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This is a simple indirect threaded Forth for the ARM Cortex M0/M3. It can currently run on the the Olimex STM32-P103 and the Olimexino STM32/LeafLabs Maple and supports generic STM32F103 and STM32F030 boards. Other targets boards should be easy to add. CoreForth consists of a small core written in ARM assembler, and additional Forth words which are compiled on the host for the target using a slightly modified version of thumbulator.

CoreForth started based on JonesForth, but evolved to be closer to Forth-79. Some words and ideas were taken also directly from CamelForth and the excellent article series by CamelForth's author Brad Rodriguez.

The motivation behind CoreForth is to provide a simple platform to explore Cortex M0/3 based development boards, not so much to be a fully fledged Forth implementation (neither is ANS Forth compliance a goal), but there is nothing preventing CoreForth to be taken into that direction.

CoreForth SE

CoreForth SE is a subroutine threaded variant of CoreForth. It is under active development and allows easier debugging and better performance.

Forth Implementation

CoreForth is an indirect threaded Forth. Register r7 is holding the instruction pointer, register r6 is the return stack pointer, and the parameter stack is handled via register sp (r13).

Four macros are used to define words within the assembler source:

  • defcode: Define a word implemented in assembler. The code field pointer points to the words' body.
  • defword: Define a word implemented in indirected threaded code, each cell contains the code field pointer of the word to invoke. The code field pointer points to the DOCOL function
  • defvar: Define a variable, the space is allocated at HERE from RAM.
  • defconst: Define a constant

Board Dependent Code

The CoreForth source is split into two parts. The actual Forth implementation under generic in CoreForth.s and a number of Forth source files, and the board dependent code under e.g. olimexino-stm32. The board dependent code uses .include to bring in the Forth kernel, this is neccessary in order to be able to add new words to the board code due to the way the words are defined using macros.


CoreForth is written in GNU Assembler, and requires the ARM GCC cross compiler as well as thumbulator. The Makefile will generate ELF and binary files, which can be flashed using e.g. OpenOCD. A good overview of bare metal programming and qemu can be found on Franceso Balduzzi's blog, and using OpenOCD with hardware is explained in more detail on Johan Simonsson's pages.

CoreForth makes use of Forth words for the non-core functionality. The Forth sources are compiled into binary form using thumbulator as a runtime environment. The compiled assembler code and Forth sources are loaded into the thumbulator memory. The Forth core executes the Forth sources, building the target dictionary in memory as it processes the input data. At the end of the compilation, the Forth core will dump itself plus the compiled dictionary into one binary. For that purpose, thumbulator supports semihosting, including console output.

This target compilation step is automated, adding new Forth sources for target compilation is as simple as adding them as parameters to the thumbulator invocation in the Makefile.

Previous Versions

The branch "1.0" contains the previous, qemu based version of CoreForth. That branch is not actively maintained anymore, but can serve as an example for an alternative cross compilation approach.