Invalid blocks

NAND Flash memory devices have invalid memory blocks that cannot be used for storing data because some memory cells inside of the device have physical defects - either inherent in a process of the device manufacturing or acquired in a process of the device exploitation and reprogramming in the user's equipment. Since a percentage of invalid blocks is pretty small inside of the chip (usually less than 1%) it is possible to use the device for  data storing. In order to use NAND devices with bad blocks these blocks should be marked in a certain way to prevent fetching data from these blocks or writing in it. This document equally uses both known terms for such blocks: invalid and bad.

Locations of the invalid blocks or the invalid blocks map should be accessible by the application for skipping the bad blocks or handling them in other way. To keep the invalid block map every NAND Flash device has a special cell array, known as the Spare Area, for storing addresses of invalid blocks. See the Spare Area location in the NAND Flash memory architecture diagrams.
 
The Spare Area in "small page" 8-bit devices is 16  large, 16-bit devices - 8 Words. The Spare Area in "large page" devices - 64 Bytes and 32 Words respectfully. Though the Spare Area is dedicated for marking bad blocks it can be also used as a general purpose memory for storing the user's data. To avoid accidental losing of the bad block map it is recommended to assign a whole entire Spare Area for storing the invalid block map and do not write in this area anything else.