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Double Power, DDR2 Memory Controller, And Variable Memory Bandwidth

AMD's Dual Core Laptops Have Arrived: Introducing the Turion 64 X2
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The most important new features of the Turion 64 X2 are the two processor cores and the integrated memory controller. Unlike its predecessor, the Turion 64, the memory controller supports dual-channel operation and the use of fast, power-saving DDR2 memory, resulting in a theoretical maximum memory bandwidth of 10.7 GB/s (Turion 64: 3.2 GB/s).

The value is "theoretical" because it depends on the memory clock rate - DDR2-667, DDR2-533, or DDR2-400 - and on the CPU clock (a peculiarity of the AMD processor architecture). The following example briefly demonstrates how this works, and points out the positive and negative effects of this technology.

High Performance Only With The "right" Memory

The processor's clock generator delivers a base rate of 200 MHz; the CPU clock rate is generated from this base rate using an integer multiplier. For example, the multiplier 10 delivers a frequency of 2000 MHz on the Turion X2 TL-60. However, in line with JEDEC's DDR2 specification, the SO-DIMM memory modules of the laptop and the associated interface currently only support maximum rates of 400, 533, or 667 MHz.

To make sure that the RAM and the interface comply with the specification, the high CPU clock rate must be reduced by a frequency divider. In the case of the Turion X2 TL-60, the divider is 6 for DDR2-667 MHz. Accordingly, the memory is operated at 333 MHz in line with the specification.

However, not every Turion 64 X2 CPU has a divider that enables operation of the installed memory with the specified clock rate. Therefore, the actual memory clock rate may be much lower than specified. For example, a Turion 64 X2 TL-56 used with DDR2-667 SO-DIMMs delivers a real memory clock rate of only 300 MHz (DDR2-600 MHz). Ultimately, the CPU selects the divider and the memory clock rate on the basis of the memory module's SPD data transmitted by the BIOS.

The following table shows the relationship between the utilized memory, the CPU model, and the resulting real memory clock rate.

Applied Memory Module
CPU-Model Core Clock DDR2-667 DDR2-533 DDR2-400
Turion 64 X2 TL-60 2000 MHz 667 MHz 500 MHz 400 MHz
Turion 64 X2 TL-56 1800 MHz 600 MHz 514 MHz 400 MHz
Turion 64 X2 TL-52 1600 MHz 640 MHz 533 MHz 400 MHz
Turion 64 X2 TL-50 1600 MHz 640 MHz 533 MHz 400 MHz
Idle Frequency 800 MHz 320 MHz 320 MHz 320 MHz

To avoid wasting money and performance, ask for the number and specification of the memory modules in the laptop you plan to purchase. As you can see in the chart, a Turion 64 X2 TL-60 shipped with DDR2-533 MHz memory only runs at 500 MHz. In contrast, the same laptop equipped with a Turion 64 X2 TL-56 processor and the same memory runs at 514 MHz - despite the fact that the Turion 64 X2 TL-60 is 200 MHz faster and $90 more expensive!

Thus, when using memory-intensive applications, the Turion 64 X2 TL-60 with DDR2-533 SO-DIMMs is barely faster than the far less expensive Turion 64 X2 TL-56 in combination with the same memory!

The connection between the CPU frequency and the real clock rate also has a positive side effect. The CPU clock rate changes dynamically depending on the load on the individual cores. When CPU load is low, its clock rate goes down to a minimum of 800 MHz, and the memory clock rate recedes to a minimum of 320 MHz. This smart technology helps save power.

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