A live CD, live DVD, or live disc is a complete bootable computer installation including operating system which runs in a computer's memory, rather than loading from a hard disk drive; the CD itself is read-only. It allows users to run an operating system for any purpose without installing it or making any changes to the computer's configuration. Live CDs can run on a computer without secondary storage, such as a hard disk drive, or with a corrupted hard disk drive or file system, allowing data recovery. A live ISO is an ISO image of a Live CD which can be used in virtual machine environments, mounted as if it were a CD/DVD and used as the virtual machine's boot CD. Live CDs, ISOs, and images usually include an operating system available without charge or restrictive licence such as Linux, rather than a commercial one such as Microsoft Windows, for legal rather than technical reasons.The functionality of a live CD is also available with a bootable live USB flash drive, or even an external USB drive. These may have the added functionality of writing changes on the bootable medium. Write-locked Live SD WORM systems are the direct solid-state counterpart to live CDs, and can be booted natively in a media card slot or by using a USB adapter. Write-locked Live SD systems avoid excessive write cycles or corruption by ill-conditioned software, such as malware.While a live CD typically does not alter any operating system or files already installed on a computer's secondary storage (such as hard disk drives), many live CDs include software mechanisms and utilities for altering the host computer's data stores, including installation of an operating system. This is important for the system management aspect of live CDs, which can be useful for removing malware, for disk imaging, and for system recovery. Unless such software is used, at the end of a live CD session the computer remains as it was before. The live system is able to run without permanent installation by placing the files that normally would be stored on a hard drive into RAM, typically in a RAM disk. The computer must have sufficient RAM both to store these files and maintain normal operation.