RAM Drive - History and Operating System Specifics

History and Operating System Specifics

The first software RAM drive 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 drive 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 drive to MS-DOS (version 2.0) in 1983. Originally named VDISK.SYS and later replaced by RAMDRIVE.SYS, it was discontinued in Windows 7. DR-DOS and the DR family of multi-user operating systems also came with a RAM disk named VDISK.SYS. In Multiuser DOS, the RAM disk defaults to the drive letter M: (for memory drive). AmigaOS has had a built in RAM drive since the release of version 1.1 in 1985 and still has it in AmigaOS 4.1 (2010). Apple Computer added the functionality to the Apple Macintosh with System 7's Memory control panel in 1991, and kept the feature through the life of Mac OS 9. Mac OS X users can use the hdid, newfs (or newfs hfs) and mount utilities to create, format and mount a RAM drive.

A RAM drive innovation introduced in 1987 by Perry Kivolowitz for AmigaOS was the ability of the RAM drive to survive most crashes and reboots. Called the ASDG Recoverable Ram Disk, the device survived reboots by allocating memory dynamically in the reverse order of default memory allocation (a feature supported by the underlying OS) so as to reduce memory fragmentation. A "super-block" was written with a unique signature which could be located in memory upon reboot. The super-block, and all other RRD disk "blocks" maintained check sums to enable the invalidation of the disk if corruption was detected. At first, the ASDG RRD was locked to ASDG memory boards and used as a selling feature. Later, the ASDG RRD was made available as shareware carrying a suggested donation of 10 dollars. The shareware version appeared on Fred Fish Disks 58 and 241.

Many Unix and Unix-like systems provide some form of RAM drive functionality. In Linux and similar systems, besides the more traditional /dev/ram, another way to store files in RAM is provided by tmpfs. RAM drives are particularly useful in high-performance, low-resource applications for which Unix-like operating systems are sometimes configured. There are also a few specialized "ultra-lightweight" linux distributions which are designed to boot from removable media and stored in a ramdisk for the entire session. Puppy Linux is the best-known of these.