Why are memory modules still only 64-bit?

[NiHaoMike]

The first SIMMs were 8-bit. So in a 32-bit PC, they had to be installed in sets of 4. Then came the 32-bit SIMMs that could be installed one at a time.

That worked until the Pentium, which has a 64-bit bus. In that case, SIMMs had to be installed in pairs. Then with the Pentium 2 also came the 64-bit DIMMs, which could be installed one at a time.

For a long time, that remained enough. Then dual channel chipsets came out and it's back to installing in pairs. With modern high end CPUs, DIMMs often have to be installed in sets of 3 or 4.

So why aren't memory modules expanding to 128-bit or even 256-bit? The move to DDR2 and DDR3 was a perfect time to do that.

 

[wingman99]

Maybe they think the bus with is fine so they just need to work on memory speed.

Allot of data is wasted when the data you need per fetch is less than 64bit.

 

[NiHaoMike]

If a 64-bit bus is enough, we wouldn't have multiple channel motherboards.

 

[wingman99]

Take a look after the dual channel there is no benefit. http://www.legitreviews.com/article/1779/3/

 

[Bobnova]

Think about how long they would have to be. Now think about designing a pcb that keeps all signal traces within 1mm of each other in length. That right there is my guess. Motherboard manufacturers would say 'lolno'.
Alternatively you could increase pin density like sodimms, and force them to use more layers. To which they say lolno again.

 

[RGone]

Now think about designing a pcb that keeps all signal traces within 1mm of each other in length. That right there is my guess. Motherboard manufacturers would say 'lolno'.

That trace length thing is exactly right. A number of years ago, DFI came with a second REV of a mobo just to address the trace length situation The later REV was a much better motherboard for benching.
RGone...