Bottom-Boot and Top-Boot Flash Devices

Some devices are organized as a few small sectors at the bottom address space,
followed by large sectors that fill the remaining space. Some devices have a few small
sectors at the top of the device’s address space and use large sectors in the lower
address space. Since boot code is typically placed in small sectors, flash memory is
some times described as bottom-boot or top-boot, depending on where the smaller
sectors are located.

Ultimately, one sector contains the memory space that the CPU accesses as a
result of a power cycle or reset (boot). This sector is usually referred to as the boot
sector. Because some CPUs have a reset vector that is at the top of memory space and
others have a reset vector at the bottom of memory space, the flash devices come in
bottom-boot and top-boot flavors. A processor that boots to the top of memory
space would probably use a top-boot device, and a processor that boots to the bottom
of its memory space would be more suited to a bottom-boot device.

When the boot sector of the flash device exists in the boot-time address space of
the CPU, it can be protected by making the boot sectors unmodifiable. Since only a
small amount of code is typically needed to provide a basic boot, there is little
wasted space if you dedicate a small sector to an unmodifiable boot routine. This
supports the ability to make as much of the flash space in-system reprogrammable
without losing the security of knowing that if all of the reprogrammable flash was
accidentally corrupted, the system would still be able to boot through the small
amount of code in the unmodifiable boot sector.