DDR2 @ 400Mhz?

[godbrother]

Hi,

I always found this a little strange. When I first ordered my Inspiron 1300 Laptop from Dell 5-6 years ago, it came with DDR2 ram however it was running @ only 400Mhz. I just assumed this was the lowest DDR2 RAM available at the time (according to Wiki, they had actually made PC2-3200)

Last week I ordered some Kingston HyperX DDR2 RAM for my laptop, I received and fitted it and to my surprise, it is still running at 400Mhz (in the BIOS)

What does this mean exactly? Dell made a motherboard using old hardware but with a new interface? If the max the board supported was 400, then why not simply build it on the older DDR1 platform?

 

[C12Friedman]

I take it you are running DDR2 PC2-6400 ram. Because your ram is double data rate (DDR), BIOS, CPU-Z and a few others will report your single rate speed (such as 400MHz for PC2-6400 ram). But doubling that number you get 800, the rated MHz of the ram. You really are running at 800MHz even though it is reported to you as 400.

 

[4745454b]

That, or you really are running DDR2-400. If its hard coded in the bios to run at 200MHz actual/400MHz effective then adding ram won't change it. You can go into the bios and change this if this is the case.

 

[Pinhedd]

Hi,

I always found this a little strange. When I first ordered my Inspiron 1300 Laptop from Dell 5-6 years ago, it came with DDR2 ram however it was running @ only 400Mhz. I just assumed this was the lowest DDR2 RAM available at the time (according to Wiki, they had actually made PC2-3200)

Last week I ordered some Kingston HyperX DDR2 RAM for my laptop, I received and fitted it and to my surprise, it is still running at 400Mhz (in the BIOS)

What does this mean exactly? Dell made a motherboard using old hardware but with a new interface? If the max the board supported was 400, then why not simply build it on the older DDR1 platform?

Memory ratings are fun... not.

There are three different but closely related numbers that you need to be aware of.

First is the IO bus transfer rate. This is the number of memory transfers that are made from the DRAM chip buffers to the memory controller per second.

Second is the IO bus clock frequency. This is the number of physical oscillator cycles that the IO bus reference clock performs in one second.

Third is the core memory clock frequency. This is the number of core memory operations that each DRAM chip performs in one second.

SDR SDRAM (Single Data Rate) transfers one memory word per IO bus clock and has one IO bus clock per core memory clock.

DDR SDRAM transfers two memory words per IO bus clock (rising and falling edge) and has one IO bus clock per core memory clock for a total of two transfers per core clock cycle.

DDR2 SDRAM transfers two memory words per IO bus clock and has two IO bus clocks per core memory clock for a total of four transfers per core clock cycle.

DDR3 SDRAM transfers two memory words per IO bus clock and has four IO bus clocks per core memory clock for a total of eight transfers per core clock cycle.

Through some simple math we can derive that DDR3-1600, DDR2-800, DDR-400, and SDR-100 have 1600, 800, 400 and 100 million IO bus transfers per second. They also have IO bus reference clock frequencies of 800 Mhz, 400 Mhz, 200 Mhz and 100 Mhz respectively. Lastly, they have core memory clock frequencies of 200 Mhz, 200 Mhz, 200 Mhz, and 100 Mhz respectively.

By looking at the last number you can see that SDRAM really hasn't gotten significantly more responsive over the years. Manufacturers have gotten better at scaling it horizontally but the real time latency hasn't changed significantly.

Marketing teams have however gotten better at selling memory. There's no standard way for motherboard manufacturers to report memory operating speeds. Some report it as the transfer rate, others report it as the IO bus frequency. This can lead to confusion such as this in which the generation spreads to the point where the IO bus frequency and the transfer rate overlap numerically, as is the case with DDR2-400 which has a transfer rate of 400 Megatransfers per second, and DDR2-800 which has an IO bus frequency of 400 Megahertz.