Bent Pins

tl;dr Apparently I ran into what is known as “bent pins” issue on my S1156-based Gigabyte GA-P55M-UD2 board the other day.

4GB of RAM started to feel cramped and adding another 4GB 1333MHz DDR3 for 20€ sounded reasonable for a 4x2GB configuration.

So I types

shutdown -h now

off comes Antec Remote Fusion top cover, in go the new memory modules, finger hits Power button.. only to see BIOS showing I still have 4GB of total memory.


After one or two CMOS resets, power cycles and a bit of BIOS-exploration, I noticed Gigabyte BIOS M.I.T. Status screen show something strange: all memory sticks were listed for 2048MB, but only two of them were shown as being “Enabled”.

OK, foreseeing some more power cycles here, so disconnecting HDD power cables for a bit seemed reasonable. Let the memory slot testing bonanza begin! Couple of interesting behaviors surfaced.

GA-P55M-UD2 has the following memory slot layout:

|-----------------------------------------| DIMM2
|-----------------------------------------| DIMM1

|-----------------------------------------| DIMM4
|-----------------------------------------| DIMM3

Motherboard manual says two-stick Dual Channel is achieved by placing DIMMs into slot 1 and 3. Yet mine were sitting in slots 1 and 2 instead. At that point I seemed to vaguely recall some issues getting slots 1 and 3 to work together and since 1 and 2 Just Worked(tm), I left them there at the time.

But as we need to know whether Slot 3 actually works at all, I set the layout up again. All I got was a pre-POST reboot loop, which was a pretty good indication of some deeper problem. It’s Google-time for “ga-p55m-ud2 8gb memory” and themsuch queries.

Surprise surprise, turns out this behavior has been seen before and one of the first recommended debugging operations seemed to always be “reseat CPU, check for bent pins”. First I had ever heard of this, you (read: I) would think that if a pin was non-functional, you wouldn’t even get to POST. But this box has been rolling along for a few years now, although with memory sticks possibly being in “wrong” non-Dual-Channel slots.

It was clear now, there was no way to avoid disconnecting the bazillion cables sticking out from the back of the server and digging through desk drawers for that Arctic Silver MX-2 tube. Yawn.

Intel’s instructions for checking sockets:

Looking carefully at the socket pin grid, there indeed seemed to be a spot in upper left corner where a pin seemed to reflect light at a different angle being slightly out of the straight line everybody else was standing in. So I augmented it to the best of my ability with the smallest flathead screwdriver I could find in the house (1mm) and using my girl’s reading glasses as magnifying help. While the pin looked somewhat more in line post-op than before, I am still not entirely sure it’s exactly factory-precision.

In the lower right part there seemed to be something that looked like a small part of a fingernail stuck between pins. So for good measures and while I was at it anyway, I patted myself on the shoulder for job obviously well done on the previous CPU installation and dug the unidentified object out of the socket. Was kinda disappointed the cooking heat of the CPU didn’t turn it into a precious jewel of some kind. Would’ve felt nice to get profit out of gcc and kernel compilations.

After some cursing at S1156 HSF mounting mechanism when putting Scythe Shuriken back on top of the CPU, machine was ready to be tested again.

  • #1 Slot 1 and 3 Dual Channel test: machine now booted instead of reboot-loop, PASSED
  • #2 Slot 2 and 4 Dual Channel test: pre-POST reboot-loop, although I’m not entirely sure this is even supposed to work, FAILED
  • #3 Install all four 2GB sticks: BIOS shows 8GB of memory, BIOS M.I.T. Status screen shows all four sticks as enabled, PASSED

So all in all, happy days.

Based on the above, I’d say bent pins inside the CPU socket causing your memory problems is a real good possibility. Definitely check socket, adjust pins and reseat CPU before you convert the motherboard into being a part of your frisbee equipment.

What I’m wondering now is what happens when different pins are out of alignment. Is it always memory issues, or different bad things happen or to what extent the CPU is capable of functioning in general?