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How Solid-state Drives Work


What Is a Solid-state Drive?

In computer lingo, there's a difference between memory and storage. Random-access memory, or RAM (or simply memory), holds the program a computer is executing, as well as any data. Like a person's short-term memory, RAM is fleeting and requires power to do its job. Storage, on the other hand, holds all the stuff of your digital life -- apps, files, photos and music. It retains that stuff even if the power is switched off. Both RAM and storage boast their capacity based on the number of bytes they can hold. For a modern computer, RAM typically comes in 4, 6 or 8 gigabytes. Storage can have almost 100 times more capacity -- the hard drive of a typical laptop, for example, can hold 500 gigabytes.

Here's where it gets a little sticky. Some storage devices have what's referred to as flash memory, a confusing term that blurs the line between RAM and storage. Devices with flash memory still hold lots of info, and they do it whether the power's on or not. But unlike hard drives, which contain spinning platters and turntable-like arms bearing read-write heads, flash-memory devices have no mechanical parts. They're built from transistors and other components you'd find on a computer chip. As a result, they enjoy a label -- solid state -- reserved for devices that take advantage of semiconductor properties.

There are two types of flash memory: NOR and NAND. Both contain cells -- transistors -- in a grid, but the wiring between the cells differs. In NOR flash, the cells are wired in parallel. In NAND flash, the cells are wired in a series. Because NOR cells contain more wires, they're bigger and more complex. NAND cells require fewer wires and can be packed on a chip in greater density. As a result, NAND flash is less expensive, and it can read and write data much more rapidly. This makes NAND flash an ideal storage technology and explains why it's the predominant type of memory in solid-state drives. NOR flash is ideal for lower-density, high-speed, read-only applications, such as those in code-storage applications.

Armed with this background, we can offer a more precise definition of a solid-state drive: It's a device that uses NAND flash to provide non-volatile, rewritable memory. In computers, a solid-state drive can be used as a storage device, replacing the traditional hard disk drive. In fact, manufacturers produce SSDs with shapes and footprints that resemble HDDs so the two technologies can be used interchangeably. But that's where the similarities end. If you cracked open the shell of a solid-state drive, you wouldn't see platters and actuator arms. Let's do that next.