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Image File Background

Our Forth system consists not only of primitives, 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 primitives and the Forth source alone. Therefore we provide the Forth code as an image file in nearly executable form. At the start of the system 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 starts executing Forth code.

The image file variants represent different compromises between the goals of making it easy to generate image files and making them portable.

Win32Forth 3.4 and Mitch Bradleys 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).

By contrast, our loader performs relocation at image load time. The loader also has to replace tokens standing for 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:

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