I was reading about UEFI capsule update, and as I understood it correctly OS saves new BIOS image into DRAM and passes a pointer to the new image via NVRAM variable. Then the OS causes system reset. BIOS in PEI stage sees the NVRAM variable and gets data from DRAM for update. I'm confused with this mechanic. Doesn't the system reset clear DRAM content? Even if it doesn't, doesn't the BIOS perform memory training at its start? I thought that the memory training reads/writes patterns that would mess DRAM content.
