Does advertised RAM size include the bits used for Parity or ECC?

Discussion in 'General Electronics Chat' started by HunterDX77M, Feb 13, 2016.

  1. HunterDX77M

    Thread Starter Active Member

    Sep 28, 2011
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    I've been thinking about RAM sizes lately and wonder: does advertised RAM size include the bits used for Parity or ECC?

    Generally, ECC is 8 bits for every 64 bits of data. Does that mean that ECC takes up 8/(64 + 8) = 1/9 of the total RAM size? For example, if I'm looking to buy 4 GB of RAM, am I really getting 3.56 GB of usable RAM with 0.44 GB of it taken by ECC? Or does that mean I'm getting 4 GB of RAM that has an additional 0.5 GB for ECC?
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    Yes. They're separate (additional) chips.

    A 4GB ECC module would have 4GB of available memory.
     
  3. hp1729

    Well-Known Member

    Nov 23, 2015
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    I think the question is asking if the byte count is just the data bits with parity and error detection and correction there but not apparent to the user.
     
  4. HunterDX77M

    Thread Starter Active Member

    Sep 28, 2011
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    Yes, that's correct.
     
  5. GopherT

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    Nov 23, 2012
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    Would you decide you don't need it if the answer is "yes"? Will you feel ripped off? Even if that 1/9 lost space would be the industry norm? Are you managing your ram so carefully in a specific project that an 11% deficiency in RAM space would cause your project to fail?

    Do you really need an answer or are you just searching for something to complain about?
     
  6. HunterDX77M

    Thread Starter Active Member

    Sep 28, 2011
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    I'm sorry. I didn't realize asking questions about something related to electronics that I'm just curious about is against forum rules. I'll try not to do that anymore.
     
  7. Veracohr

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    Jan 3, 2011
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    I agree with you Hunter. Some people on this forum have a pretty narrow specification of how a question should be asked.
     
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  8. hp1729

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    If you were considering designing a new RAM what might you consider?
     
  9. GopherT

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    I would consider what motivated the thread starter to ask his question, if he shares it.
     
  10. hp1729

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    There are not many advances you can make in RAM. How about including Error detection and Correction in the RAM chip itself? That can't be a new idea, can it?
     
  11. GopherT

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    Maybe a type of error correction more associated with the user than the bits. Something like making sure the computer does what the user MEANT for it to do rather than what the user SAID it should do. Now that I think about it, my wife would like to have that feature installed on me, too.
     
  12. HunterDX77M

    Thread Starter Active Member

    Sep 28, 2011
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    I was just reading about DRAM design and architecture in one of my books. After reading a section about parity and ECC I started thinking about my own PC and I was just curious if manufacturers advertised it. That's all.

    As I've stated before, I was just curious. There is no grand scheme or hidden agenda behind my knowing this. I had a question pertaining to electronics. I came to a forum that is "all about circuits" and posted a thread with my question hoping to get insight from people more knowledgeable that I am. I thought that was the point of a forum like this.

    If I am wrong about the purpose of this site and it is not about wanting to learn things related to electronics and that you must already have a very specific project in mind when asking anything, please let me know and I will take my questioning to another site.

    I'm sorry to have bothered you.
     
  13. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    @HunterDX77M

    Parity bits are calculated on the fly based on the data of each byte. No need to store anything for that. Odd parity = total number of 1s in a byte is odd, then odd. Total number of 1s in a byte is even, then even.

    EEC ships have one more chip that is used to store the ninth bit of each byte. The extra bit is not counted as total storage space when the chip is advertised (or you will end up with some weird sizes for those chips.).

    [​IMG]
     
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  14. HunterDX77M

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    Sep 28, 2011
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    @GopherT

    Thank you. That's exactly what I wanted to know. :)
     
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  15. hp1729

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    Nov 23, 2015
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    Well I thought it was a good idea. What will RAM manufacturers do after they have reached the next limit on memory size?
     
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