What are IOT and EMT instructions?

[estes at tty3b.UUCP]

	I recently have been trying to port some software to run under
	BTL Unix 4.0 and 5.0.  In these attempts, I have experienced
	both "IOT trap - core dumped" and "EMT trap - core dumped".
	Referring to signal(2), I find that those result from IOT and
	EMT instructions, respectively.  What the manual doesn't say
	is what these instructions are.

	How do these signals occur? and what do they mean?  Am I doing
	something wrong?

	The other signals are fairly easy to figure out, but these two
	have me puzzled.

	Ted Estes
	Teletype Corp., Skokie, Il
	{otuxa, we13}!tty3b!estes