Information Technology Reference
In-Depth Information
Addressing Methods
All USB drives and other drives of recent vintage support logical block
addressing (LBA). With LBA, blocks of storage capability are numbered
sequentially beginning at zero. All blocks have the same size, again typically
512 bytes. The logical block address is often referred to as a sector address
because the block size equals the capacity of a sector in a hard drive. To
access the media, software specifies the logical block address to read or write
to. For hard drives, the drive's controller translates each LBA to a cylinder,
head, and sector on the drive. For flash drives, the drive's controller trans-
lates each LBA to a block, page, and column in the memory array. The
sequence of logical block addresses doesn't have to correspond to the physi-
cal locations of the sectors in a drive or the memory in a chip. All that mat-
ters is that the media's controller knows what area of storage corresponds to
each address.
In older systems, software accessed storage media using CHS addressing,
where the software specifies a cylinder, head, and sector number to read or
write to. A storage device can support both CHS addressing and LBA.
Compared to CHS addressing, LBA is simpler, more flexible, and supports
larger capacities. File-system drivers in embedded systems are unlikely to
need to use CHS addressing.
Reading and Writing Considerations
Storage media varies in the available methods of write-protecting the con-
tents, support for erasing, and copy-protection technologies.
Write Protection
The storage media, drive mechanism, circuits, or a manual switch can per-
mit or forbid writing to the media. For example, a flash-memory controller
can forbid writing to all or a portion of the memory. Or a manual switch on
a flash-memory card can inform the host that the media shouldn't be erased
or overwritten. Higher-level software in the mass-storage master can also
control access to data on a storage device.
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