# 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 its 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 "compiled"
type (regular nonnative word), 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 compiled word, 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 compiled word is an EXIT. This pops RS, sets
IP to it, and continues.

A compiled word is simply a list of wordrefs, but not all those
wordrefs are 2 bytes in length. Some wordrefs are special. For
example, a reference to (n) will be followed by an extra 2 bytes
number. It's the responsibility of the (n) word to advance IP
by 2 extra bytes.

# Stack management

In all supported arches, The Parameter Stack and Return Stack
tops are tracked by a registered assigned to this purpose. For
example, in z80, it's SP and IX that do that. The value in those
registers are referred to as PS Pointer (PSP) and RS Pointer
(RSP).

Those stacks are contiguous and grow in opposite directions. PS
grows "down", RS grows "up".

Stack underflow and overflow: In each native word involving
PS popping, we check whether the stack is big enough. If it's
not we go in "uflw" (underflow) error condition, then abort.

We don't check RS for underflow because the cost of the check
is significant and its usefulness is dubious: if RS isn't
tightly in control, we're screwed anyways, and that, well
before we reach underflow.

Overflow condition happen when RSP and PSP meet somewhere in
the middle. That check is made at each "next" call.

# Dictionary entry

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 name size + IMMEDIATE flag (7th bit)
- 1b entry type
- Parameter field (PF)

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

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

The entry type is simply a number corresponding to a type which
will determine how the word will be executed. See "Word types"
below.

# Word types

There are 6 word types in Collapse OS. Whenever you have a
wordref, it's pointing to a byte with numbers 0 to 5. This
number is the word type and the word's behavior depends on it.

0: native. This words PFA contains native binary code and is
jumped to directly.

1: compiled. This word's PFA contains a list of wordrefs and its
execution is described in "Execution model" above.

2: cell. This word is usually followed by a 2-byte value in its
PFA. Upon execution, the address of the PFA is pushed to PS.

3: DOES>. This word is created by "DOES>" and is followed
by a 2-bytes value as well as the address where "DOES>" was
compiled. At that address is an wordref list exactly like in a
compiled word. Upon execution, after having pushed its cell
addr to PSP, it executes its reference exactly like a
compiled word.

4: alias. See usage.txt. PFA is like a cell, but instead of
pushing it to PS, we execute it.

5: switch. Same as alias, but with an added indirection.

# System variables

There are some core variables in the core system that are
referred to directly by their address in memory throughout the
code. The place where they live is configurable by the SYSVARS
constant in xcomp unit, but their relative offset is not. In
fact, they're mostly referred to directly as their numerical
offset along with a comment indicating what this offset refers
to.

This system is a bit fragile because every time we change those
offsets, we have to be careful to adjust all system variables
offsets, but thankfully, there aren't many system variables.
Here's a list of them:

SYSVARS   FUTURE USES          +3c       BLK(*
+02       CURRENT              +3e       A@*
+04       HERE                 +40       A!*
+06       C<?                  +42       A,*
+08       C<* override         +44       FUTURE USES
+0a       NLPTR                +51       CURRENTPTR
+0c       C<*                  +53       (emit) override
+0e       WORDBUF              +55       (key) override
+2e       BOOT C< PTR          +57       FUTURE USES
+30       IN>
+32       IN(*                 +70       DRIVERS
+34       BLK@*                +80       RAMEND
+36       BLK!*
+38       BLK>
+3a       BLKDTY

CURRENT points to the last dict entry.

HERE points to current write offset.

C<* holds routine address called on C<. If the C<* override
at 0x08 is nonzero, this routine is called instead.

IN> is the current position in IN(, which is the input buffer.

IN(* is a pointer to the input buffer, allocated at runtime.

CURRENTPTR points to current CURRENT. The Forth CURRENT word
doesn't return RAM+2 directly, but rather the value at this
address. Most of the time, it points to RAM+2, but sometimes,
when maintaining alternative dicts (during cross compilation
for example), it can point elsewhere.

NLPTR points to an alternative routine for NL (by default,
CRLF).

BLK* see B416.

DRIVERS section is reserved for recipe-specific drivers.

FUTURE USES section is unused for now.          

# Initialization sequence

(this describes the z80 boot sequence, but other arches have
a very similar sequence, and, of course, once we enter Forth
territory, identical)

On boot, we jump to the "main" routine in B289 which does
very few things.

1. Set SP to PS_ADDR and IX to RS_ADDR.
2. Set CURRENT to value of LATEST field in stable ABI.
3. Set HERE to HERESTART const if defined, to CURRENT other-
   wise.
4. Execute the word referred to by 0x04 (BOOT) in stable ABI.

In a normal system, BOOT is in core words at B396 and does a
few things:

1. Initialize all overrides to 0.
2. Write LATEST in BOOT C< PTR ( see below ).
3. Set "C<*", the word that C< calls, to (boot<).
4. Call INTERPRET which interprets boot source code until
   ASCII EOT (4) is met. This usually initializes drivers.
5. Initialize rdln buffer, _sys entry (for EMPTY), prints
   "CollapseOS" and then calls (main).
6. (main) interprets from rdln input (usually from KEY) until
   EOT is met, then calls BYE.

# Stable ABI

The Stable ABI lives at the beginning of the binary and prov-
ides a way for Collapse OS code to access values that would
otherwise be difficult to access. Here's the complete list of
these references:

04 BOOT addr         06 (uflw) addr      08 LATEST
13 (oflw) addr       1a next addr

BOOT, (uflw) and (oflw) exist because they are referred to
before those words are defined (in core words). LATEST is a
critical part of the initialization sequence.

All Collapse OS binaries, regardless of architecture, have
those values at those offsets of them. Some binaries are built
to run at offset different than zero. This stable ABI lives at
that offset, not 0.
