Exception Handling

When an operation traps, the conceptual model is that everything stops and the trap handler is executed; since the underlying assumption is that of serial execution, traps are harder to implement on machines that use pipelining or have multiple ALU. Hardware support for identifying exactly which operation did trap may be needed. For example,

both operations can be physically executed in parallel; if the multiplication traps, the handler will need the value of the arguments y and z. But the value of z is modified by the addition, which started in the same time as the multiply and eventually finished first. IEEE supporting systems must either avoid such a situation, or provide a way to store z and pass the original value to the handler, if needed.