6809 Boot code

Let's walk through 6809 Boot code at B280. This walkthrough is
a bit less detailed that the "canonical" z80 one, which is
contains comments that are common to all CPUs.

This assembles the boot binary. It requires the 6809 assembler
(B50) and cross compilation setup (B200). It requires some
constants to be set. See doc/bootstrap.txt for details.

RESERVED REGISTERS: 

* D is the Work register
* S points to PSP
* U points to RSP
* Y holds IP (Interpreter Pointer)

The boot binary is loaded in 2 parts. The first part, "decla-
rations" before xcomp overrides, with the loader word 6809M. The
rest, after xcomp overrides, with 6809C.

As with any boot binary, it begins with the Stable ABI (see
doc/impl.txt), all of it at this point being a placeholder.

Right after that comes next and execute routines, the heart of
Collapse OS' runtime. 6809 adressing mode come handy here and
it allows us to have quite compact code.

In next, we can read wordref from (Y) and increase IP by 2 in
a single op, then continue to exec, which expects a wordref in
X.

Then, it's a matter of reading the first byte and to bit-
fiddling along with conditional jumps to get to the proper logic
for the word contents, which begins 1 byte after the initial X
position. TFR ops used in XT and DOES are a bit expensive, but
they're hardly avoidable.

Then comes the initialization code, that is, set PSP, RSP, and
call BOOT from the stable ABI.

Then come the base native words. They're all straightforward
and we can see that we benefit greatly from 6809's superior
indexing ops. We rarely use PSH/PUL. We work directly with S
because it's generally faster for what we want to do.

Sometimes, we lack register space so we use the zero page as
a temporary holding area (<> indexing).

FIND: something not so straightforward happens here. Unlike in
z80, we don't hold our string by the tail, so comparison happens
in "forward" mode. We even re-use code from []= for this.
String length, which is held in B, is re-used in the "length
matched!" part of the code (because, you know, it matched...).
However, to go to the beginning of the string in the dict entry,
we need LEAX to go backward, so we NEGB. However, because B hold
our reference length, we need to NEGB again afterwards.