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I was reading that in old i386 based systems, the ram bus used to be the same speed as the CPU but when CPU frequencies increased, it became too hard to make RAM the same speed. Why can't we have 2GHz RAM? Is there some component that can't scale?

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At one time it was easy to make a bus that would handle the really low speeds CPUs and memory ran at. Since CPUs occupy a much smaller space, it was easier to make them run at the higher frequencies and run the memory and its bus at a lower attainable frequency. As the technology improves for creating motherboards and the faster memory gets cheap enough, you see the memory bus speeds increase to parity with the processors. You can have 2GHz ram, you just have to pay for the motherboard and memory sticks that are capable of this. –  Fiasco Labs Oct 2 '11 at 6:29
    
@FiascoLabs, can you make your comment into an answer? There's no accept button on comments and I think your answer is the best. –  MattSmith Oct 3 '11 at 11:03
    
Done! See below. –  Fiasco Labs Oct 6 '11 at 2:45
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At one time it was easy to make a bus that would handle the really low speeds CPUs and memory ran at. Since CPUs occupy a much smaller space, it was easier to make them run at the higher frequencies and run the memory and its bus at a lower attainable frequency. As the technology improves for creating motherboards and the faster memory gets cheap enough, you see the memory bus speeds increase to parity with the processors. You can have 2GHz ram, you just have to pay for the motherboard and memory sticks that are capable of this.

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Historically, memory clock speeds were actually faster then the processor clock speed, so the processor would run at a fraction of the memory clock (which would allow for reads and writes to be performed in a single cycle). This situation did change beginning in the early 90's.

As of writing this, we do have memory modules operating in the +2 GHz range. My new build uses 1866 MHz memory, and I have run the system with the memory at 2100 MHz (although you do need to reduce the memory response time to compensate for the increased clock speed).

Now, clock speed is only a small part of the battle when it comes to RAM (like most other electronics). A simple thing to look at is the width of the data bus. If you have some simple RAM clocked at 100 MHz, and my data bus is 64 bits wide, I can get up to 800 MB/s transfer rate. However, if I had a 128-bit data bus, I could transfer up to 1600 MB/s.

This comes back to the whole "clock speed vs. performance" debate - there is more then you can determine just via how fast the clock runs. There's also a technology difference. Whereas most components in a CPU are discrete transistors (including the memory in the CPU, which is SRAM, not DRAM), most memory modules are dynamic RAM.

DRAM uses far less transistors in it's design, but to accomplish this, requires that a capacitor hold some charge to keep the bit held in the transistor. This also requires DRAM to be periodically refreshed, limiting the maximum clock speed (if only the laws of physics supplied us with perfect insulators...).


And, if you're wondering why we use DRAM instead of SRAM, the issue is cost. SRAM is really fast and really expensive, and comparatively, DRAM is slower (although more than fast enough for our needs) albeit significantly cheaper. This is why SRAM is used in your processor's cache in a relatively low capacity, while DRAM is used for the computer memory in a much bigger capacity.

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Although it does as told by David...

It's not necessary given that not every instruction executed needs to access the memory.

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We do. Kingston Hyper-X DDR3 RAM and Corsair's latest top out at 2,533 MHz.

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