14.2 Image File Background

Gforth consists not only of primitives (in the engine), but also of definitions written in Forth. Since the Forth compiler itself belongs to those definitions, it is not possible to start the system with the engine and the Forth source alone. Therefore we provide the Forth code as an image file in nearly executable form. When Gforth starts up, a C routine loads the image file into memory, optionally relocates the addresses, then sets up the memory (stacks etc.) according to information in the image file, and (finally) starts executing Forth code.

The default image file is gforth.fi (in the GFORTHPATH). You can use a different image by using the -i, --image-file or --appl-image options (see Invoking Gforth), e.g.:

gforth-fast -i myimage.fi

There are different variants of image files, and they represent different compromises between the goals of making it easy to generate image files and making them portable.

Win32Forth 3.4 and Mitch Bradley’s cforth use relocation at run-time. This avoids many of the complications discussed below (image files are data relocatable without further ado), but costs performance (one addition per memory access) and makes it difficult to pass addresses between Forth and library calls or other programs.

By contrast, the Gforth loader performs relocation at image load time. The loader also has to replace tokens that represent primitive calls with the appropriate code-field addresses (or code addresses in the case of direct threading).

There are three kinds of image files, with different degrees of relocatability: non-relocatable, data-relocatable, and fully relocatable image files.

These image file variants have several restrictions in common; they are caused by the design of the image file loader:



In my opinion, though, you should think thrice before using a doubly-linked list (whatever implementation).