Single sided/double sided

The terms single-sided and double-sided refer to types of computer data storage media, and can have one of two meanings depending on the type of media to which they refer.

Removable Media

For removable media, the terms literally refer to the number of sides of the media that are used for storing data.

Early floppy disks only used one surface for recording. The term "single sided" was not common until the introduction of double-sided disks, which offered double the capacity in the same physical size. Initially, the double-sided floppy disks had to be removed and flipped over to access data on the other side, but eventually devices were made that could read both sides without the need to eject the disk.

DVDs also are available in single-sided and double-sided formats. When used for movie releases, double-sided DVDs typically have the widescreen (or letterbox) version of the movie on one side, and the pan and scan (sometimes called "fullscreen") version on the opposite side. Some releases place the feature on one side, and the "extras" on the opposite. It is more common, however, for movies to be released on single-sided, dual-layer discs, with the film and extras on the same side, and widescreen and letterbox versions packaged separately.

When DVDs are used for computer data storage, dual-layer discs are preferred over double-sided ones, to avoid the inconvenience of needing to remove and re-insert discs. However, there is some interest, especially in the development of discs that are both double-sided and dual-layer, effectively quadrupling the amount of information that can be stored as compared to the original, single-sided recordable DVDs. Because of the increased storage available, and the increasing number of computers equipped with drives capable of reading and writing DVDs, many new games are starting to be released on DVDs, and the format is becoming common for personal and business backup needs.

RAM Modules

When referring to RAM modules, the terms take on a less literal meaning. "Single sided" refers to a RAM expansion with a single "bank" of chips, which the computer can access all at once. The physical expansion card may have chips on both of its sides, or only on one side, but is considered to be single sided because the computer can "see" all of its memory at once.

"Double sided" RAM has its chips divided into two sides (called "banks"), only one of which can be seen at a time by the computer. Initially, these were created by essentially attaching two single-sided SIMM cards to the same PCB, but more modern chips use different wiring. Pins 33 and 45 on the board are used by double-sided memory, and can sometimes be an indicator as to whether a given module is single or double-sided. To use the second half of the storage available, the computer must switch to the second bank, and can no longer read or write to the first half until it switches back again.

Higher-end computer systems may wish to avoid using double-sided memory because of the reduced performance that results from the time needed to switch banks, however it is not a noticeable decrease for the majority of personal uses.