answer to "what is a JTAG"

A JTAG is a developers tool that plugs into a board you are developing
via a ribbon cable and allows the developer to
perform operations on the system under development,
including debugging by singlestepping code and reading
and writing memory locations.

So for our purposes it lets you load a new image into flash
even if the existing board has NOTHING running on it. If you mess up
the existing bootloader you can still recover the system.

JTAG requires hardware support inside the processor so you can't
just hang a JTAG on anything (for example, a '486 has no support).

Another competing standard is BDM which is supported
in (some) Motorola processors including PowerPC and
Coldfire. Does pretty much the same things. (Standards!
So many to chose from; aren't we lucky?)

There is a a homebrew JTAG design at the site !3runo sent out,
http://hri.sourceforge.net/tools/index.html and more information
at http://www.ocdemon.net/ (Macraigor Systems) including a good
white paper on all the debugging standards.

The Macraigor site has lots of info on what processors support each
standard.

-- 
Brian Wilson
Corvallis, Oregon
541-757-2045