https://devzone.nordicsemi.com/f/nordic-q-a/19981/dfu-app-data-and-fdsHi, I have initial flashed boot loader and application based on SD version 3.1 and SDK 12.2.0 with DFU_APP_DATA_RESERVED CODE_PAGE_SIZE * 3 FDS_VIRTUAL_PAGES 3 later updated both boot loader and Application with below change DFU_APP_DATA_RESERVEDen-USTelligent Community 13Fri, 24 Feb 2017 15:41:04 GMThttps://devzone.nordicsemi.com/thread/77755?ContentTypeID=1Fri, 24 Feb 2017 15:41:04 GMT137ad170-7792-4731-bb38-c0d22fbe4515:215c0fa9-d4a0-49ad-8037-5f0bd98fc540bjorn-spockeli<p>DFU_APP_DATA_RESERVED only referes to how many pages below the bootloader start address it shoudl preserve during a DFU, i.e. not erase, see <a href="http://infocenter.nordicsemi.com/topic/com.nordic.infocenter.sdk5.v12.2.0/lib_bootloader_dfu_banks.html?cp=4_0_1_3_5_1_2_2#lib_bootloader_dfu_appdata">this</a> page. Whereas, FDS_VIRTUAL_PAGES defines how many pages below the bootloader start address that are reserved for the FDS module. So if you want to preserve the data stored by FDS you should set DFU_APP_DATA_RESERVED equal to FDS_VIRTUAL_PAGES.</p><div style="clear:both;"></div>