How Does Flash Memory Work? We’ve all transferred digital photos from a camera to a computer via an SD card or used a USB jump drive to share files with colleagues. While memory cards, SSDs, jump drives and other media clearly differ, they all function using similar forms of storage. Flash memory is almost ubiquitous in the modern consumer electronics market, and its widespread adoption is driven by the unique principles that govern its operation.
What Is Flash Memory?
Flash memory is a form of digital storage that is non-volatile, or able to retain its contents even after losing power. Like its sibling the EEPROM, or electrically-erasable programmable read-only memory, it can be read and erased without any moving parts. The major differences between the two are that flash memory’s dramatically lower overhead costs and ability to erase entire chunks in one go make it far more accessible for general storage.
Originally developed in the early 1980s by a Toshiba scientist, flash memory has come a long way since its inception. Today’s portable devices and drive media easily store hundreds or thousands of gigabytes apiece.
Flash memory may take various forms. Most users are familiar with portable implementations, like jump drives, SD cards and vendor-specific formats, such as Sony’s Memory Stick and Fujifilm’s xD-Picture card. Solid-state drives, or SSDs, use similar technology to facilitate permanent storage in computers, and they’ve been hailed as long-overdue successors to the conventional hard drive.
The Key Components of Flash Memory
Most flash devices feature a controller and the flash memory itself. Controllers connect the memory to the outside world via interfaces like USB buses and SATA connectors. They also perform vital operations like encrypting data before storing it and translating logic signals into the specific voltages needed to perform erasure, reading and writing.
Controllers check for errors and monitor usage patterns to ensure that all of the different memory cells get worn down at similar rates and deliver consistent operation throughout a device’s lifetime. As you might expect, some controllers feature higher quality engineering, which partially accounts for variances in device behavior.
What about the memory itself? Each bit, which represents a 0 or a 1, is stored by a device referred to as a cell, and cells are grouped together in strings called pages. Multiple pages make up a grid called a block. The cells, which are composed of floating gate field effect transistors, can each store a capacitively induced electrical charge, with the absence of charge representing a 1 and the existence of a charge representing a 0. The connections between cells are comprised of signal lines that eventually lead back to the controller.
How Does Flash Memory Work?
When a controller stores or reads a bit in memory, it relies on the fact that flash cells are wired together like logic gates; their output depends on the state of their inputs. By sending a signal and sampling the responses from the various connecting signal lines, the controller can discover where information resides or change stored values.
Of course, this is a major simplification, as manufacturers don’t all follow the same game plan. Different wiring configurations permit flash memory to exhibit unique behaviors and specific data access modes. While the majority of flash adheres to a NAND logic pattern that permits access at the page, word or bit level, some flash works on a NOR scheme that only accesses cells by bits and words.
Because NAND flash can be accessed in larger chunks, it’s commonly favored for consumer media devices. The word and bit-level access patterns and low-fault characteristics of NOR flash, on the other hand, make it more suitable for certain critical applications, such as the read-only memory that stores your computer’s BIOS. Some devices even increase storage density by storing multiple or triple voltage levels in each cell. Although each scheme comes with its own data-loss and recovery challenges, many flash implementations are significantly faster and more portable than hard drives.
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