M-Systems Flash Disk Pioneers Flash Memory - Manual (Page 11)

Implementing MLC NAND Flash for Cost-Effective, High-Capacity Memory
91-SR-014-02-8L 11
Efficient Bad Block Handling
x2 technology handles bad blocks, which can be randomly present in flash media, by enabling
unaligned block access to two planes. Bad blocks are mapped individually on each plane, as shown
in Figure 4. Good units can therefore be aligned or unaligned, minimizing the effects of bad blocks
on the media. Without this capability, a bad block in one plane would cause a good block in the
second plane to be tagged as a bad block, making it unusable. This customized method of bad block
handling for two planes enhances data reliability without adversely affecting performance.
Bad Unit Good Unit
Good Unit
16
-bi
t Dat
a B
us
Bad Unit
Un
ali
gn
ed
Un
it
16
-bi
t Dat
a B
us
Flash Plane 2 Flash Plane 1
Internal Bus
Good Unit Good Unit Aligned Unit
Good Unit Good Unit Aligned Unit
~
~
~
~
~
~
~
~
Good Unit Good Unit Aligned Unit
Good Unit Good Unit Aligned Unit
Good Unit Good Unit Aligned Unit
Figure 4: Unaligned Multiplane Bad Block Access