Hardware Reference
In-Depth Information
Today, solid-state drives are used for many of the tasks magnetic and optical drives have traditionally
performed, including system drives, primary and secondary data storage, and removable-media
storage.
Virtual SSD (RAMdisk)
Although most people think of a physical drive when they discuss SSDs, these drives are available in
both physical and virtual form. A virtual SSD is traditionally called a RAMdisk because it uses a
portion of system RAM to act as a disk drive. The benefits are incredible read/write performance (it
is RAM, after all), whereas the drawbacks are the fact that all data is lost when the system powers
down or reboots, and that the RAM used for the RAMdisk is unavailable for the operating system
(OS) and applications.
RAMdisk software has been available for PCs since right after the PC debuted in late 1981. IBM
included the source code to a RAMdisk program (later called VDISK.SYS) in the March 1983 PC
DOS 2.0 manual, as part of a tutorial for writing device drivers. (Device driver support was first
implemented in DOS 2.0.) IBM later released VDISK.SYS as part of PC DOS 3.0 in August 1984.
Microsoft first included a RAMdisk program (called RAMDRIVE.SYS) with MS-DOS 3.2 (released
in 1986). Versions of RAMDRIVE.SYS were included in DOS and Windows versions up to
Windows 3.1, and a renamed version called RAMDISK.SYS has been included with Windows XP
and Windows 7/Vista. However, they are not automatically installed, and they are not well
documented. These DOS- or Windows-based RAMdisk programs are useful for creating high-speed
SSDs using existing RAM. As an alternative to using RAMDRIVE.SYS, you can use a variety of
commercial and freeware utilities available for Windows and for Linux. (See
http://en.wikipedia.org/wiki/List_of_RAM_drive_software for links to some of these utilities.)
Flash-Based SSDs
Shortly after the release of the IBM PC in 1981, several companies developed and released physical
solid-state drives that could function as direct hard drive replacements. Many of these used
conventional dynamic or static RAM, with an optional battery for backup power, whereas others used
more exotic forms of nonvolatile memory, thus requiring no power to retain data. For example, Intel
had released “bubble” memory in the late 1970s, which was used in several SSD products. Bubble
memory was even included in the Grid Compass in 1982, one of the first laptops ever released.
Although SSDs can use any type of memory technology, when people think of modern SSDs, they
think of those using flash memory. Flash-based SSDs more recently started appearing in
commercially available laptop PCs from Dell, Asus, Lenovo, and others in 2007-2008. Since then,
many other laptop and desktop PC manufacturers have introduced systems with flash-based SSDs.
Ever since SSDs first became available for PCs in the early 1980s, many have thought that they
would universally replace hard drives. Well, it has been nearly 30 years since I first heard that
prediction, and it is just now becoming partially true. Until recently, the principle barriers preventing
SSDs from overtaking hard disks has been cost per GB and performance. Early SSDs were slower
than HDDs, especially when writing data, and performance would often fall dramatically as the drive
filled up. The development of controller hardware and operating systems optimized for SSDs have
enabled recent SSDs to surpass conventional hard disk drives in performance. Although SSDs are
still more expensive per GB than traditional hard disk drives, SSDs are now widely used for
applications where cost is not as important as performance and durability: Tablets, smartphones,
netbooks, and Ultrabooks use SSDs.
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