SimForth is the Forth langage used in SimTaDyn for replacing the Visual Basic in traditional spreadsheet (I got fun when scripting in VB for spreadsheet). SimForth can be used as standalone forth (see here for more details how to compile, install and use it).

SimForth is based on standard Forth 1978 ... 2012 like Gforth, pForth, 4th, reforth but with some incompatibilites or missing words with the standard Forth 2012 but with words specialized for the SimTaDyn project. SimForth has not been written in assembly langage (because it was not created to be embedded for a real time system) but in C++ (for a more readable code than C). C++ helped to have both a classic Forth and a specialized Forth for the SimTaDyn project.

I feeled concern by the 4th philosophy: having a Forth that never crashes. This is currently not yet the case for SimForth but I would to reach the same goal in the future.

SimForth, like gforth can import, C functions in the aim to call existing code and helping adding plugins for SimTaDyn.

There are several good websites and books introducing standard Forth:

• Starting Forth well written for discovering this language.
• Forth, W.P. Salman, O. Tisserand, B. Toulout Edition Macmillan (english version) or Eyrolles (french version) 1983. An excellent book for understanding how Forth works internaly. The 1983 french version has more typo than the english version (the 2nd revision is 1984 but I do not have it). ISBN-13: 978-0387912561, ISBN-10: 0387912568
• JonesForth excellent tutorial on how to implement a Forth (in assembly) and how Forth is implemented.
• Thinking Forth how to well write Forth code.

The second link is my best book followed by the JonesForth tutorial: they complete themselves.

Standard Forths give words than can self modifying definition. These words are great for embedded systems where RAM manipulation is important but for GIS application this is less important and even dangerous when badly used. For this reason some words in SimForth do not follow the standard or are inexistant.

Standard Forths user variables are not present in SimForth but either used as "pure function" (as refrence to functional programming).

• HERE gives the address of the 1st free slot in the dictionary and in standard Forth, a variable DP is also given for changing HERE. In SimForth DP does not exist and the only way to deplace HERE is to use words like , and ALLOT (note ALLOT take into account negative values, will this method is not recommended). This avoid erroneous code like this 0 DP ! erasing the whole dictionary.

• Words @, '!, ,, ALLOT and equivalent have safe guards to be sure to not leave the dictionary or smash previous definition.

SimForth data numbers (that can be placed in data stack) are 32-bits length but tokens (aka addresses) are 16-bits length in the aim to minimize the dictionary size. However a SimForth dictionary cannot be greater than 64 kilo bytes (= 2^16) but this can be improved in future.

Addresses are relative specially LFA (Link Field Address). The advantage with relative addresses compared to absolute addresses is that you can "concat" several Forth dictionaries together or translate them into memory without effort.

SimForth dictionary is architecture independant for two reasons:

• Numbers are stored internaly in big endian and then transformed in the current endian of the architecture when reading.
• Dictionary datum do not need to be aligned (the word ALIGN is not needed). This simplifies the sharing of dictionaries between architecture.

Therefore users can share the byte code of compiled SimForth scripts (as a single binary file) and append it in their dictionary instead of sharing (several) ASCII code source and compile them. This makes sharing executable extremly easy in comparaison of sharing Unix binaries for example.

SimForth uses the old standard: 31 caracters max for a name. Gforth, pForth removed this limitation. All SimForth words stored in the dictionary are upper cased while the user can type mix of lower and upper cases, SimForth will transform words internaly.

Forth is born as language for constrain embedded systems where the memory was costly. A block is a chunck of 1024 bytes data for storing and loading the source code. This is still used while in modern computers with huge quantities memories directly managed by the operating system. Therefore we prefer using file streams instead. Words like LIST, LOAD are not defined in the core.

Known words are stored through a linked list (from the last inserted). Vocabular allows in standard Forth to minize research in existing words using the principe of trees (but instead of searching from the root) Forth search from a leaf.

SimForth will use a trie tee for fasten the research.

SimForth has not been made to be the faster Forth on the market but to try to be portable and safer as possible in execution. Forths like Gforth, JonesForth are written in assembly to manipulate directly CPU registers like interpreter pointer (IP) or stack pointer (SP). SimForth get inspiration from pForth, reforth and is written in C++ to be portable, Therefore stacks are "simulated" by an array and registers by variable, implying more overhead than direct assembly.

TODO