dealing with exceptions computer architecture part 1

Lecture 7

Dealing with Exceptions

Computer ArchitectureCNE-301

Lecturer: Irfan Ali1

7

Exception Categories

Two Types of Exceptions: Interrupts and Traps• Interrupts

• Caused by external events:

• Network, Keyboard, Disk I/O, Timer

• Page fault - virtual memory

• System call - user request for OS action

• Asynchronous to program execution

• May be handled between instructions

• Simply suspend and resume user program

• Traps

• Caused by internal events

• Exceptional conditions (overflow)

• Undefined Instruction

• Hardware malfunction

• Usually Synchronous to program execution

• Condition must be remedied by the handler

• Instruction may be retried or simulated and program continued or program may be aborted8

Synchronous vs Asynchronous• Definition: If the event occurs at the same place

every time the program is executed with the same data and memory allocation, the event is synchronous. Otherwise asynchronous.

• Except for hardware malfunctions, asynchronous events are caused by devices external to the CPU and memory.

• Asynchronous events usually are easier to handled because asynchronous events can be handled after the completion of the current instruction.

9

Exceptions in Simple five-stage pipeline• Instruction Fetch, & Memory stages

• Page fault on instruction/data fetch

• Misaligned memory access

• Memory-protection violation

• Instruction Decode stage• Undefined/illegal opcode

• Execution stage• Arithmetic exception

• Write-Back stage• No exceptions!

10

What happens during an exceptionIn The Hardware

• The pipeline has to 1) stop executing the offending instruction in midstream,

2) let all preceding instructions complete,

3) flush all succeeding instructions,

4) set a register to show the cause of the exception,

5) save the address of the offending instruction, and

6) then jump to a prearranged address (the address of the exception handler code)

In The Software

• The software (OS) looks at the cause of the exception and “deals” with it

• Normally OS kills the program

11

Exceptions

Exception = non-programmed control transfer• system takes action to handle the exception

• must record the address of the offending instruction

• record any other information necessary to return afterwards

• returns control to user

• must save & restore user state

user program

normal control flow:

sequential, jumps, branches, calls, returns

System

Exception

HandlerException:

return from

exception

12

must record

15

What Makes Pipelining Hard?

Examples of interrupts:

• Power failing,

• Arithmetic overflow,

• I/O device request,

• OS call,

• Page fault

Interrupts (also known as: faults, exceptions, traps) often require

• surprise jump (to vectored address)

• linking return address

• saving of PSW-Program Status Word (including CCs-Condition Codes)

• state change (e.g., to kernel mode)

Interrupts cause

great havoc!

There are 5 instructions executing in 5 stage pipeline when an interrupt occurs:

• How to stop the pipeline?

• How to restart the pipeline?

• Who caused the interrupt?

16


Interrupts cause

great havoc!

What happens on interrupt while in delay slot ?

• Next instruction is not sequential

solution #1: save multiple PCs

• Save current and next PC

• Special return sequence, more complex hardware

solution #2: single PC plus

• Branch delay bit

• PC points to branch instruction

Stage Problem that causes the interrupt

IF Page fault on instruction fetch; misaligned memory access; memory-protection violation

ID Undefined or illegal opcode

EX Arithmetic interrupt

MEM Page fault on data fetch; misaligned memory access; memory-protection violation

17


• Simultaneous exceptions in more than one pipeline stage, e.g.,

• Load with data page fault in MEM stage

• Add with instruction page fault in IF stage

• Add fault will happen BEFORE load fault

• Solution #1

• Interrupt status vector per instruction

• Defer check until last stage, kill state update if exception

• Solution #2

• Interrupt ASAP(as soon as possible)

• Restart everything that is incomplete

Another advantage for state update late in pipeline!

Interrupts cause

great havoc!

20


Here’s what happens on a data page fault.

1 2 3 4 5 6 7 8 9

i F D X M W

i+1 F D X M W < page fault

i+2 F D X M W < squash



i+5 trap > F D X M W

i+6 trap handler > F D X M W

Interrupts cause

great havoc!

Exceptions - “Stuff Happens”

• Exceptions definition: “unexpected change in control flow”

• Another form of control hazard.

For example:

add $1, $2, $1; causing an arithmetic overflow

sw $3, 400($1);

add $5, $1, $2;

Invalid $1 contaminates other registers or memory locations!

21

Additions to MIPS ISA to support Exceptions• EPC (Exceptional Program Counter)

• A 32-bit register

• Hold the address of the offending instruction

• Cause• A 32-bit register in MIPS (some bits are unused currently.)

• Record the cause of the exception

• Status - interrupt mask and enable bits and determines what exceptions can occur.

• Control signals to write EPC , Cause, and Status

• Be able to write exception address into PC, increase mux set PC to exception address (MIPS uses 8000 00180hex ).

• May have to undo PC = PC + 4, since want EPC to point to offending instruction (not its successor); PC = PC – 4

• What else?

flush all succeeding instructions in pipeline

22

Flush instructions in Branch Hazard

36 sub $10, $4, $8

40 beq $1, $3, 7 # taget = 40 + 4 + 7*4 = 72

44 and $12, $2, $5 if($1==$2)

48 or $13, $2, $6 go(PC+4+PC*C)

52 ….

….

72 lw $4, 50($7)

23

Pipelining in MIPS

• MIPS architecture was designed to be pipelined• Simple instruction format (makes IF, ID easy)

• Single-word instructions

• Small number of instruction formats

• Common fields in same place (e.g., rs, rt) in different formats

• Memory operations only in lw, sw instructions (simplifies EX)

• Memory operands aligned in memory (simplifies MEM)

• Single value for Write-Back (limits forwarding)

• Pipelining is harder in CISC architectures

25

27

Key observation: architected state only change in memory and

register write stages.

Introduction to ILP

• What is ILP?• Processor and Compiler design techniques that speed up execution by causing

individual machine operations to execute in parallel

• ILP is transparent to the user• Multiple operations executed in parallel even though the system is handed a

single program written with a sequential processor in mind

• Same execution hardware as a normal RISC machine• May be more than one of any given type of hardware

34

This allows us to replace the loop above with the following code sequence, which makes

possible overlapping of the iterations of the loop:

a[1] = a[1] + b[1];

for (i=1; i<=99; i= i+1){

b[i+1] = c[i] + d[i];

a[i+1] = a[i+1] + b[i+1];

}

b[101] = c[100] + d[100];

Example 3

for (i=1; i<=100; i= i+1){

a[i+1] = a[i] + c[i]; //S1

b[i+1] = b[i] + a[i+1]; //S2

}

This loop is not parallel it has cycles in the dependencies, namely the statements S1 and S2

depend on themselves!

35

There are a number of techniques for converting such loop-level

parallelism into instruction-level parallelism. Basically, such

techniques work by unrolling the loop.

An important alternative method for exploiting loop-level

parallelism is the use of vector instructions on a vector

processor.

http://web.cs.iastate.edu/~prabhu/Tutorial/PIPELINE/loopUnrolling.html

dealing with exceptions computer architecture part 1

Technology