RAM disk

From Wikipedia, the free encyclopedia

This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (June 2009)

A RAM disk or RAM drive is a block of RAM (primary storage or volatile memory) that a computer's software is treating as if the memory were a disk drive (secondary storage). It is sometimes referred to as a virtual RAM drive or software RAM drive to distinguish its use of "primary storage" from a "hardware RAM drive" that uses separate hardware containing RAM, such as a solid-state drive.

Because the speed of RAM is so much faster for most kinds of storage, files on a RAM disk can be accessed much more quickly. Also, because the storage is actually in RAM, it is volatile memory, which means it will be lost when the computer powers off. This is sometimes desirable: for example, when working with a decrypted copy of an encrypted file. In many cases, the data stored on the RAM disk is created, for faster access, from data permanently stored elsewhere, and is re-created on the RAM disk when the system reboots.

[edit] Implementation

Software RAM disks use the normal RAM in main memory as if it were a partition on a hard drive rather than actually accessing the data bus normally used for secondary storage. Though RAM disks can often be supported directly from the operating system via special mechanisms in the operating system kernel, it is possible to also create and manage a RAM disk by way of a user space application process.^[1] Usually no battery backup is needed due to the temporary nature of the information stored in the RAM disk, but an uninterruptible power supply can keep the entire system running during a power outage, if necessary.

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

[edit] Usage for web caches

It is possible to store a web cache on a RAM disk and this can improve the speed of loading pages.^[2] Due to the volatility of RAM disks, using a RAM disk has privacy advantages.^[3]

[edit] History

The first software RAM disk for microcomputers was invented and written by Jerry Karlin in the UK in 1979/80. The software, known as the Silicon Disk System was further developed into a commercial product and marketed by JK Systems Research which became Microcosm Research Ltd when the company was joined by Peter Cheesewright of Microcosm Ltd. The idea was to enable the early microcomputers to use more RAM than the CPU could directly address. Making bank-switched RAM behave like a disk drive was much faster than the disk drives - especially in those days before hard drives were readily available on such machines.

The Silicon Disk was launched in 1980, initially for the CP/M operating system and later for MS-DOS. Due to the limitations in memory addressing on Apple II series and Commodore computers, a RAM disk was also a popular application on Commodore 64 and Commodore 128 systems with RAM Expansion Units and on Apple II series computers with more than 64kB of RAM. Apple Computer supported a software RAM drive natively in ProDOS: on systems with 128kB or more of RAM, ProDOS would automatically allocate a RAM drive named /RAM.

Microsoft added a RAM disk to MS-DOS (version 2.0) in 1983. AmigaOS has had a built in RAM disk since the release of version 1.2 in 1986. Apple Computer added the functionality to the Apple Macintosh with System 7 in 1991. In addition, many Unix and Unix-like systems provide some form of RAM disk functionality; where it is particularly useful in high-performance, low-resource situations that some Unix-like operating systems can be configured for.

[edit] See also

• Cache, an area to store transient copies of data being written to, or repeatedly read from, a slower device
• Solid-state drive, which often involves the use of RAM on a physical drive
• tmpfs, a different way to store files in RAM

[edit] References

[edit] External links

• Sample code to create RAM disk drivers on Win32 and classic Mac OS
• Unix shell script and command line to create a RAM disk on Mac OS X
• Ramdisk/Ramdrive - See how it works and how it can help you (translated version)