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In Detail

At the heart of Athlon 64 are two core technologies, AMD's AMD64 architecture and HyperTransport. The former, specifically designed for AMD’s 64bit range of processors requires the latter as a fundamental part of its design.

Every processor architecture is given a name or model number, although traditionally people have been able to class processors as either RISC (Reduced Instruction Set Computer) or CISC (Complex Instruction Set Computer).
However recent processor design is blurring the gap between the two with Intel's Itanium using VLIW (Very Long Instruction Word). Manufacturers have their own names for specific architectures, ignoring references to RISC or CISC.
Intel's Itanium architecture is called IA-64 and thus AMD spawned x86-64, although this was later changed to AMD64.

The AMD64 builds on current x86 architecture which has been present in the vast majority of desktops computers over the past decade. To that, AMD has added extensions enabling programmers to easily produce programs that take full advantage of the Athlon 64.
A similar technique of adding extensions was applied to 16bit processors when elevating them to 32bit.
The key advantage of adding new technology on top of existing foundations is compatibility with current 32bit applications.

Although x86 is by far the most popular architecture it certainly isn't pretty from a chip designer's point of view. By building on top of x86 architecture, AMD has inherited many of it’s less savoury traits. But any problems with using the x86 architecture as a base were AMD’s and will not in any way bother the end user. All the PC buyer will see is a processor that works, just like any other that they’ve used in the past.

One of the main reasons why Intel's Itanium 64bit processor had a slow take-up was due to a lack of software support.
Itanium, unlike the Opteron or Athlon 64 can only run 64bit applications and this meant that Intel had to spend millions on providing extended support to key developers. Although the cost of this support is really seen as a secondary issue next to time.
Applications take time to ’port’ to new architectures and there's nothing worse than buying into a system architecture that can run little or no software.

Developers will be happy to know that porting applications to run natively on AMD64 isn’t an arduous task. IBM for example, managed to port its relational database software suite, DB2, over to AMD64 in just two days.
Ease of use for consumers, developers and integrators is the order of the day for AMD and the Athlon 64.

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Probably the most interesting aspect of the AMD64 architecture is its onboard memory controller. This has several advantages but most prevalent is performance.
In current x86 systems the memory controller is situated on the "Northbridge", a chipset that is placed on the motherboard external to the CPU.
The Northbridge interfaces directly between the CPU, RAM and AGP slot. By bringing the memory controller physically closer to the processor you are reducing latency between the CPU and memory and thus increasing overall system performance. The Athlon 64 sports a radically designed memory management unit, which allows for large linear addressing capabilities, ensuring developers easy access to the vast amounts of memory that become available once 64bit mode is enabled.

Having a large amount of Level 2 cache means the core of the CPU has to be physically large. A physically large core raises the possibility of low production yields and consequently raises the overall cost. Cache size will play a part in the final pricing of this already large chip.

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To ensure backward compatibility AMD64 architecture has two master running modes, legacy and long mode. Legacy mode is active when you are running a 32bit operating system such as Windows XP where the Athlon 64 acts just like a Pentium 4 or Athlon XP, albeit faster.
Once you load an operating system that supports 64bit processors, long mode becomes active. This is the first of two criteria that have to be met in order for the Athlon 64 to operate in 64bit mode.

Within long mode there are two sub modes, 64bit and compatibility. If a program that hasn't been recompiled to take advantage of AMD64 technology is executed then it will be run in compatibility mode. The Athlon 64 will be seen as a legacy x86 protected environment and function perfectly well without the user requiring any special interaction.
When a program compiled specifically for the Athlon 64 is run the second criteria will be met, only then will you experience true 64bit mode.

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