Dig into nvm implement

As far as I know from the stm32_nvm.c implement
here we use 2 flash sector to store config parameters .
I find out descrition

* We consider each sector as an array of 64-bit fields except the first N bytes, which we

* instead use as an allocation block. The allocation block is a compact bit-field (2 bit per entry)

* that keeps track of the state of each field (erased, invalid, valid).

It tell me that the first N bytes used to indicate the state of field …

But when we write some buffer eg. buf[10]={0..10} to one sector . I find out the fist byte will be modify as 0b11110000 instead of 0 . because of the sector alloc_table and data are both point to the FLASH_SECTOR_A_BASE address . so , how to distinguish the first N bytes?

sector_t sectors[] = { {

    .sector_id = FLASH_SECTOR_A,

    .n_data = FLASH_SECTOR_A_SIZE >> 3,

    .n_reserved = (FLASH_SECTOR_A_SIZE >> 3) >> 5,

    .alloc_table = FLASH_SECTOR_A_BASE,

    .data = (uint64_t *)FLASH_SECTOR_A_BASE

}, {

    .sector_id = FLASH_SECTOR_B,

    .n_data = FLASH_SECTOR_B_SIZE >> 3,

    .n_reserved = (FLASH_SECTOR_B_SIZE >> 3) >> 5,

    .alloc_table = FLASH_SECTOR_B_BASE,

    .data = (uint64_t *)FLASH_SECTOR_B_BASE


Am I miss something ? It is normal for that implement?