Computers as Components, 2nd Edition

112 CHAPTER 3 CPUs

with changes in the flow of control of a program, it makes sense to use similar

mechanisms. However, exceptions are generated internally.

Exceptions in general require both prioritization and vectoring. Exceptions must

be pr ioritized because a single operation may generate more than one exception—

for example, an illegal operand and an illegal memory access. The priority of

exceptions is usually fixed by the CPU architecture. Vectoring provides a way for

the user to specify the handler for the exception condition. The vector number for

an exception is usually predefined by the architecture;it is used to index into a ta ble

of exception ...