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The Divider Problem

AM2: AMD Reinvents Itself
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The DDR2 memory speeds we set, such as 736 MHz for the Athlon 64 X2 4400+, weren't chosen at random, but were assigned by the CPU.

Looking for a moment at the operation of the DDR1 interface on the old Socket 939, we see that the processor converts the CPU clock speed using a divider, in order to come up with the correct speed to use for addressing the memory. Internal memory interfaces have used DDR400 (200 MHz) since they were invented.

Athlon 64 X2 4200+: 2200 MHz / 11 = 200 MHz (DDR400)


Athlon 64 X2 3200+: 2000 MHz / 10 = 200 MHz (DDR400)

For this reason, AMD only sells CPUs with even clock speeds, which are divisible by 200 MHz, in its range.

With the switch to DDR2, AMD runs up against a problem: DDR2-800 has a clock speed of 400 MHz and can therefore no longer be divided by all previously envisaged CPU clock speeds. And the highest speed possible is not divisible by 400.

But how does the CPU react if a memory timing is set which cannot be divided by the CPU clock speed?

AMD came up with something smart here: if a divider is produced that exceeds the next possible JEDEC-compliant standard memory clock (400, 533, 667, 800), the next lower divider is selected automatically.

Athlon 64 X2 4800+: 2400 MHz / 6 = 400 MHz (DDR2-800)


Athlon 64 X2 4000+: 2000 MHz / 5 = 400 MHz (DDR2-800)
Athlon 64 X2 5000+: 2600 MHz / 7 = 371 MHz (DDR2-742)
Athlon 64 X2 4400+: 2200 MHz / 6 = 366 MHz (DDR2-733)

This produces highly curious memory clocks like DDR2-742 and DDR2-733. The automatic selection of the divider cannot be deactivated or influenced.

We therefore advise the ambitious user to grab a pocket calculator before making a purchase, and check whether the divider for the CPU tallies or not. It may be that the dependence of the memory interface on clock speed means that, despite its 200 MHz higher speed, a CPU may actually be slower when running some applications. For example:

Athlon 64 X2 4200+: 2200 MHz with DDR2-733


Athlon 64 X2 4000+: 2000 MHz with DDR2-800

If you now consider the fact that the lower memory clock speed already forced on the 2200 MHz CPU is compounded by a cache merely half the size of the less expensive 4000+ at 2000 MHz, you have to ask yourself: who can really understand all of this?

An overview of all possible memory configurations at standard clock speeds

The memory clock speeds in red are not standard and are therefore rounded down, resulting in a performance loss.
Summary
  1. The AMD Generation Comes Of Age
  2. New Socket AM2 With DDR2
  3. Memory Speed: Socket AM2 Vs. Socket 939
  4. Write Performance
  5. Write Performance, Continued
  6. Write Performance, Continued
  7. Memory Speed: AMD Vs. Intel
  8. Memory Speed: AMD Vs. Intel, Continued
  9. The Divider Problem
  10. DDR2 SLI Memory: 10.3 GB Per Second
  11. Automatically Overclocking SLI Memory
  12. Automatically Overclocking SLI Memory, Continued
  13. All New AM2 Processors
  14. The New Socket AM2 With 940 Pins
  15. New Retention Module Requires New Heatsink
  16. Low Dissipation Guaranteed
  17. New Energy-efficient Processors
  18. Heatsinks: Things Gets Louder, Continued
  19. Features: Virtualization And TCPA
  20. Price Comparison: AMD More Expensive Than Intel
  21. Motherboards
  22. Gigabyte
  23. Epox
  24. The New nForce5 Chipset For AM2
  25. LAN Interface With 2 Gbit/s
  26. Test Setup
  27. Benchmarks And Settings
  28. Benchmarks Socket 939 Vs. Socket AM2
  29. Games - DirectX
  30. Games - DirectX, Continued
  31. Video
  32. Video, Continued
  33. Video, Continued
  34. Video, Continued
  35. Audio
  36. Applications
  37. Applications, Continued
  38. Applications, Continued
  39. Multitasking, Continued
  40. Synthetic Benchmarks
  41. Synthetic Benchmarks, Continued
  42. Synthetic Benchmarks, Continued
  43. Synthetic Benchmarks, Continued
  44. Synthetic Benchmarks, Continued
  45. Evaluating The Benchmark Scores
  46. Athlon 64 X2: Now Somewhat Slower
  47. Benchmarks AMD Vs. Intel
  48. Games - DirectX
  49. Games - DirectX, Continued
  50. Video, Continued
  51. Audio
  52. Applications
  53. Applications, Continued
  54. Multitasking
  55. Synthetic Benchmarks
  56. Synthetic Benchmarks, Continued
  57. Synthetic Benchmarks, Continued
  58. Synthetic Benchmarks, Continued
  59. Synthetic Benchmarks, Continued
  60. Synthetic Benchmarks, Continued
  61. AMD Athlon 64 FX-62 Vs. Intel Extreme Edition 965
  62. Conclusion: Good Energy Efficiency, But A Bad Value For Money
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