Exceptions and Interrputs

CS 321 – Introduction to Computer Architecture and Machine-Level

Programming

Barry Britt, Systems Analyst IIDepartment of Computer Science

Iowa State University

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.

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.

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.

Examples

Events From? MIPS Terminology

Arithmetic overflow Internal (to program) Exception

Undefined instruction Internal Exception

I/O device request External Interrupt

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.

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.

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.

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.

Registers

Register Name Register Number Usage

Status Reg #12 Interrupt mask and enable bits

Cause Reg #13 Exception type and pending interrupt bits

EPC Reg #14 Address of instruction that caused the exception

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

Implementation, cont.

Vectored interrupts: A table that exists within the hardware. Table can be indexed by exception type

Pseudocode???

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();

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!

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.

Critical Thinking Questions

Why do we clear the interrupt before executing the interrupt handler?

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.

Critical Thinking Questions

Why do we increment the program counter of the process before handing off execution back to the interrupted process?

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.

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

MIPS Instructions, cont.

Mfc0 $k0, $13 #move cause into register $k0Mfc0 $k1, $14 #move EPC into register $k1Addi $k1, $k1, 4 #Do not re-execute exception instr................rfe #Restore the interrupt statejr $k1 #Transfer control to the original proc