DDR or SDRAM or RDRAM

[Chrissirhc]

Hello, can someone sum up the differences between DDR SDRAM RDRAM in a succinct way?

My understanding SDRAM today has a bandwidth of 133hz with a data rate of 900Mb/s???? and DDR effectively doubles the bandwidth because data can travel both ways the same time???(I need help with that point). RDRAM I haven't got a clue.

Thanks

Chris

 

[rjkrash]

check out

http://www.webopedia.com/TERM/S/SDRAM.html

The two rules for success are:
1. Never tell them everything you know.

 

[cdogg]

That link above is good for a quick reference, but here's a website that goes into good detail of the difference between SDRAM and RDRAM:

http://www.hardwarecentral.com/hardwarecentral/reviews/1787/19/

As far as DDR-SDRAM, check out:

http://www.hardwarecentral.com/hardwarecentral/reviews/2445/1/

*********************

After reading both, you can compare and contrast the advantages of both RDRAM and DDR SDRAM

 

[paparazi]

You must also realize some important issues about Motherboard chipset performance.
Basically when DDR came out there wasn't a great deal of differance in performance between a SDR System and a DDR one!! reason for this was that the motherboard chipsets were NOT allowing the DDR memory to perform to it's maximum capacity!!so DDR systems were at best 10% quicker and at worst no better than SDR.
I can happily report that in the last 6 months things have improved alot.
I will cut to the chase!!
The fastest DDR chipset is the VIA KT266a (note*a) with this chipset on your Motherboard expect a 15% performance gain over the equivillant SDR system.
It is also worth noting that SIS has a BUDGET offering in the shape of it's 635 chipset, nearly as quick and half the price and can be found on some cheaper but very fast motherboards from the ECS Elite group.
Pitfalls*** to avoid. ALI magic chipset equipt boards
The older Via KT266, which was very disapointing.
AMD 760 Northbridge with Via southbridge, common choice for many board manufactures untill recently with fair performance only.
Hope this has helped. Martin

 

[paparazi]

RDram is the king with it's extremely wide band width, sorry don't know the specs off the top of my head but I know it's something in excess of 4 times faster than DDR.
So why aren't RDram systems ruling the PC market?
Two reasons firstly this type of ram is very expensive and secondly it is only supported by Intels Pentium 4, and as we all know BANG for BUCK the P4 is knocked well and truely into touch by the Athlon/Palemino processors from AMD.
I also think there is another issue here, Ram is just one aspect of a system, we have to choose a set of ballanced components based around a given platform either AMD or Intel, It is the choice of platform which ultimately decided which Ram type you end up with.
Interestingly this has been mixed up a bit with the introduction of SDR and soon to be DDR chipsets that will support the P4, these should give the Intel chip a fighting chance against the AMD Palemino, not so much in the performance stakes but hopefully the P4 might start to compete directly in the VALUE market. Martin Vote if you found this post helpful please!!

 

[Jakespeare]

SiS chipset is the 735 (not the 635)

 

[cdogg]

Just to clear one thing up. RDRAM actually runs internally at double the bus speed of the system. The 400 MHz is the speed of the frontside bus. The internal clock speed of RDRAM is actually 800MHz, or it's "effective" speed. Hence PC800!!

http://www.amptron.com/html/ProductNews/RDRAM.html

Now, you'll notice that the bandwidth is greater in DDR than in RDRAM when comparing a single channel. However, RDRAM is coupled into a parallel configuration, so the bandwidth is actually 3.2 GB/sec (1.6 x 2).

The real advantages to RDRAM are noticed only when using bandwidth-intensive apps and dealing with large files (such as for Movie or Sound editing). Even though RDRAM has the better benchmark, the margin of victory is too narrow to want to spend the extra buck!

 

[Chrissirhc]

Didn't really expect to get many responses to this question glad I posted it though.

Originally my intention was to get an easy to understand yet low level explanation of why DDR is faster than say SDRAM (but I guess it is good to hear answers at a high level as well). I'm confused with how the bus speed bandwidth relates the max data rate. And this is so with hard disks and pci/agp bus as well. I expect it will be universally applied.

Looking at the link (cdogg's) it seems that the link is quite simple it is the bus width i.e how many bits that can travel at the same time. I assume it is one bit per a cycle. Therefore getting the data rate is simple bandwidth*buswidth.

For DDR it just says it transfers at the rise and fall of each cycle (something brought up previously) I still don't get that I would have thought that you can only encode one bit i.e. a 1 or 0 by having a full cycle. E.g start with high voltage or start with no voltage.

Then looking back at some networking stuff I see that the max data rate is 2 times the bandwidth, in which case why didn't all ram work like this.

Can anyone clear up any of my misunderstandings?

Cheers

Chris

 

[cdogg]

Here's a follow-up article of the one I posted previously. It better explains the "latency" effect of RDRAM vs. SDRAM:

http://www6.tomshardware.com/mainboard/00q1/000308/fsb-133-2-04.html

Here's a good place to start when it comes to "how" DRAM actually works:

http://www.howstuffworks.com/ram.htm