forthstrap

forthstrap is a simple Forth with the goal of being used in the Linux bootstrap process.

The goal of forthstrap is to be able to start from a small hex0 binary image and then end with a Linux-esque operating system and a capable C compiler. The overarching goal is to replace the early stages of the Linux bootstrap process with something a little faster, easier to understand, and with less code.

forthstrap starts from a hex0 loader image. This image is highly platform dependent; however, it is only required to implement the Forth interpreter, compiler and a small number of built-in words. In addition, the resulting Forth interpreter is somewhat slow. From here, we eventually move to:

• Jumping to Protected Mode from Real Mode (where applicable).
• Moving to a more advanced interpreter with better performance.
• Implementing memory management, virtual memory, paging and file system access.
• Setting up a small operating system, with a user mode and basic drivers.
• Setting up a more advanced Forth compiler, used to compile userspace applications.
• A simple userspace with some Unix-like utilities, like an assembler and a linker.
• Setting up a C compiler.

Implementations of the base image exist for the following platforms:

• i386 on bare-metal BIOS
• In this case, the hex0 image also only takes up one sector of disk space.

Words

Only the following words are implemented:

Primitives

• @ (addr -- x) - Fetch memory contents at a specific address.
• ! (x addr --) - Store a value at a specific address.
• sp@ ( -- addr) - Fetch the current stack pointer.
• rp@ ( -- addr) - Fetch the current return stack pointer.
• 0= (x -- flag) - Compare a value to zero, -1 if the top of the stack is zero, 0 otherwise.
• + (x y -- z) - Add two numbers together.
• nand (x y -- z) - Perform a bitwise NAND operation on two numbers.
• exit (r:addr --) - Exit the current function and return to the caller.
• : ( -- ) - Start defining a new word.
• ; ( -- ) - End defining a new word.
• key ( -- c) - Read a character from the keyboard.

Variables

• state - 0 to execute words, 1 to compile words.
• here - Pointer to the next free location in memory.
• latest - Pointer to the most recently defined word.

Constants

• ws - Size of a word in bytes.

Inspiration

Inspiration is taken from sectorforth, jonesforth and eulex.