Tag: Virtualization

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This article is about the computer memory management technique. For the technique of pooling multiple storage devices, see Storage virtualization.

“In computing, virtual memory, or virtual storage^[b] is a memory management technique that provides an “idealized abstraction of the storage resources that are actually available on a given machine”^[3] which “creates the illusion to users of a very large (main) memory”.^[4]” (WP)

“The computer’s operating system, using a combination of hardware and software, maps memory addresses used by a program, called virtual addresses, into physical addresses in computer memory. Main storage, as seen by a process or task, appears as a contiguous address space or collection of contiguous segments. The operating system manages virtual address spaces and the assignment of real memory to virtual memory. Address translation hardware in the CPU, often referred to as a memory management unit (MMU), automatically translates virtual addresses to physical addresses. Software within the operating system may extend these capabilities to provide a virtual address space that can exceed the capacity of real memory and thus reference more memory than is physically present in the computer.” (WP)

“The primary benefits of virtual memory include freeing applications from having to manage a shared memory space, ability to share memory used by libraries between processes, increased security due to memory isolation, and being able to conceptually use more memory than might be physically available, using the technique of paging or segmentation.” (WP)

1962

Virtual Memory

Fritz-Rudolf Güntsch (1925–2012), Tom Kilburn (1921–2001)

Memory was one of the key limitations of early computers. To deal with this limitation, programmers would split code and data into many different pieces, bring them into memory for processing, and then swap them back out to the auxiliary storage device when the computer’s memory was needed for something else. All of this movement was controlled by the programmer, and it took a lot of effort to get everything correct.

Working on his doctoral thesis at Technical University Berlin, Fritz-Rudolf Güntsch proposed having the computer automatically move data from storage into memory when it was referenced, and automatically save it back when the memory was needed for something else. To make this work, the computer’s data would be assigned to different portions of a larger virtual memory address space, which the computer would map, or assign, to much smaller pages of physical memory as needed.

Even by the standards of the day, the machine that Güntsch designed was tiny: it had just six blocks of core memory, each 100 words in size, that it used to create a virtual address space of 1,000 blocks. But he accurately described the idea.

Meanwhile, at the University of Manchester, a team led by Tom Kilburn was working to build what was a massive and fast machine by the day’s standards. For that machine, they designed and implemented a virtual memory system with 16,384 words of core memory and an auxiliary storage of 98,304 words. Each word had 48 bits, large enough to hold a floating-point number or two integers. Originally called MUSE, for “microsecond engine,” and later renamed Atlas, the transistorized computer took six years to create, with the British electronics firms Ferranti and Plessey joining in.

Today virtual memory is a standard part of every modern operating system: even cell phones use virtual memory. Seymour Cray (1925–1996), known for creating the world’s fastest computers in the 1960s, ’70s, ’80s, and ’90s, famously eschewed virtual memory because moving data between auxiliary storage and main memory takes precious time and slows down a computer. “You can’t fake what you don’t have,” Cray often said.

When it comes to virtual memory, it turns out that you can.

SEE ALSO Floating-Point Numbers (1914), Time-Sharing (1961)

Console of the Atlas 1 computer, used to provide information on the operation of the machine. The computer was installed at the Atlas Computer Laboratory in Britain and was mainly used for particle physics research.

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