What is a Lisp image?

Question

Essentially, I would like to know what a Lisp image is? Is it a slice of memory containing the Lisp interpreter and one or more programs or what?

Answer 1

The Lisp image as dumped memory

The image is usually a file. It is a dump of the memory of a Lisp system. It contains all functions (often compiled to machine code), variable values, symbols, etc. of the Lisp system. It is a snapshot of a running Lisp.

To create an image, one starts the Lisp, uses it for a while and then one dumps an image (the name of the function to do that depends on the implementation).

Using a Lisp image

Next time one restarts Lisp, one can use the dumped image and gets a state back roughly where one was before. When dumping an image one can also tell the Lisp what it should do when the dumped image is started. That way one can reconnect to servers, open files again, etc.

To start such a Lisp system, one needs a kernel and an image. Sometimes the Lisp can put both into a single file, so that an executable file contains both the kernel (with some runtime functionality) and the image data.

On a Lisp Machine (a computer, running a Lisp operating system) a kind of boot loader (the FEP, Front End Processor) may load the image (called 'world') into memory and then start this image. In this case there is no kernel and all that is running on the computer is this Lisp image, which contains all functionality (interpreter, compiler, memory management, GC, network stack, drivers, ...). Basically it is an OS in a single file.

Some Lisp systems will optimize the memory before dumping an image. They may do a garbage collection, order the objects in memory, etc.

Why use images?

Why would one use images? It saves time to load things and one can give preconfigured Lisp systems with application code and data to users. Starting a Common Lisp implementation with a saved image is usually fast - a few milliseconds on a current computer.

Since the Lisp image may contain a lot of functionality (a compiler, even a development environment, lots of debugging information, ...) it is typically several megabytes in size.

Using images in Lisp is very similar to what Smalltalk systems do. Squeak for example also uses an image of Smalltalk code and data and a runtime executable. There is a practical difference: most current Lisp systems use compiled machine code. Thus the image is not portable between different processor architectures (x86, x86-64, SPARC, POWER, ARM, ...) or even operating systems.

History

Such Lisp images have been in use since a long time. For example the function SYSOUT in BBN Lisp from 1967 created such an image. SYSIN would read such an image at start.

Examples for functions saving images

For an example see the function save-image of LispWorks or read the SBCL manual on saving core images.