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A system image in computing is the state of a computer or software system stored in some non-volatile form. The form of storage is often a file. A system is said to be capable of using (or dumping) system images if it can be closed down and later restored to exactly the same state. System images are sometimes used for backup.
There are generally two types of system image:
- An image of a whole computer system;
- An image of some part of a system, and often a particular program.
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System images
Images of the first type are normally created by copying the contents of the computer's mass storage to a file elsewhere, often with disk cloning programs. On many systems a complete system image cannot be created by a disk cloning program running within that system because information can be held outside of disks and volatile memory, for example in non-volatile memory like boot ROMs.
Non-system images
Images of the second type generally have special purposes, often related to persistence. A common example is a database management system (DBMS). Most DBMS can store the state of its database or databases to a file before being closed down (see database dump). The DBMS can then be restarted later with the information in the database intact and proceed as though the software had never stopped. Another example would be the hibernate feature of many operating systems. Here the state of all RAM memory is stored to disk, the computer is brought into an energy saving mode, then later restored to normal operation.
Some emulators provide a facility to save an image of the system being emulated. This is often called a savestate.
Program-based images
Some programming languages provide a command to take a system image of a program. This is normally a standard feature in Lisp and Smalltalk, sometimes other languages provide it. Development in these languages is often quite different from many other programming languages. For example in Lisp the programmer may load packages or other code into a running Lisp implementation using the read-eval-print loop, which usually compiles the programs. The programmer may then dump a system image, containing that pre-compiled and possibly customized code. Often this image is an executable, and can be run on other machines. This system image can be the form in which executable programs are distributed — this method has often been used by programs (such as TeX and Emacs) largely implemented in Lisp, Smalltalk, or idiosyncratic languages to avoid spending time repeating the same initialization work every time they start up.
Although its purpose is different, a "system image" is often similar in structure to a core dump.
