Flash Memory

Flash memory can be written to and erased under software control. Speed is considerably slower than hard disk. Flash memory is divided into relatively large erasable units (blocks). In a NOR flash memory chip, data can be changed from a binary 1 to 0directly to the cell address, one bit or word at a time. However, to change from 0 to 1, an entire erase block must be erased using a sequence of control instructions to the flash chip.

Flash memory erase block can be uniform in size or variable. The smaller block can store the bootloader and the kernel or data are kept in larger block. This is commonly called boot block or boot section chip.

To modify data stored in Flash memory array, the block in which the modified data resides must be completely erased. As the block size for Flash is much larger than typical hard disk (512 or 1K bytes), the wrtie time of Flash can be many times of hard disk.

Another limitation for Flash is there is write lifetime. Write may fails after the lifetime (100K) is exceeded.

NAND Flash is newer technology. It has smaller block. While NOR flash uses parallel address and data lines. NAND flash use proprietary serial interface. Lifetime for NAND fash is also significantly higher.