Flash Memory

Flash Memory is a type of erasable ROM intended for use as Mass Storage. For note, ever since The Eighties there's virtually no ROM chips produced that cannot be somehow erased and rewritten. It's just that most of them could be only erased as a whole: either by exposure to the UV light or by the high voltage pulse. After erasing the chip could be written to, quite similar to the normal RAM, but it will hold the information after the power would be switched off. This technique of erasing and writing is commonly called "flashing" in the technical lingo, a rudiment of the "ancient" times when ROM chips were written by burning the specific connections inside them by these pulses. Flash memory takes its name from this process, but differs from these early chips by the simple fact that it can be "flashed" only partially, leaving other parts of the chip untouched.

It is also structured much like the common DRAM chips, using capacitors to store the data, though they are designed differently from the capacitors in the DRAM, and are able to hold the informations for years. This means that it can be read and written quite similarly, so it is much faster than the older magnetic or optical drives, and as it has no moving parts, these devices are much more durable and they also require little physical space, electrical power, or cooling. Because, unlike a hard drive, flash memory has no seek time, flash media deals much better with data fragmentation.

Not all things are great in flash land, though. When the chip is "empty", data can be read and written to any part of at random and without causing hardly any fatigue, but it is the erasing part that causes much of its problems. While flash chips can be erased partially, it must be done in blocks, by using high-voltage pulses to reinitialize the capacitors in the whole sections of the chip, commonly called "pages". It is this sectional nature that means that Flash cannot be used as a normal non-volatile RAM similar to core, and that they need to have additional controller chips or built-in circuits to handle the erasing and addressing, and that makes them more similar to disk drives in actual usage.

To add an injury to the insult, the high voltage needed to erase the flash memory degrades the structure of the semiconductor itself, so it wears out much faster than most other types of Mass Storage, making flash (much like CD/DVD-RW discs) generally unsuitable for use as RAM, and as writing data to it is generally slower than reading (if the chip is full and needs erasing first -- much slower). It usually cannot be used for a swap file, which usually need much to be written on. Finally, because it's a semiconductor chip and requires complex and expensive tools to manufacture, flash memory is closer to the DRAM in cost, at roughly $1 per GB in the 2010s. It thus is still enormously expensive compared to magnetic and optical storage, rather severely limiting its usefulness in Real Life. Things are getting better, but SSDs still cost about ten times more than good old HDDs of similar capacity. And the USB thumb drives can look very cute.

Common form factors:

 * Solid State Drives are the HDD-sized devices that hold quite a few Flash chips and are roughly similar in capacity to the modern HDDs. They are usually marketed as direct replacements for hard drives for people that prefer their speed and indifference to the rough handling, and are not afraid of their price, small sizes, and the whole "fatigue" thing, so they are typically found in standard laptop HDD sizes, and use the common HDD IDE or SATA interfaces. Slightly easing the "fatigue anxiety", though, modern SSDs have roughly the same mean time between failures as the modern hard drives. On the other hands, they're still much more expensive. Nintendo's Wii has a rather small 512MB SSD instead of its competitors' multigigabyte hard drives. To offset the cost problem, it's common for homebuilt PCs to have a relatively small SSD to hold key programs (including the operating system) and one or more larger HDDs for general storage.
 * Thumb Drives are rather similar, differing only in that they have smaller size (only slightly larger than the plug -- typically male USB -- they are integrated with) and capacity, and generally aren't designed to be the main system volume, but intended to to replace removable media, similar to the early Floppy Disks. They usually use not the SATA interface, like the SSDs, but one of the common outside buses like USB. Most (but not all) thumb drives contain flash memory. Most thumb drives use memory soldered to the drive's PCB; others are bridges to read and write flash cards.
 * And then there's the USB bracelet.
 * And USB Flash watches, James Bond style! Some Swiss Army knives come with a USB drive attachment as well.
 * Flash Cards are simply boards with flash chips and the minimum number of support chips needed to make the memory work all squeezed into a standard-sized Cartridge. Numerous formats are available, with varying degrees of physical and logical compatibility, in approximately chronological order:
 * Numerous game consoles use proprietary memory card formats to save game progress between play sessions separately from the game itself. Since consoles don't use rewriteable optical drives, this became pretty much mandatory once games started coming on optical disc instead of magnetic disk or cartridge.
 * Several flash card standards have a rather bizarre origins from the laptop extension buses that make them somewhat similar to modern SSDs
 * PCMCIA, first of them, was originally a laptop memory card format, based on a pin-reduced version of the good, ol' ISA (technically, on its hard-drive subset, the IDE interface) bus. Later it became "PC Card", because People Can't Memorize Computer Industry Acronyms, and explicitly started allowing IDE drives and other peripherals. Has 68 pins compared to the 16-bit ISA's 100.
 * Its successor, CardBus, is commonly called "PCI for laptops", as it could switch back and forth between ISA/IDE and PCI modes on-the-fly. Also 68 pins.
 * CompactFlash is a tiny version of PCMCIA and is electrically compatible with IDE, requiring only a pin converter. Has 50 pins.
 * PCMCIA's successor, ExpressCard, is based on PCI Express and USB, and is being used for some truly enormous flash cards that are basically a whole bunch of readers for other formats squeezed into one card. 26 pins.
 * The SIM cards in many cellphones are used as their primary (sometimes ONLY) mass storage device
 * MMC (MultiMediaCard), RS-MMC and MMCmicro.
 * SD (Secure Digital), miniSD and microSD are an updated version of MMC that, as of the 2010s, is supported by the vast majority of consumer electronics devices that take flash memory cards.
 * xD Picture Cards, a variant developed by Olympus and Fujifilm specifically for digital cameras. (Currently being phased out in favor of the better-supported SD format.)
 * Memory Stick and Memory Stick PRO/PRO Duo/Micro are Sony's proprietary format.
 * Many MP3 players, digital cameras and camcorders, and smartphones have built-in flash memory. This is often eMMC, a chip using the same signals as MMC in a far smaller package that's soldered to the device's PCB.