this post was submitted on 04 Aug 2023
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Explain Like I'm Five
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Modern RAM just needs to be told to refresh. The device itself will go through the refreshing process. But the whole array needs to be refreshed, there's no LRU scheme to tell what bank or row was last accessed.
Starting with DDR3 it's not so easy. Density is so high that reading or writing one row affects cells in adjacent rows. Partial target row refresh (PTRR) counters this, where any access of a row is followed by a refresh of adjacent rows. Flaws in this process in early DDR3 controllers was at the heart of rowhammer exploits, where repeated accesses to a memory location could work out what's stored in physically adjacent memory, even if it's not privileged. IIRC DDR4 pulled the PTRR process into the RAM's built in refresh circuitry so it's transparent to the memory controller.