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Intel Core 2 Duo - Test
by Franck Delattre et Marc Prieur
Published on July 4, 2006

Windows x64 & EM64T
Introduced by AMD in 2003, AMD64 ISA took a long time working its way onto the computer desktop market. It’s a 64 bit extension of the x86 instruction set. So, general registers, small memory areas, which temporary store memory addresses and whole numbers, are increased from 32 to 64 bits.

Intel released in early 2005, a comparable and compatible function, the EM64T, but this function was only available for Netburst and not Mobile. With Core, the EM64T is extended to all platforms.

Processing 64 bits data isn´t an innovation by itself. Since its introduction, x87, which is in charge of floating point calculations goes up to 80 bits internally. Also, some MMX/SSE/SSE2 instructions give us the possibility of working with 64 bit whole numbers. However, the use of this type of data is now generalised to all data, that is stored in the GPR and this bring two advantages :
  • An acceleration of calculations with whole numbers. Indeed, for applications requiring calculations with very significant whole numbers (the limit is 4.29e9 in 32 bits, and reaches 1.84e19 in 64 bits), encoding the whole number in 64 bits makes it possible for the processor to manipulate it more easily and faster without the necessity of having to double the number of registers and clock cycles required for calculations. This should only concern very specific applications such as data encryption or scientific calculations.

  • storing addresses in 64 bits makes it possible to exceed the 4 GB limitation due to 32 bit binary encoding and increases it to 256 terabytes because of a "limitation" at 48 bits for virtual memory coding. We noted however that Intel exceeded this 4GB limitation with the Xeon to reach 64 GB and this even if this mode has limitations. There again, this won´t be really too useful for most users.

  • In fact, the main benefit of EM64T like AMD64 is the number of registers. Indeed, in x86, processors have eight 80 bits x87 registers, eight general 32 bits registers and eight SSE 128 bits registers. Increasing the number of available registers makes it possible to restrict the number of instructions intended to free and copy the latter in memory and in consequence to increase performances.

    Finally, the release of EM64T AMD64 creates a break with the sacrosanct x86 compatibility. Many executables are still compiled with the x86 instruction set such as it was with the 386. There has been some improvement since, but they aren´t necessarily used by developers during compilation. From now on, improvements will be automatically included.

    What are performance gains in practice? To find out, we installed Windows XP on a Core 2 Duo E6600, Pentium D 950 and Athlon FX-60 x64 and tested three 32 bit software in 32 and 64 bits: Mathematica 5.2 (scientific calculations), CineBench 9.5 (3d rendering) and Far Cry (game).

    With Mathematica, performances were very variable since the gain was 2.7% for Core, 8.6% for Netburst and the speed of the K8 was reduced by 2.9%. Cinebench provided better results with AMD with a performance gain of 11.5% as compared to 8.6% with the Pentium D and 4.6% for the Core 2 Duo. It was finally with the Pentium D that Far Cry benefited the most from the 64 bits with 6.5% improvements, as compared to 3.2% for the Athlon 64 FX and 0.3% (!) for the Core.

    Performances were very variable from one processor to another and we even observed a performance drop in one case. Overall (and ironically), the Pentium 4 provided the most homogeneous performance gains. This could be explained by the presence of the Trace Cache that stores instructions as they are decoded.

    On the contrary, with more classic architecture such as the Core or K8, instructions are stored before decoding in the L1I, whereas AMD64 and EM64T have a negative impact on decoding performances, because these instructions are coded on more octets than standard x86 instructions. This increases the decoding load. It seems by the way that with the Core 2 Duo many cases of fusion aren´t activated in 64 bits.

    Of course, this load for decoders is generally compensated by the diminution of the total amount of 64 bit instructions. However, in the end, performances gains can change from one architecture to another. In fact, Core seems to benefit less than other architectures from the 64 bits. This isn´t really that dramatic as 32 bit performances and overall low performance gains obtained with 64 bits regardless of the architecture.
    Test configurations
    After specific tests, we put the Core 2 Duo through our usual test suite. For all DDR2 configurations we used DDR2-667 4-4-4-12, as well as DDR2-800 4-4-4-12 for the highest end AM2 and Core 2 Duo solutions.

    We used the following configurations:

    Common :
    - ATI Radeon X850 XT PE
    - 2 x Raptor 74 GB
    - Windows XP SP2 French

    Intel Socket 775 Core 2 Duo :
    - ASUSTeK P5W DH (i975X) motherboard
    - 2 x 512 MB DDR2-667 4-4-4
    - 2 x 512 MB DDR2-800 4-4-4

    Intel Socket 775 :
    - ASUSTeK P5WD2-E motherboard (i975X)
    - 2 x 512 MB DDR2-667 4-4-4

    Intel Socket mPGA479 :
    - Gigabyte GA-I8I945GTMF-YRH motherboard
    - 2 x 512 MB DDR2-667 4-4-4

    AMD Socket AM2 :
    - ASUS M2N32-SLI Deluxe motherboard
    - 2 x 512 MB DDR2-667 4-4-4
    - 2 x 512 MB DDR2-800 4-4-4

    AMD Socket 939 :
    - ASUS A8N SLI Premium motherboard
    - 2 x 512 MB DDR-400 2-2-2

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