the HTT operates at is simply a multiple of that 200MHz base or 'reference' frequency. Many other clock frequencies are also derived from this 200MHz reference clock, such as CPU clock speed. Most Socket 754 A64s, for example, operate at an HTT speed of 800MHz. A clock multiplier of 4x was used to obtain this. So 200x8 = 800MHz. You may be asking why AMD lists an HTT speed of 1600MHz in the specifications for these processors. HTT is a 'double pumped' or 'double data rate' technology, much like DDR RAM. So, simply double the final result. 200x8 = 800MHz x 2 = 1600MHz. I'll get more into A64 Mathematics later on.