 While EPROMs had to be completely erased before being rewritten, NAND-type flash memory may be erased, written and read in blocks (or pages) which are generally much smaller than the entire device.NOR-type flash allows a single machine word (byte) to be written to an erased location or read independently.A flash memory device typically consists of one or more flash memory chips (each holding many flash memory cells) along with a separate flash memory controller chip.