Final answer:
Illegal addresses are trapped using the valid-invalid bit, which indicates whether a memory address is valid or invalid. If invalid, the system prevents illegal memory access through a trap such as a page fault.
Step-by-step explanation:
Illegal addresses in a computing context are typically trapped using the valid-invalid bit. This bit is a flag associated with a memory address in a page table, indicating whether the address is valid (i.e., a legal address that points to a location within the process's address space) or invalid (i.e., an illegal address attempting to access memory outside of the allocated space). When the CPU tries to access memory, the valid-invalid bit is checked: if it's set to invalid, the system triggers a trap such as a page fault, preventing the illegal memory access.