1. Exceptions and Interrputs CS 321 – Introduction to Computer Architecture and Machine-Level Programming Barry Britt, Systems Analyst II Department of Computer Science Iowa State University

2. Who am I? Systems Administrator for 8 years  Bachelors in CS, 2002  Bachelors in Music, 2002  Masters in CS, 2007 Work:  Computer technician in industry for 3 years.  Research support for 3 years.  University support for 5 years.

3. Definition An exception, or interrupt, is an unexpected event that changes the normal flow of instruction execution (excluding branches and jumps). Exceptions can have both internal and external causes. Interrupts have only external causes.

4. NOTE Some authors do not distinguish between interrupts and exceptions and will use the term interrupt for both types of events. One decent rule of thumb:  Exceptions generally handle events intrinsic to a program (think Java try- catch blocks).  Interrupts handle user or device requests.

5. Examples EventsFrom?MIPS Terminology Arithmetic overflowInternal (to program)Exception Undefined instructionInternalException I/O device requestExternalInterrupt

6. Further Examples Imagine your cell phone ringing  The phone causes an interrupt.  The phone ring-tone is like a “signal”  When you acknowledge the interrupt, a hardware “signal” is stopped.  The ringtone and the caller ID can give you about where the call originates.  Answering the phone is “servicing” or “handling” the interrupt.

7. Control Flow The most challenging part of processor design is implementing the interrupt and exception handlers. Interrupts must be acknowledged:  Interrupt source  Interrupt priority level  Interrupt security Both hardware AND software are required to handle exceptions.

8. Handling an Exception (High-Level) Requirements  A mechanism to indicate an exception.  A mechanism to specify the nature of the exception.  A mechanism to specify the requested service.  A mechanism to specify the priority of the exception.  A mechanism to inform the requester that the exception is being handled.  A mechanism to restore normal activity after handling the exception.

9. Implementations Implementations are different for different computers. MIPS:  Part of the CPU (coprocessor 0) records information needed to handle interrupts and exceptions.  This is kept in 32-bit registers that are NOT part of the general register set. Access requires special or privileged instructions.

10. Registers Register NameRegister NumberUsage StatusReg #12Interrupt mask and enable bits CauseReg #13Exception type and pending interrupt bits EPCReg #14Address of instruction that caused the exception

11. Implementation, cont. Exception is in the Cause register.  Contains a field which indicates the reason for the exception. This field is used to determine which handler to execute.  To execute, Save program state update the Next-Instruction to the first address of the exception handler, and execute. After execution, restore program state

12. Implementation, cont. Vectored interrupts:  A table that exists within the hardware.  Table can be indexed by exception type Pseudocode???

13. Pseudocode class HandleError: Program program = new Program(); Exception exceptionToHandle = new Exception(); Main: program.getCurrentlyRunningProgram(); exceptionToHandle = program.getException(); Handler handler = new Handler(exceptionToHandle); handler.saveProgramState(program); handler.HandleException(); program = handler.restoreProgramState();

14. Exception Handlers An exception handler is a software routine invoked to handle exceptions when they occur. Even though this is a software routine, hardware actions are required to successfully deal with exceptions!

15. Required Actions for Handlers Save execution state of currently running process Clear interrupts Handle the interrupt Restore execution state of program Increment the process' program counter and hand control back to interrupted process.

16. Critical Thinking Questions Why do we clear the interrupt before executing the interrupt handler?

17. Critical Thinking Questions Why do we clear the interrupt before executing the interrupt handler?  If we don't clear the interrupt, the next program to execute will experience the same interrupt request and try to handle an invalid interrupt.  This would effectively deadlock the system.

18. Critical Thinking Questions Why do we increment the program counter of the process before handing off execution back to the interrupted process?

19. Critical Thinking Questions Why do we increment the program counter of the process before handing off execution back to the interrupted process?  Otherwise, the process would execute the same interrupted statement as before, preventing further program execution.

20. MIPS Instructions Mfc0 -> move from coprocessor 0 Mtc0 -> move to coprocessor 0 Lwc0 -> load word to register of coprocessor 0 Swc0 -> store word from register of coprocessor 0

21. MIPS Instructions, cont. Mfc0$k0, $13#move cause into register $k0 Mfc0 $k1, $14#move EPC into register $k1 Addi$k1, $k1, 4#Do not re-execute exception instr................ rfe#Restore the interrupt state jr$k1#Transfer control to the original proc