RAM question.

Discussion in 'NZ Computing' started by Warwick, Nov 16, 2004.

  1. Warwick

    Warwick Guest

    If you have two 256 mB ram cards and want to upgrade,
    two 512 cards == $300ish and one 1gB card is about $320 ish.

    Man in shop ways two 512 is better than one 1gig by 10%.

    I had always thought that it was called RAM because the time taken to get
    the data at any address is the same as the time taken to get the data at
    any other address.

    He is also pushing the cheaper deal.

    I was hoping somewhere here could tell me the why of all this.



    cheers
    Warwick, Nov 16, 2004
    #1
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  2. Warwick

    Bok Guest

    Warwick wrote:
    > If you have two 256 mB ram cards and want to upgrade,
    > two 512 cards == $300ish and one 1gB card is about $320 ish.
    >
    > Man in shop ways two 512 is better than one 1gig by 10%.


    I assume your talking DDR RAM here?
    The advantage of two separate memory modules over a single module is the
    *potential* to run them in dual channel mode (if your M/B and chipsets
    support it). If you don't achieve dual channel mode, then there is no
    performance advantage.

    > I had always thought that it was called RAM because the time taken to get
    > the data at any address is the same as the time taken to get the data at
    > any other address.


    That's roughly correct assuming access from main memory and not from cache.

    Dual channel memory does not improve the access time (or latency) ,
    what it does is double the amount of data transferred at a time from 64
    bits to 128 bits. The net effect of this is to double the peak memory
    bandwidth. For example: the peak bandwidth of PC3200/DDR400 RAM is
    3.2GB/s, so the peak bandwidth in dual channel mode would be 6.4GB/s

    Peak bandwidth = (memory speed) x (bytes transferred) x (channels)
    = 400M/s x 8bytes x 2 = 6.4GB/s for DDR400 RAM

    The other factor to consider is processor FSB (front side bus) speed.
    For example, a 533MHz FSB has a peak bandwidth of 4.2GB/s and a 800Mhz
    FSB 6.4 GB/s. This means a 533 or 800MHz FSB will be subutilised with
    DDR400 RAM in single channel mode (all you can do with a single memory
    module).

    Doubling the *peak* memory bandwidth doesn't translate to doubling the
    peformance of applications though. It will benefit apps that move a lot
    of data sequentially most.

    The 10% figure your man in the shop gave is probably a reasonable
    average to expect (for dual channel over single channel), but expect the
    improvement to vary anywhere from 0% to perhaps 30% or more.
    Bok, Nov 16, 2004
    #2
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  3. Warwick

    William Bell Guest

    On Tue, 16 Nov 2004 20:34:49 +1300, Warwick <> wrote:

    >If you have two 256 mB ram cards and want to upgrade,
    >two 512 cards == $300ish and one 1gB card is about $320 ish.
    >
    >Man in shop ways two 512 is better than one 1gig by 10%.
    >
    >I had always thought that it was called RAM because the time taken to get
    >the data at any address is the same as the time taken to get the data at
    >any other address.
    >
    >He is also pushing the cheaper deal.
    >
    >I was hoping somewhere here could tell me the why of all this.
    >
    >
    >
    >cheers



    If you are running Dual Channel you will need 2, plus 2 is also better as if
    one fails you still have the other..


    Supply and demand plus technology improvements always cost more.
    William Bell, Nov 16, 2004
    #3
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