First Stage Bootloader

The First Stage Bootloader reads software images from flash memory, utilizing redundant copies of the images, and launches the operating system. This bootloader finds a valid copy of the OS image and the ram filesystem image in flash memory. If none of the copies are fully valid, the bootloader attempts to construct a fully valid image by validating the images in small sections and piecing together validated sections from multiple copies. Periodically, throughout this process, the First Stage Bootloader restarts the watchdog timer. The First Stage Bootloader reads a boot table from a default location in flash memory. This boot table describes where to find the OS image and its supporting ram filesystem image in flash. It uses header information and a checksum to validate the table. If the table is corrupt, it reads the next copy until it finds a valid table. There can be many copies of the table in flash, and all will be read if necessary. The First Stage Bootloader reads the ram filesystem image into memory and validates its contents. Similar to the boot table, if one copy of the image is corrupt, it will read the remaining copies until it finds one with a valid header. If it doesn't find a valid copy, it will break the image down into smaller portions. For each section, it checks each copy until it finds a valid copy of the section and copies the valid section into a new copy of the image. The First Stage Bootloader reads the OS image and interprets it. If anything in the image is corrupt, it reads the remaining copies until it finds a fully valid copy. If no copy is fully valid, it will use individual valid records from multiple copies to create a fully valid image.