This library provides a token threaded Forth kernel that may be embedded in Arduino sketches. The kernel can be used as a debug shell or even full scale Forth applications.
The Forth Virtual Machine allows multi-tasking which makes it easy to integrate with the Arduino core library functions. Context switch to and from the virtual machine is as low as 8 us (halt) and 10 us (yield/branch).
A code generator, token compiler, example sketch is available. Forth declarations are translated to the Forth Virtual Machine instruction set and dictionary format. The token compiler is also an excellent example of mixing forth, C/C++ and Arduino library functions in the same sketch.
The Forth Virtual Machine (FVM) with 130 instructions is approx. 5.4 Kbyte without kernel dictionary table and strings. This adds approx. 1 Kbyte. The instruction level trace adds an additional 500 bytes. Many of the kernel instructions are defined in both C++ and FVM instructions. This allows tailoring for speed and/or size.
The Forth Virtual Machine is byte token threaded. Most kernel and application tokens are a single byte. A prefix token is used to extend beyond 256 tokens. A total of 512 tokens are allowed; 0..255 are kernel tokens and 256..511 are application tokens. Kernel tokens are operations codes. These can be C++ code and/or threaded code (in FVM.cpp). The application tokens are always threaded code.
Kernel tokens 0..127 are direct operation codes while tokens 128..255 require a prefix token (OP_KERNEL). Application tokens 256..383 (-1..-128) are direct nested by the kernel. The token value are the one-complement index to threaded code in a table in program memory.
Application tokens 384..511 require a token prefix (OP_CALL) to the mapped values 0..127 which are the index to threaded code in a table in data memory (i.e. token minus 384).
The token threading inner interpreter uses several optimizations to reduce calls, i.e. return stack push and pop of return addresses.
The first optimization, the token mapping (-1..-128) allows the inner interpreter to directly handle calls of threaded code. The inner interpreter will perform the nesting.
The second optimization, tail call reduction, allows the inner interpreter to perform a jump to threaded code instead of call when the succeeding operation is EXIT. This reduces return stack depth, pushing and poping return address. The optimization is also used for prefix operations (OP_KERNEL and OP_CALL).
Download and unzip the Arduino-FVM library into your sketchbook libraries directory. Rename from Arduino-FVM-master to FVM.
The FVM library and examples should be found in the Arduino IDE File>Examples menu.