The fundamental thing I don't get yet is how is any particular core ("tile"?) identified so it is addressable by other cores.
Register 5 in the sswitch of each node (not core) is the node id.
If you want to plug two existing networks together, they need to:
a) Decide which of the two sides is going to be master for this network
reconfiguration;
b) Figure out if they already _are_ one network;
c) If not, one side needs to renumber its nodes so they don't overlap
with the other side;
d) Enable links, set up routing.
If you only want to add single, not-yet-connected boards, it is easier:
(more detailed description, it's a lot less work really ;-) ):
-- The new board initialises its node it to 0 (or any other particular value);
-- Both sides enable the link;
-- The "accepting" side sets up its routing table so it can access the new
board, at that particular node id;
-- Now the "accepting" side can write to the "new" sswitch, set the node id
it wants, set the routing tables; and update all routing tables in the "old"
network to make the new board reachable.
[I haven't yet done this with xmos chips, mind -- there might well be
mistakes/oversights/terrible errors/dragons].