Exceptions in Programming Languages

What?

There’s many times that exceptional events interrupt normal program flow (E.G. In MIPS). if they’re internal (called by program execution - E.G. syscall, TLB Miss), they’re known as traps. Otherwise, they’re interrupts (E.G. Network packet arrives, O keyboard click).

The Exception Mechanism:

1. Save Address of Current Instruction.

• Typically into an Exception Program Counter (EPC)

2. Transfer Control to the OS at a known address

• Approach 1 (Doesn’t work for external interrupts):
  • Jump to a predefined (OS) address
  • Use the Cause Register to then branch to the right handler
  • This method is simply like calling a function
  • Works well for syscall as Cause register is explicitly set.
• Approach 2 (Vectored Interrupt):
  • A bunch of little, physical, wires that corresponds to a specific exception.

3. Handle Exception

• All registers must be preserved, similar to a procedure call

4. Return to User Program Execution:

• Handler restores user program’s registers and jumps back using EPC
• Relies on special instruction eret (“exception return”)