The main functionality of the MBR is to provide an interface to allow in-system updates of the SoftDevice and bootloader firmware.
The Master Boot Record (MBR) module occupies a defined region in the SoC program memory where the System Vector table resides.
All exceptions (reset, hard fault, interrupts, SVC) are, first, processed by the MBR and then are forwarded to the appropriate handlers (for example the bootloader or the SoftDevice exception handlers). See Interrupt model and processor availability for more details on the interrupt forwarding scheme.
During a firmware update process, the MBR is never erased. The MBR ensures that the bootloader can recover from any unexpected resets during an ongoing update process.