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  • SharkyForums.Com - Print: Rambus technology

    Rambus technology
    By Sketch October 19, 2000, 11:02 AM

    All I have seen from Intel and Rambus dealing with RDRAM is marketing hype. Does anyone know how Rambus actually works? What differentiates it from SDRAM in terms of signalling, addressing, etc.

    By Arcadian October 19, 2000, 11:52 AM

    quote:Originally posted by Sketch:
    All I have seen from Intel and Rambus dealing with RDRAM is marketing hype. Does anyone know how Rambus actually works? What differentiates it from SDRAM in terms of signalling, addressing, etc.

    Rambus is more than a memory interface. It is a memory protocol. It sends data that descibes memory functions in addition to simple reads and writes. Unlike SDRAM, it contains configuration registers. It also has a 16bit data path as opposed to the 64bit data path that SDRAM uses. This allows for lower pin count, which is a good thing. To make up the bandwidth, Rambus is double data rate, and runs up to 400MHz, while SDRAM is only double data rate in the DDR versions, and it has a maximum frequency of 133MHz. In addition, it is easier to double Rambus bandwidth by adding additional channels, while SDRAM has a problem with pin count if you try to add channels. Hope this answers your questions.

    By Moridin October 19, 2000, 12:39 PM

    quote:Originally posted by Sketch:
    All I have seen from Intel and Rambus dealing with RDRAM is marketing hype. Does anyone know how Rambus actually works? What differentiates it from SDRAM in terms of signalling, addressing, etc.

    The basic difference is that RDRAM uses a serial interface between the RAM and the memory controller while SDRAM and DDR SDRAM use parallel interfaces. There are so many other differences it would be hard to describe them all completely, so instead I will point you to some excellent articles discussing memory technologies. You would do well to start at the beginning of each and work your way through until you get to the stuff on RDRAM since the background and theory are critical to understanding RDRAM.


    Hannibal's articles at ars

    part 1 http://arstechnica.com/paedia/r/ram_guide/ram_guide.part1-2.html

    part 2 http://arstechnica.com/paedia/r/ram_guide/ram_guide.part2-1.html

    part 3 http://arstechnica.com/paedia/r/ram_guide/ram_guide.part3-1.html


    Johan's articles at aces

    part 1 http://www.aceshardware.com/Spades/read.php?article_id=5000172

    part 2 http://www.aceshardware.com/Spades/read.php?article_id=5000183

    By Sketch October 19, 2000, 02:00 PM

    Thanks for the links. I'll check them out a bit later. I already have a good bit of background, as I am a recently graduated Computer Engineer and had to take a few hardware design courses and we delved into memory technology a good bit in one of them (a motherboard design class), but the most we talked about was SDRAM, RDRAM wasn't even mentioned.

    By Moridin October 19, 2000, 03:02 PM

    quote:Originally posted by Sketch:
    Thanks for the links. I'll check them out a bit later. I already have a good bit of background, as I am a recently graduated Computer Engineer and had to take a few hardware design courses and we delved into memory technology a good bit in one of them (a motherboard design class), but the most we talked about was SDRAM, RDRAM wasn't even mentioned.

    The links I gave are still well worth reading but I will try cover some of it.

    What it boils down to is that RDRAM has a serial interface that sends data in 16 byte packets on 16 data lines.

    It's channel starts at the chipset and runs through all the chips on that channel and then back to the chipset. (The closest analogy I can think of is a token ring network) The memory address you want to access is sent out on that channel and eventually reaches the chip that has the desired data. The data is placed on the bus and continues around the loop until it reaches the chipset. Unlike SDRAM each chip operates independently.

    The data bus itself is 16 bits wide and clocked at 400 MHz DDR for PC800. Therefore each packet is 10ns long (100 MHz). Since the channel is quite long the time it takes the signals to propagate along the channel contribute to the latency seen by the CPU. The time it takes to put the packet on the channel also contributes to the latency. On top of this, RDRAM does not support critical first word bursting which makes matters even worse.

    RDRDAM uses the same DRAM cells as SDRAM, but uses 16 or 32 banks each of which shares sense amps with its neighbors. (SDRAM uses 4 banks and each bank has its own sense amps) Since each chip is independent, this means that you can have up to 16 open pages per chip. SDRAM can have 4 per DIMM. The problem with this is that active chips (that have open pages) consume a lot of power so heat could be a problem keeping many banks open. In theory RDRAM supports hundreds of open pages, in practice the i820 supports 8 and the i840 supports 32. Most SDRAM chipsets support 4 but the BX chipset can support 8 under some conditions.

    (Data stored in an open page has lower latency then data stored in one that is not open, so more open pages means lower latency)

    I think that about covers it, I just hope I got all my terms right.


    By 100%TotallyNude October 19, 2000, 07:11 PM

    quote:Originally posted by Moridin:
    I think that about covers it, I just hope I got all my terms right.

    I bet you did, but does it matter? With DDR DRAM out, and cheaper than Rambus, is Rambus another solution in search of a problem?

    By Bash October 19, 2000, 11:48 PM

    quote:Originally posted by 100%TotallyNude:
    I bet you did, but does it matter? With DDR DRAM out, and cheaper than Rambus, is Rambus another solution in search of a problem?

    Last I checked DDR wasn't what I'd consider out & available to the general public. (i.e. not listed on pricewatch) Maybe I'm wrong... If it is indeed out, how much does it go for? You can get 800mhz Samsung RDRAM for $180. That's only about $35 more than the good name brand PC133.

    -Bash

    By Humus October 20, 2000, 07:57 AM

    DDR prices: http://www.crucial.com/ddr/order.asp

    By Bash October 20, 2000, 02:43 PM

    quote:Originally posted by Humus:
    DDR prices: http://www.crucial.com/ddr/order.asp

    The web site says, "Qualified hardware developers may order DDR SDRAM parts online by contacting Crucial's DDR sales team."

    i.e. not publically available.

    -Bash

    By Humus October 20, 2000, 05:02 PM

    quote:Originally posted by Bash:
    The web site says, "Qualified hardware developers may order DDR SDRAM parts online by contacting Crucial's DDR sales team."

    i.e. not publically available.

    -Bash

    Nope, but since there's actually no point in buying DDR until we see some mobos for it it doesn't matter much.

    By Elxman October 21, 2000, 12:51 PM

    only thing I think RDRAM is better than SDRAM is multitasking..

    By Intercollector October 23, 2000, 06:40 PM

    Everyone compares RDRAM to DDR DRAM for some reason. I'm not sure why people do it, other than the fact they are both next generation (post SDRAM) solutions. But everyone keeps saying how DDR DRAM is soooo much faster than RDRAM yet RDRAM came out about a year ago and we're yet to see a DDR DRAM solution. As for prices go RDRAM is falling dramatically. Take a look at www.atic.ca. 128Megs PC800 for 260 Candadian. Thats $169 american. Pretty damn cheap if you ask me. If DDR DRAM comes out at that price I'll be very impressed.


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