In your day-to-day life you probably come across many ways to store information. You may write a note on a whiteboard or on a piece of paper. You may write a letter. You may read a book. You may also store a letter in a filing cabinet.
Each of these forms of storage is better for some purposes and worse for others. That is to say, they have tradeoffs. Perhaps most volatile of the examples above is the whiteboard. This is the same as saying that it is the easiest to change and the fastest to access. The least volative is the book.
The examples above also vary in how dense they are. The least dense is the whiteboard. In other words, it takes up a lot of space relative to how much information you can write on it. The most dense of those examples is the book.
We can see a trend here. The most volatile forms of storage tend to be the least dense. They also tend to be more expensive for the amount of information stored.
| Most Dense | Least Dense | ||
| Least Volatile | Most Volatile | ||
| Book | Filing Cabinet | Paper or Letter | White board |
The relationship shown tends to hold true in computer storage as well. In the case of electronic storage, some of the more volatile forms of memory lose their contents when power is removed.
| Most Dense | Least Dense | ||||
| Least Volatile | Most Volatile | ||||
| Tape | Optical Disk | Disk | Flash | RAM | Registers |
Note that the ideal form of memory is most dense, most fast, and doesn't lose contents when power is removed. As you can see there is not ideal memory. But as our technology develops the memories move closer to the ideal, supplanting older technologies. An example of this is the floppy disk, which is neither dense nor slow by todays standards.
RAM comes in several forms which vary in how dense they are and how quickly they can be read and written. But all RAM forms are faster than disk and slower than CPU registers. In 2005 desktop computers typically have hundreds of megabytes of RAM. Information in RAM is lost when the power to the computer is removed.
CPU registers are very fast memory, largely by virtue of being on the same chip as the CPU. They are also large and may be power hungry compared to RAM. Typical computers will have a hundreds of bytes of registers. Information in registers is lost when the power to the computer is removed.
Disks rely on moving parts and are much slower than RAM. Typical computers will have a hundreds of gigabytes of disk. Information in disk is not lost when power is removed.
Flash is a semiconductor technology which superficially resembles RAM, at least more so than the other technologies listed here. But unlike RAM, flash retains its data when power is removed. But flash is much slower than RAM, almost as slow as disk. Note that Compact Flash cards are often used in cameras.
CDRWs and DVDs are very dense if we simply consider the raw disks. Consider how many DVDs can be stacked in the approximately one inch thickness of a hard disk. CDRWs and DVDs are rather slow to read and write. They are also dirt cheap storage.
Computer magnetic tape is only seen in industrial and educational computer centers, not in the home. That is because the manufacturers of the tape drives haven't recognized the opportunity of selling huge quantities of the drives. Instead the tape drives are more expensive than typical desktop computers. But the media is very dense, and very slow.
There are many other technologies that are contenders for the memory market. It would be risky to predict which of these technologies will achieve any significant market presence. When they do, it will be necessary to update this page.