A RAM disk is a software abstraction that treats a segment of random access memory (RAM) as secondary storage, a role typically filled by hard drives.

Access time is greatly improved to files held on RAM disks. However, the volatility of RAM means that data will be lost if power is lost, e.g. when the computer is turned off. This is sometimes desirable, for example when working with a decrypted copy of an encrypted document. In many cases, the data stored on the RAM disk is only a copy of or is derived from data permanently stored elsewhere, andthereforewill be re-created when power is restored and the system reboots.

RAM disks can also be used to hold uncompressed programs for short periods.

Implementation

Software ramdisks use the normal RAM in main memory as if it were a partition on a hard drive, using special mechanisms in the operating system kernel, instead of actually accessing the data bus normally used for secondary storage. Usually no battery backup is needed due to the temporary nature of the information stored in the ramdisk, but an uninterruptible power supply can keep the entire system running during a power outage, if necessary.

Some ramdisks use the cramfs filesystem, to allow compressed data to be accessed on the fly, without uncompressing it first. This is convenient because ramdisks are often small due to the higher price per megabyte than conventional hard drive storage.

The first commercially available software RAM disk for microcomputers was the Silicon Disk System from Microcosm Ltd. This appeared in 1980, initially for the CP/M operating system and later for MS-DOS.