What are traps?

Question

There are many different types of traps listed in processor datasheets, e.g. BusFault, MemManage Fault, Usage Fault and Address Error.

What is their purpose? How can they be utilized in fault handling?

Answer 1

Traps are essentially subroutine calls that are forced by the processor when it detects something unusual in your stream of instructions. (Some processors make them into interrupts, but that's mostly just pushing more context onto the stack; this gets more interesting if the trap includes a switch between user and system address spaces).

This is useful for handling conditions that occur rarely but need to be addressed, such as division by zero. Normally, it is useless overhead to have an extra pair of instructions to test the divisor for zero before executing a divide instruction, since the divisor is never expected to be zero. So the architects have the processor do this check in parallel with the actual divide as part of the divide instruction, and cause the processor to trap to a divide-by-zero routine if the divisor is zero. Another interesting case is illegal-memory-address; clearly, you don't want to have to code a test to check each address before you use it.

Often there are a variety of fault conditions of potential interest and the processor by design will pass control to a different trap routine (often set up as a vector) for each different type of fault.

Once the processor has a trap facility, the CPU architects find lots of uses. A common use is debugger breakpoints, and trap-to-OS for the purposes of executing a operating system call.