EMULATION
A different processor, imitated
An emulator runs on a current computer but provides the same facilities and behaviour as an earlier one. None of the old machine's instructions run directly on the new one.
Pick a methodSome files only make sense in the program that made them.
Libraries and archives can keep old software running inside a virtual machine: a computer that exists only as software on a modern one. It is how a 1990s disc can still be opened, clicked and explored today.
- Keep the original file
- Rebuild the old computer
- Open it as it was
- Emulation in one line
THE SHORT ANSWER
Rebuild the old computer
in software.
A virtual machine has no physical existence.
It is part of the behaviour of a host computer.
VIRTUALIZATION
The same processor, shared
Virtualization needs the host's instruction set to match the virtual machine's, so almost all instructions run directly on the host.
See the numbers
WHY ARCHIVES CARE
The look and feel survive
Emulation tries to preserve original behaviour as well as content, which matters most for games and interactive apps.
What gets in the way
HOW AN ARCHIVE OPENS AN OLD DISC
From disc
to screen.
A National Library of Scotland trial, written up by the Digital Preservation Coalition, followed roughly these steps with a couple of old CD-ROMs.
Read the clues
Metadata on the disc covers suggested which old operating systems were needed.
Build the old environment
The team built virtual machines for each system. Configuring them took a whole afternoon.
Make a disc image
Copying each disc to an image file lets it be mounted virtually, the main way to reach content on old optical discs.
Open it as it was
With the image mounted, the original software could be installed and the content explored.
TWO NUMBERS WORTH KNOWING
An old idea,
a growing service.
One date from the start of the debate,
one figure from a shared library service.
1995
Year emulation for preservation reached the public eye
Rosenthal credits Jeff Rothenberg's 1995 Scientific American article, Ensuring the Longevity of Digital Documents, with first bringing emulation's importance for preservation to public attention.
THE IDEA · ROSENTHAL 2015
39
Emulated computer environments in one test sandbox
Yale Library's Emulation as a Service sandbox, built with the Software Preservation Network, offered 39 emulated operating environments to librarians and archivists.
SHARED SERVICE · YALE 2021
Yale has used emulation for CD-ROM textbook supplements, 1980s video games and architectural plans drawn with early design software.
PICK A METHOD
What would an archive
do with it?
Pick an item to see how a digital archive might open it. The bar shows how much of the old computer has to be recreated.
Based on the Digital Preservation Handbook (DPC) and Rosenthal, Emulation and Virtualization as Preservation Strategies (2015).
WHAT GETS IN THE WAY
The hard part
is not the code.
Rosenthal lists two main barriers: inadequate tools for creating preserved system images, and a lack of legal clarity around software licences. He calls the greatest barrier economic.
Web-based services now let people use emulators in a browser, and national libraries including the British Library and the German National Library use such frameworks for born-digital content on physical media.
In conversation
Questions people ask
What is a virtual machine?
A computer with no separate physical existence: it is part of the behaviour of a physical host computer, and mimics the instruction set and hardware of some physical or abstract machine.
Is emulation the same as virtualization?
Not quite. Emulation runs a virtual machine on a host with a different instruction set; virtualization needs a matching one. Rosenthal notes the difference between them is not absolute.
Why not just convert old files?
Format migration is widely used, but any change to a file's structure can introduce errors or lose information. Emulation keeps the original file and opens it in its original software.
Why is emulation hard for libraries?
Emulators must be built and maintained by skilled people, new ones are needed as hardware becomes obsolete, and software licensing raises rights questions.
Can anyone try emulated software?
Some libraries offer it. Yale Library launched an emulation viewer for CD-ROMs in 2021, and web-based services let emulators run in a browser.
Sources
Each figure on this page comes from the sources below.
- Digital Preservation Coalition: Digital Preservation Handbook, Preservation action
- Digital Preservation Coalition: Emulation research at the National Library of Scotland
- Rosenthal, Emulation and Virtualization as Preservation Strategies (Mellon Foundation, 2015), via UNT Digital Library
- Yale University Library: Library sandbox lets researchers test software emulation (2021)