Collapse OS' Forth implementation notes

*** EXECUTION MODEL

After having read a line through readln, we want to interpret it. As a general
rule, we go like this:

1. read single word from line
2. Can we find the word in dict?
3. If yes, execute that word, goto 1
4. Is it a number?
5. If yes, push that number to PS, goto 1
6. Error: undefined word.

*** EXECUTING A WORD

At it's core, executing a word is pushing the wordref on PS and calling EXECUTE.
Then, we let the word do its things. Some words are special, but most of them
are of the compiledWord type, and that's their execution that we describe here.

First of all, at all time during execution, the Interpreter Pointer (IP) points
to the wordref we're executing next.

When we execute a compiledWord, the first thing we do is push IP to the Return
Stack (RS). Therefore, RS' top of stack will contain a wordref to execute next,
after we EXIT.

At the end of every compiledWord is an EXIT. This pops RS, sets IP to it, and
continues.

*** Stack management

The Parameter stack (PS) is maintained by SP and the Return stack (RS) is
maintained by IX. This allows us to generally use push and pop freely because PS
is the most frequently used. However, this causes a problem with routine calls:
because in Forth, the stack isn't balanced within each call, our return offset,
when placed by a CALL, messes everything up. This is one of the reasons why we
need stack management routines below. IX always points to RS' Top Of Stack (TOS)

This return stack contain "Interpreter pointers", that is a pointer to the
address of a word, as seen in a compiled list of words.

*** Dictionary

A dictionary entry has this structure:

- Xb name. Arbitrary long number of character (but can't be bigger than
  input buffer, of course). not null-terminated
- 2b prev offset
- 1b size + IMMEDIATE flag
- 2b code pointer
- Parameter field (PF)

The prev offset is the number of bytes between the prev field and the previous
word's code pointer.

The size + flag indicate the size of the name field, with the 7th bit being the
IMMEDIATE flag.

The code pointer point to "word routines". These routines expect to be called
with IY pointing to the PF. They themselves are expected to end by jumping to
the address at (IP). They will usually do so with "jp next".

That's for "regular" words (words that are part of the dict chain). There are
also "special words", for example NUMBER, LIT, FBR, that have a slightly
different structure. They're also a pointer to an executable, but as for the
other fields, the only one they have is the "flags" field.