Useful Notes / Flash Memory
Various formats of flash memory media.

Flash Memory is a type of erasable ROM intended for use as Mass Storage. Ever since The '80s, virtually all ROM chips produced could be somehow erased and rewritten, though most of them could be only erased as a whole: either by exposure to UV light, or by high voltagenote  pulse. After erasing the chip, it could be written to, quite similar to normal RAM, but it would hold the information after the power was switched off. This technique of erasing and writing is commonly called "flashing" in technical lingo, a rudiment of the "ancient" times when ROM chips were written by burning the specific connections inside them using 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" partially, leaving other parts of the chip untouched.

It is also structured much like common DRAM chips, using capacitors to store the data, though they are designed differently from the capacitors in DRAM, and are able to hold the information for years. This means that it can be read and written to 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. Since (unlike a hard drive) flash memory has no seek time, flash media deals much better with data fragmentation. Because of these things, flash memory is ideal for portable devices (i.e. tablets, smartphones, etc.).

Not all things are great in flash land, though. When the chip is "empty", data can be read and written to any part of it at random and hardly causing 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 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. The demand for improved efficiency in handling memory and 'cleaning up' memory cells resulted in a number of handling tricks the industry calls "Garbage Collection" being introduced, to help ensure that the lifespan of memory cells was used as efficiently as possible to maximise the overall operational lifespan. A further technique called 'wear levelling' ensures that wear and tear on flash memory cells is evenly distributed, rather than concentrated on a potential point of weakness.

To add insult to injury, 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. This makes 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 also cannot be used as a boot volume, which usually need a lot of writing. Finally, because it's a semiconductor chip and requires complex and expensive tools to manufacture, flash memory is closer to DRAM in cost, and 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 HDD-sized devices that hold quite a few Flash chips and are roughly similar in capacity to modern HDDs. They are usually marketed as direct replacements for hard drives for people that prefer their speed and indifference to rough handling, and are not afraid of their price, small sizes, and the whole "fatigue" thing. As such, they are typically found in standard laptop HDD sizes, and use common HDD IDE or SATA interfaces. Slightly easing the "fatigue anxiety", though, modern SSDs have roughly the same mean time between failures as modern hard drives note  On the other hand, they're still much more expensive. Nintendo's Wii has a rather small 512MB SSD instead of its competitors' multigigabyte hard drives. (The Wii U upped it to 8/32 GB, depending on the configuration purchased.) 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. Some more specialized devices, like the Apple Macbook Air and certain extra-thin models of Ultrabook laptops, utilize an extremely minimized form factor for their solid state drives that connects to the motherboard using a very slim PCI-E connector.
  • Solid State Hybrid Drives strive to combine both magnetic turn-tables and solid state memory into a single unit. They can simply be a traditional hard drive with a secondary solid state partition on board, sharing the same SATA port (rarely); or they can be a seam-less unit that appears like a traditional hard drive, with an invisible cache of solid state memory that the firmware uses for small, frequently-used files.

    These units have added complexity, but are typically much less expensive than an SSD of the same capacity. They are still maturing in reliability, but they can speed up boot-times tremendously, and launch your favorite applications almost like a pure SSD. Huge files, like High-Definition video are stored on the turn-table like normal, as the disk-platters are excellent for continuous access to huge data chunks. Hybrid Drives often utilize optimization techniques to allocate the flash memory space provided for the most-accessed files to speed up disk operations.
  • Thumb Drives explanation  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 internal HDD interface, like SSDs, but one of the common outside buses like USB. Most (but not all) thumb drives contain flash memory. This is usually permanent, but upgradeable ones instead contain flash readers that take a standard flash card.
    • 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.
    • At the extreme, some manufacturers like Sandisk have made their Cruzer Fit USB flash drives so tiny they barely stick out of the port when plugged in and can be easily mistaken for something like one of Logitech's nano-receivers for wireless mice.
    • As with floppy disks, USB flash drives can be configured to serve as boot disks and installers. There are utilities to facilitate booting into Windows, Linux, and even Mac OS.
  • 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 rewritable 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" 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.
    • 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.
  • Nearly all modern MP3 players and all smartphones (and whatever you call their non-cellular-capable brethren) and tablets use internal flash memory, similar to an SSD. In a similar vein, Apple laptops with solid-state storage as their only storage option, like the MacBook Air, have it on several chips that are soldered directly onto the motherboard wherever the designers could make them fit.