Detecting exact bad ram chip.

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  • zpi
    Senior Member
    • Feb 2018
    • 132
    • Lithuania

    #1

    Detecting exact bad ram chip.

    Hey guys,

    Maybe somebody can share how and if it is at all possible (it should be, as apple refurbishes boards in this way) to find exact bad ram chip in macbook or any other boards ? As from memtest we can get bad response from exact address, but how to know in which chip it is ?
  • stj
    Great Sage 齊天大聖
    • Dec 2009
    • 30964
    • Albion

    #2
    Re: Detecting exact bad ram chip.

    the ram is often handling a number of the bits per chip - a schematic can show you how they relate to the bad bits in the diagnostic.

    it's perticularly easy for apple because they are soldered down so every board is the same.
    unlike if they had dimm sockets

    you may also find something with a current-probe or thermal camera

    Comment

    • WilsonEvans
      Banned
      • Dec 2020
      • 1
      • United States

      #3
      Re: Detecting exact bad ram chip.

      Not sure if this is the right place to ask, but I'll be happy to move it elsewhere if needed. A few weeks back, I purchased 16GB of RAM for my desktop system. Everything worked well for quite some time until this past weekend. I started having a ton of BSODs and random apps crashing. I ran Memtest86+, and the RAM came back with a ton of errors. I didn't check it when I first bought it, so it may have been problematic to start with. But my question is, does RAM degrade over time? I've heard about buying bad RAM, but never RAM that works, but eventually just goes kaput.

      Thoughts?

      Comment

      • eccerr0r
        Solder Sloth
        • Nov 2012
        • 8688
        • USA

        #4
        Re: Detecting exact bad ram chip.

        Yes they can go bad over time like any other IC.
        However bad power is more likely.

        Comment

        • televizora
          ghettomodmaster
          • Nov 2016
          • 957
          • Bulgaria

          #5
          Re: Detecting exact bad ram chip.

          Usually you can locate bad chip or slot by sequence. First chip or slot are usually at the beginning of the memory map, lower addresses.
          If you know the sequence, the first chip and the capacity of the memory chip, usually you can guess.
          Useful conversions. I don't "speak" imperial. Please use metric, if you want to address me.
          1km=1000m=100000cm, 1inch=2.54cm, 1mile=1609.344meters, 1ft=30.48cm 1gal(US)=3.785liters, 1lb=453grams, 1oz=28.34grams

          Comment

          • eccerr0r
            Solder Sloth
            • Nov 2012
            • 8688
            • USA

            #6
            Re: Detecting exact bad ram chip.

            It's always been a problem that bit number is not always the same as bit number according to the CPU. And the fact that any particular chip can be anywhere in the address space...

            I guess sometimes it's easier to work with SOCs when there's only one memory chip... but that's no fun.

            Comment

            • televizora
              ghettomodmaster
              • Nov 2016
              • 957
              • Bulgaria

              #7
              Re: Detecting exact bad ram chip.

              Originally posted by eccerr0r
              And the fact that any particular chip can be anywhere in the address space...
              This is not exactly true. There is a logic. For example - you have 4GB RAM.
              4 sticks x 1GB. In slots DIMM 0-3
              Memtest reports errors at first 500MB. Then it is the stick in slot 0.
              Useful conversions. I don't "speak" imperial. Please use metric, if you want to address me.
              1km=1000m=100000cm, 1inch=2.54cm, 1mile=1609.344meters, 1ft=30.48cm 1gal(US)=3.785liters, 1lb=453grams, 1oz=28.34grams

              Comment

              • ChaosLegionnaire
                HC Overclocker
                • Jul 2012
                • 3264
                • Singapore

                #8
                Re: Detecting exact bad ram chip.

                yes but it also depends on how the memory bus is organised. if its single channel (64-bit mode), dual channel (128-bit mode) and so on for triple and quad channel. the data is often striped (in a raid 0 fashion) across different memory ic chips and/or modules depending on the organisation of the memory bus.

                so u can find the faulty module but the exact faulty ram ic chip may be tricky like what the OP is asking for.

                Comment

                • televizora
                  ghettomodmaster
                  • Nov 2016
                  • 957
                  • Bulgaria

                  #9
                  Re: Detecting exact bad ram chip.

                  Originally posted by ChaosLegionnaire
                  so u can find the faulty module but the exact faulty ram ic chip may be tricky like what the OP is asking for.
                  If you have a schematic and details about the architecture I think that this is not really a problem. Also, soldered ram usually uses higher density chips and usually the chips are not numerous. But this depends on the mobo.
                  Useful conversions. I don't "speak" imperial. Please use metric, if you want to address me.
                  1km=1000m=100000cm, 1inch=2.54cm, 1mile=1609.344meters, 1ft=30.48cm 1gal(US)=3.785liters, 1lb=453grams, 1oz=28.34grams

                  Comment

                  • eccerr0r
                    Solder Sloth
                    • Nov 2012
                    • 8688
                    • USA

                    #10
                    Re: Detecting exact bad ram chip.

                    Originally posted by televizora
                    This is not exactly true. There is a logic. For example - you have 4GB RAM.
                    4 sticks x 1GB. In slots DIMM 0-3
                    Memtest reports errors at first 500MB. Then it is the stick in slot 0.
                    No it is true, you don't know ahead of time. Unless you have schematics for the board, any particular bit can be connected to any line and the computer really won't know a difference. That's why you never know.

                    For instance: there's no reason D0 needs to be connected to D0 of the memory chips, the memory chips don't care what's being stored - you can swap D0 and D6 and the computer wouldn't care.

                    The address bits *somewhat* have don't cares too. Because the low order bits are used for refresh, it matters which are connected to what. However the bits that aren't dealing with refresh, once again the memory chip doesn't care where things are being stored, it just needs to return the right data when the cpu requests it.

                    The only thing that may enforce bit ordering is multiplexed lines and there is some ordering that's involved of refresh.. But for old fashioned memory chips and especially nonmultiplexed SRAMs, they don't care, coupled with chip selects and output enables... without the schematics, who knows where a bit is stored.

                    Comment

                    • televizora
                      ghettomodmaster
                      • Nov 2016
                      • 957
                      • Bulgaria

                      #11
                      Re: Detecting exact bad ram chip.

                      Originally posted by eccerr0r
                      But for old fashioned memory chips and especially nonmultiplexed SRAMs, they don't care, coupled with chip selects and output enables... without the schematics, who knows where a bit is stored.
                      Even seen an old BIOS, where if you don't put a stick in the first memory slot, DIMM0, the BIOS tell you that memory configuration is wrong and there is no memory module in DIMM0? You cant be sure, but this is the assumption with which I work and more often than not it is right. For soldered chips this doesn't work that way. You need to have a schematic. Because only the one that designed the pcb knows where each trace goes.
                      I have used this logic multiple times while diagnosing faulty DIMM memory modules.
                      If I know which soldered chip is the start of the sequence and which are the next one, know their capacity, I can guess if there are 8 chips and total of 4GB memory for example, and I during testing I have error at 250MB address for example, that this is *perhaps* chip 0.
                      Otherwise you are right. You address the memory and write data in it. The memory itself doesn't care how it is arranged,
                      unless significant mistakes are made.
                      Originally posted by ChaosLegionnaire
                      if its single channel (64-bit mode), dual channel (128-bit mode)
                      You are right about this. These modes themselves make diagnosing problematic.
                      Because one 2GB DIMM and 2x1GB DIMMs logically are equal address space, while being at different physical locations.
                      This depends so much on the architecture.
                      Last edited by televizora; 12-17-2020, 06:16 PM.
                      Useful conversions. I don't "speak" imperial. Please use metric, if you want to address me.
                      1km=1000m=100000cm, 1inch=2.54cm, 1mile=1609.344meters, 1ft=30.48cm 1gal(US)=3.785liters, 1lb=453grams, 1oz=28.34grams

                      Comment

                      • SMDFlea
                        Super Moderator
                        • Jan 2018
                        • 20534
                        • UK

                        #12
                        Re: Detecting exact bad ram chip.

                        The member at post #3 has been banned as a spammer. We are seeing a lot of these lately where a post has been copied from reddit. Post #12 was deleted ,the post said solved and included spam links in the quote.

                        https://www.reddit.com/r/hardware/co...bad_over_time/
                        All donations to badcaps are welcome, click on this link to donate. Thanks to all supporters

                        Comment

                        • tester272001
                          Senior Member
                          • Apr 2011
                          • 91
                          • USA

                          #13
                          Re: Detecting exact bad ram chip.

                          What a waste of time and effort! Why spam garbage from Redit ? I am doumbfounded. they must make some decent $ by doing this???

                          Comment

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