iMac GT 640M Bus Width 64bit?

Discussion in 'iMac' started by WAM2, Jan 3, 2013.

  1. WAM2, Jan 3, 2013
    Last edited: Jan 4, 2013

    macrumors 6502a

    WAM2

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    #1
    Well... I was tweaking around with benchmarks and over clocking and such, and before I even started I was completely puzzled that it seems, the GT 640M in my iMac has a Memory Bus Width of 64-Bit. I Find this very weird since NVIDIA Says it only comes in 128-Bit, which I would highly prefer over this. It completely crushed this GPU.. Can someone confirm theirs is 64-Bit Bus Width also? I have checked google and everything and every one of the 640m's I see is 128-bit. even the LE edition.. This is mind boggling to me, if this really is a 64-bit gnu then apple really really screwed us base line iMac owners over..

    I checked mine In GPU-Z on Windows and CUDA-Z on OSX and both say its 64-Bit.. Bummer.


    ** The CUDA-Z Info Is my iMac - The GPU-Z Info is a Regular GT 640M.
     

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  2. macrumors 6502a

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    #2
    That's not good. My desktop PC's 4 year old nVidia GTX 260 has a memory bus width of 448 bits, and even my 8 year old HP laptop with a lowly ATI Mobility Radeon X600 has a 128 bit memory bandwidth, so 64 bits for a GT 640M doesn't sound right.
     
  3. thread starter macrumors 6502a

    WAM2

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    #3
    Exactly! Its substantially Disappointing. And I Almost feel like its only me that has it for whatever reason.
     
  4. macrumors 6502

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    #4
    The GPU-Z picture you posted shows a bus width of 128 bits. Where do you see 64 bits?
     
  5. macrumors 6502a

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    #5
    Looking at your results again, I notice that GPU-Z is reporting a memory bus width of 128 bits. Also, CUDA is showing a 64 bit bus width for 512MB, half you memory. The memory is organized into two banks with 64 bit width for a total of 128 bits. GPU-Z is showing no CUDA for Windows, so there may be a missing CUDA driver.
     
  6. thread starter macrumors 6502a

    WAM2

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    #6
    Sorry for the misconception. The 2nd Picture is a screenshot from my friend who also has a 640m just the 1gb. The CUDA-Z is my screenshot. Also Even regular GT 640M's with 512mb are still 128-Bit.

    ----------

    The GPU-Z is a screenshot of the same GPU. Just with 1gb of VRAM. Not my System. The CUDA-Z screenshot is mine. I'll post a GPU-Z Screenshot of mine, even though it says the same thing as cuda-z
     
  7. macrumors 6502

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    #7
    so are there any other base model owners to confirm 64-bit bus in 640m?
    sounds really strange, if not more..
     
  8. macrumors 601

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    #8
    Could you do some benchmarks and compare them to known 640M benchmarks? It can be that the CUDA-Z etc. utilities simply don't read the information correctly.
     
  9. thread starter macrumors 6502a

    WAM2

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    #9
    thats a good idea. I'll get on it
     
  10. macrumors member

    swarleystinson

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    #10
    And I would suggest that you do so quickly. Are you still within the 14 day window? I would definitely look to return it if that's the case.

    Even if you're outside the 14 day window and the benchmarks differ you could always call applecare and complain.


    Another thought though is that perhaps your friend's GPU-Z is displaying the total memory bus width and yours is only showing the bus width of one memory controller. It could be that you have the 640m is made up of 2 64-bit memory controllers bringing the total to 128.

    This article could potentially relate to your "problem" if it's just a matter of how it's being displayed.

    But, according to Notebookcheck.net you definitely should have a total of 128-bit bus width ... which in and of itself would be a small bottleneck. 64-bit only should considerably affect performance.
     
  11. macrumors 6502a

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    #11
    Affect performance? Probably not that much. At the same speed, 128 bit GDDR3 gives equivalent bandwidth as a 64 bit GDDR5 bus. Considering that most variants of the 640m use gddr3, your gpu should be roughly comparable to them. Your memory speed is higher as well, 2500 mhz instead of 2000 mhz in the standard 640m.

    It is still absolutely pathetic that apple cripples their computers like this. They probably bought defective chips from nvidia for really cheap prices and put them in the computer to save money, rather that buying standard 128 bit bus chips.
     
  12. macrumors member

    swarleystinson

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    #12
    Right, 64-bit GDDR5 = 128-bit GDDR3. But, didn't he say his buddy's iMac? i.e., assuming apple tech specs are correct, they're both GDDR5. So his buddy has 128-bit bus width of GDDR5 and he potentially has 64-bit GDDR5. Which mean he may have an effective 128-bit GDDR3 and his buddy has a effectively a 256-bit GDDR3. GDDR5 is quad-pumped, so for each clock cycle, 4 bits get passed instead of 2. So you don't need twice the lanes if you can more effectively move traffic on the lanes you have.

    So if he had half the bus width and the same DRAM GPU speed as his buddy, it absolutely would affect gaming performance. Especially if running at native resolution.
     
  13. WAM2, Jan 6, 2013
    Last edited: Jan 6, 2013

    thread starter macrumors 6502a

    WAM2

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    #13
    This.

    Its quite a performance hit, doing the same benchmarks the GPU tests I got ~25 FPS lower than his, and on the lighter tests where you get easily a couple hundreds of FPS i get 100 FPS and he gets ~350

    This is horrendous. I Was excited about Apple Putting NVIDIA Cards back in, but they crippled it!
     
  14. macrumors 601

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    #14
    Ouch, thats really a low one from Apple :( I think you should at least write a complain to them. This is clearly misleading advertising...
     
  15. macrumors 6502

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    #15
    Are you sure about this? No offense, but I was under the impression that all of the mobile GPUs had 64 bit memory bandwidth?

    EDIT: Nope, checked the website and it says 128 bit as you point out. Weird. GPU-z error perhaps?
     
  16. macrumors member

    swarleystinson

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    #16
    This. ;)

    Dude, I'd totally call them, be respectful and nice and explain that this is inappropriate. If they try and stiff-arm you, I'd start getting vocal and angry and demand to speak with a supervisor.

    You absolutely have a case. It clearly states in the tech specs on apple.com that the 640m has a 128-bit bus width of GDDR5 SDRAM. So I wouldn't hurt your case by being a jerk right off the bat, but I would call them and explain to them that you have been given a partially defective GPU and would like a replacement.

    I wouldn't take no for an answer unless they can somehow miraculously fix it over the phone.

    ----------

    Look at what's been written above.
     
  17. thread starter macrumors 6502a

    WAM2

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    #17

    Well, I'd like to know if anyone elses 640m is a 64bit bus also. Because if all of them are, whats the point in calling if im going to end up with the same thing. Also does it actually state 128-bit bus on apple's website?
     
  18. macrumors member

    swarleystinson

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    #18
    http://www.apple.com/imac/specs/

    Scroll down to the graphics section. :cool:
     
  19. macrumors 6502a

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    #19
    I know that.

    I doubt his buddy is using an imac because he said his buddy's computer has 1 gb vram which apple does not offer. What I mean is that most 640m's (and also some 650m's) have a 128 bit gddr3 bus and so will have the same bandwidth and similar performance.

    There should be a small difference between them.
    http://www.anandtech.com/bench/Product/622?vs=654

    Comparison between the clevo (using a gddr3 650m) and the samsung (using a gddr5 650m). Very little difference between the two, probably because the 640m is too weak to use a full 64 GB/sec bandwidth (probably needs a little more than 32 GB/sec). Run some games and see what the memory controller load in gpu-z is.

    OP, that is not a correct screenshot of the 640m btw. Kepler does not have a "shader" clock. Clock speed should be around 600 mhz. What speed does your 640m run at.
     
  20. macrumors member

    swarleystinson

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    #20
    Are you sure?. It says there that the Samsung uses gddr3 according to the link to buy.com on anandtech's site. Also, I wouldn't call some of those differences small. Plus he's saying he's seeing a huge performance hit.

    And, as I just posted, apple's own website claims he should have double the bus width.
     
  21. macrumors 6502

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    #21
  22. thread starter macrumors 6502a

    WAM2

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    #22
    Yeah I Dont see anything either.
     
  23. macrumors member

    swarleystinson

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    #23
    That's weird. I could have sworn last night I saw it say 128-bit. Did apple remove it? I was a little sleep-deprived but I'm pretty certain I didn't make that up.
     
  24. macrumors 6502

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    #24
    I can say for sure, I haven't seen that kind of information at iMac specs page, nor now, neither the day Apple presented new iMacs.
     
  25. macrumors member

    swarleystinson

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    #25
    :confused: ... :eek:
     

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