If the 'Emergency stop' input is just used to put a message up on the lcd, why are you so worried about the response time? Most code works as a loop - otherwise it would run once then do nothing! If the cpu is controlling motors one would expect it would have some input then control the output then keep on looping - one would expect you would put an abort test in that loop. Even PLCs process their ladder code as a loop. In your instance, I would expect as the 'emergency stop' is just a logic input informing the cpu that the power has been cut to the motors so it can stop whatever it was trying to do, so putting it on an interrupt I would see as being unusual. Resetting the processor or causing a context change seems the hard way of doing it - the software should test the input and process the action is the input is activated as part of its normal operation.

The term 'EMERGENCY STOP' has legal and safety implications especially where humans and machines interact. There is also laws describing the location, labelling and operation of these switches in most countries. Equipment involved in machine safety is usually specifically designed for the job - if a logic controller is involved it must be certified as a safety controller or a simple reliable device such as a relay or switch. NEVER rely on a microprocessor alone to ensure safety - it is near impossible to prove that it will do what you want every time. A microprocessor and its associated software is very complex and succeptable to a number of random failures. Even a gun has a safety catch.

Again, do a search on 'therac 25' - that machine embodied most of the common mistakes in machine design and it killed people.

I might sound a bit heavy handed about the safety issues - I wouldn't want to be blamed for a death of a person due to my negligence.