6502 Boot code

6502 boot code lives in arch/6502/blk.fs.

RESERVED REGISTERS:

* X is reserved for PS
* S is reserved for RS

The PS lives in the zero page (ZP) and begins at $ff, growing
downwards. X always points to it.

RS lives in the hardware stack (Page 1) and begins at $1ff.

The IP is held in the ZP at a hardcoded offset, defined by the
IPL (low) and IPH (high) xcomp constants.

Because 6502 has a peculiar way of indirectly addressing memory
(it needs a space in the ZP pointing to the target), we have
INDL (low) and INDH (high) hardcoded offsets in the ZP. These
are preceded by INDJ, which at initialization is filled with $6c
which is the opcode for an indirect jump.

Therefore, you have 2 levels of indirect jumping available to
you once you fill INDL/INDH: Jump to it with "INDL JMP[]," or
jump to where the address described in INDL/INDH points to with
"INDJ JMP,".

In native words, INDL/INDH is very often used as a regular
holding space, a second "N" register. That is often needed
because the 6502 is severly limited compared to other CPUs.

In 6502, the N register has to live in the ZP. It is thus
mandatory to override "'N" constant in xcomp, which by default
lives in SYSVARS (which is not necessarily in the ZP on 6502).