Why are flash sectors of ROM smaller at the starting address?

Question

Recently been working a lot with flash ROM, and I've found consistently within both within the internal flash of a chip and even with external SPI flash devices, that sectors are usually sized in a pattern like so:

I'm curious as to why the starting address space has smaller sectors than the later addresses. My suspicion is that it's more convenient when using something like a bootloader. Since bootloaders are often less than 128k, this would allow the bootloader to be written to maybe the first 2 or 3 sectors. This enables the main space application to have more room to expand into since to program it, we need to start at the beginning of a new sector. If we had only 128k sized sectors, then we'd essentially be wasting 128k - bootloader_size in space.

Is my suspicion correct? Or is there another reason this is done? Really curious to know what the design decision is here.

Answer 1

Because not all STM32F40x/41x devices have the full 1 MB of flash memory described in this table. Some, like the STM32F410C8, have as little as 64 KB -- that is, just the first four 16 KB sectors. Reducing the sector size at the start of memory allows these devices to still have multiple sectors available to work with, without leading to an excessive number of sectors on larger devices. It also makes some small sectors available on all devices for applications like EEPROM emulation, which requires two distinct flash sectors to be allocated for its exclusive use.