Hardware Drivers

To be able to run on a wide variety of hardware, Collapse OS
needs to abstract away interactions with it. It does so with
drivers, which are words that conform to a protocol and whose
job is to talk with the specific hardware they support.

This way, core words can be independant of implementation
details for particular hardware.

Running a minimal Collapse OS requires very little drivers:

(key?)     -- c? f Returns whether a key has been pressed and,
                   if it has, returns which key. When f is
                   false, c is *not* placed in the stack.
(emit)     c --    Spit a character on the console.

To have a functional (albeit minimal) Collapse OS running on
your fancy machine, all you need to do is to insert these words
in the "Drivers" layer of your xcomp (see doc/bootstrap) and
you're golden.

Be aware that these words are cross-compiled, so xcomp rules
apply. See doc/cross.

Most of the time, you'll want to implement those words in native
code. But Forth code is also an option. At the driver layer,
the whole "low" part of core words are available, which is a
majority of core words. And, because we're in xcomp, all immedi-
ate words are provided by the host. Therefore, the only words
that are off-limit for Forth driver code are non-imm words
defined in the "high" part of core words.

Subsystems

Having a minimal Collapse OS is already awesome, but maybe you'd
like to go a bit further and support fancy stuff like mass stor-
age or RS-232.

To that end, Collapse OS has subsystems, which are chunks of
logic sitting on top of wider hardware abstractions. For exam-
ple, the SD card subsystem depends on having hardware that can
somehow do SPI communication with a particular device.

So, if you make the effort of implementing the protocol required
by the SD card subsystem, then you win the prize of being able
to access SD cards!

Each subsystem in Collapse OS has its own documentation page
which details its required protocol and sub-subsystems:

* BLK subsystem (doc/blk)
* Grid subsystem (doc/grid)
* RX/TX subsystem (doc/rxtx)
* SPI protocol (doc/spi)
* PS/2 subsystem (doc/ps2)