Mass Storage Basics - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 8

data, and manage the low-level protocols for accessing the memory. Some
cards require only a few port pins to access. With the addition of a USB
device controller and supporting firmware, USB hosts can access the data in
a device's flash-memory card. Users can also remove a card from the device
and insert the card in a card reader attached to a PC or other computer.
Flash memory consumes less power than other media types. When attached
to a USB host or hub, a typical flash-memory storage device can receive all
of its power from the bus.
Hard drives are the cheapest per byte and can hold massive quantities of
data. CD and DVD drives are less common in embedded systems because
embedded applications tend to require media that is easily erased and rewrit-
ten. CD-RW, DVD-RW, and DVD+RW discs can be erased and rewritten,
but not as easily as magnetic media.
A device that contains a USB host controller and supporting firmware can
access ordinary USB flash drives and hard drives. Because a USB host must
manage the bus, USB host programming is more complex than USB device
programming. But for some applications, the ability to store data in generic
drives makes the increased complexity worthwhile.
Drive Mechanisms
Hard disks require a drive mechanism to spin the disks and position the read
and write heads (Figure 1-4). A hard drive contains a stack of platters. Each
platter has magnetic storage media arranged in concentric circles, called
tracks, on both sides. Each surface of a platter has a head positioned above
the platter's surface. The head can read or write to the bit of data directly
opposite the head.
An area on a drive can be identified by cylinder, head, and sector. A cylinder
is a stack of tracks of the same diameter. Each surface has a head, so the head
identifies a surface on a platter. A sector is a portion of a track and contains
the smallest addressable quantity of data in the media. All sectors in a drive
have the same capacity, typically 512 bytes.
The drive mechanism spins the disks and moves the heads to requested
tracks. When a requested sector on a spinning disk passes under the head,