Hardware Reference
In-Depth Information
Chapter 10. Flash and Removable Storage
Alternative Storage Devices
Since the mid-1980s, the primary storage device used by computers has been the hard disk drive.
More recently, however, flash-based storage including SSDs (solid-state drives) are increasingly
being used as hard drive replacements. Although SSDs can physically replace a hard disk drive
(HDD), they operate using a completely different set of principles that may require a treatment unlike
that used for conventional HDDs. For data backup, data transport between computers, and temporary
storage, secondary removable storage devices such as flash memory devices/drives, optical drives,
magnetic tape drives, removable media hard drives, and even floppy drives have been useful
supplements to primary storage. Cloud storage, too, now plays a major role in data transfer, storage,
and backup. Optical storage, such as CD, DVD, and BD (Blu-ray Disc), is covered in
Chapter 11
,
“
Optical Storage
.
”
Flash Memory Devices
Flash memory is a special type of solid-state memory chip that requires no power to maintain its
contents. Flash memory cards can easily be moved from digital cameras to laptop or desktop
computers and can even be inserted into photo printers or self-contained photo display devices. Flash
memory can store any type of computer data, but its original primary application was digital
photography. However, more and more digital music players have removable flash memory cards,
and so-called
thumb
or
keychain
flash memory devices that plug directly into a USB port have
helped make flash memory a mainstream storage medium and a popular replacement for some types of
magnetic removable-media storage, particularly floppy disks, Zip drives, and SuperDisk drives.
Flash memory in the form of SSDs is rapidly increasing in market share as a high-speed alternative to
conventional magnetic hard disk storage.
Flash memory was invented by Fujio Masuoka at Toshiba in the early 1980s, with the original patents
filed in late 1981. At the time Toshiba unfortunately didn't know how important this invention was,
and by 1988 Intel had introduced competitive versions and quickly took the lead in flash memory
development and production.
Flash memory is a type of nonvolatile memory that can be electrically programmed and erased. It was
originally used in PC motherboards as EEPROM (electrically erasable programmable read-only
memory) chips for storing the motherboard basic input/output system (BIOS). Flash ROMs replaced
the EPROM, which could only be programmed or erased by special equipment outside of the
motherboard.
Flash memory can be considered sort of a cross between random access memory (RAM) and read-
only memory (ROM). Just like RAM, flash memory can be written directly in the system, yet just like
ROM it is completely nonvolatile, meaning that it retains data after the power has been turned off
(and without a battery like the Complementary Metal Oxide Semiconductor [CMOS] RAM). Besides
being nonvolatile, there is one other big difference between flash memory and conventional RAM:
The system cannot rewrite Flash memory; it must always erase it first.
When erased, flash memory cells are in a low-voltage state that carries a logical 1 value. The act of
writing to (or programming) flash places a charge in the transistor's floating gate, which changes the
