Exception Processing

ECE511: Digital System & Microprocessor

What we are going to learn in this session: What are exceptions. How M68k handles exceptions. M68k execution states and how they relate

to exception handling.

Introduction

Exceptions

Out-of-ordinary events that happen during M68k run-time.

Caused by: Interrupt requests.Errors during processing cycle.Arithmetic operations that cannot be

completed. Illegal instructions.

Exception Example (Divide by Zero)

MOVE.W #0,D0 MOVE.L #500000, D1 DIVU D0,D1

500,000 / 0 = ?

* Divide by Zero Exception

Exception Example: Address Error

Memory range = $000000 to $004000 MOVE.B #$0A,$004500

$0000$0001$0002

……

$3FFE$3FFF$4000

$4500?(Address out of range)

* Address Error Exception

Exception Example: Address Error

MOVE.B #$A00A,$000001

$0000$0001$0002

……

$3FFE$3FFF$4000

Word transfer to odd address.

* Address Error Exception

External peripheral asks for attention by outputting interrupt on IPL0, IPL1, IPL2.

Exception Example: Interrupt Request

M68k External Peripheral

1 M68k is executing instructions normally.

2 External peripheral hasimportant task for M68k.

3

M68k compares interrupt level to SR.

4

T S I2 I1 I0 X N Z V CM68k services/holds interrupt based on interrupt level

5

Exception Example: Spurious InterruptS0 S1 S2 S3 S4 S5 S6 S7 S0 S1 S2

CLK

A1 – A23

AS

LDS/UDS

R/W

DTACK

D0 – D15

FC0 – FC2

DTACK never responds

Exceptions

Exceptions should be handled properly:Can cause data loss.System failure.No prioritizing of tasks.

M68k handles exceptions using Exception Handling.

M68k Execution States

M68k Execution States

M68k always functions in either of the three states:Normal.Halted.Exception.

Normal State

M68k operating normally. Executing in user mode (S=0). Has a restricted instruction set:

Can’t execute privileged instructions.Prevents from executing potentially

destructive instructions. Reserved for SV.

Restricted Commands in Normal State STOP RESET RTE Any commands that modify the SR. MOVE USP

Exception State

Handles special events (exceptions), then returns to normal execution.

Can execute all instructions. Always saves processor context before

handling exception, restored when returned to normal execution.

Always executes in SV mode (S=1).

The Status Register

T S I2 I1 I0 X N Z V C

Supervisor Mode, S = 1 (Exception State, Halted State)User Mode, S = 0 (Normal State)

Halted State

M68k stops execution of all instructions. Done by activating HALT line:

Stops execution after current instruction.Waits until HALT is inactive.Resumes normal execution.

Halted State

Reason:Catastrophic system failures (double bus

faults, hardware failure).User: intentionally activating the HALT line.

Purpose:Protect data inside memory.

How States are Changed

Normal HaltedException

Double bus fault/User-initiated

S=0, RTE, Edit SR

S=1, Exceptions, Interrupt

Reset Exception

User-initiated

Exception Handling

Exception Handling

Series of steps performed to handle exceptions. Ensures exceptions processed properly &

resume normal execution. Allows M68k to:

Save all processor status before exception. Handle the exception. Restore processor status when exception has been

handled.

What Happens During an Exception? Contents of SR saved to memory. S bit is set. Exception vector is obtained. Processor context (PC & SR) saved on software

stack. PC set to address of exception vector. Execution resumes from new PC location. Control and context restored & execution

resumes from last address before exception.

Exception Handler Operation

TRAPV

Save PC and SR to stack.

Read handler address from Vector Table.

Read original PC and SR from

stack.

Save D0-D7, A0-A7

Handler Code

Reload D0-D7, A0-A7

RTE

Next Instruction

Exception Processing Steps

Exception Processing Steps

1. Adjust SR.

2. Get Vector Number.

3. Save processor information.

4. Fetch new PC.

5. Restore context, resume last instruction in Normal mode.

Step 1: Adjust SR

To enter SV state and prepare for exception processing:Store SR internally.Set S = 1 (SV Mode).Set T = 0 (Disable Tracing). If interrupt, update Interrupt Mask Bits.

SR Bits Modified by Step 1

T S I2 I1 I0 X N Z V C

S = 1T = 0

Updated if:IREQ > ICURRENT

Step 2: Get Vector Number

Determines what type of exception, and get location of its appropriate handler.

VN: 8-bit value indicating exception type.Supplied by external device or M68k.What kind of exception is happening.Ranges from $00 (0) to $FF (255).

VN used to find Vector Address (VA).

Vector Address (VA)

Special memory location that stores addresses of exception handlers.

VA obtained by multiplying VN with 4. VN x 4 = VA.

VA contains a long-word value containing address of exception handler.

Contents inside VA loaded into PC, execution continued at this location.

How Exception Handling Works

1. VN given by external circuit/M68k. Based on VN, M68k knows what type of exception has occurred.

VA = VN x 4VN

VN

2. VN x 4 = VA. Address of exception handler contained inside VA. M68k loads value into PC.

$0000 xxx$0004 xxx$0008 $3000$000C xxx$0010 xxx

PC $3000

3. M68k saves registers into stack, execution resumes at exception handler address.

$3000$3002$3004$0006$0018

Exception handler

codeRTE

Vector Table (Antonakos, pg. 110)

VN Assignment

0 Reset: Initial SSP and PC

2 Bus Error

3 Address Error

5 Divide by Zero

8 Privilege Violation

9 Trace

24 Spurious Interrupt

25-31 Interrupt Auto-vectors

32-47 TRAP Instruction Vectors

VA = VN x 4

0 = $0000

$0064 - $007C

$0080 – 00BC

96 = $0060

36 = $0024

32 = $0020

20 = $0014

12 = $000C

8 = $0008

Step 3: Save Processor Information

Saves processor context into stack:PC: next instruction after exception

processing.SR.Bus cycle attempted, part of instruction (for

bus error and address error). SSP is used already in SV mode.

How Context Saved into Stack – Typical Exception

Status Register

PC (High Word)

PC (Low Word)

Old SSP location

New SSP location

How Context Saved into Stack

PC = $001111 SR = $0115 SSP = $002FFF

$0115

$0000

$1111

…$002FFF

$002FFD

$002FFB

$002FF8

SSP

SSP-2

SSP-4

SSP-6

Old SSP location

New SSP location

Step 4: Fetch New PC

Loads PC from vector table in Step 2. Processing resumes at exception handler. Returned to normal processing using

(Return to Exception) RTE.

Exception Handler

Contains instructions on how to process the exception.

Located in SV memory space. RTE at the end, resumes normal

execution.

Exception Handler Contents

Save Registers

Handler Code

Reload Registers

RTE (Return to Exception)

Save Registers, PC into Stack

Code to handle exception

Reload Registers from Stack

Reload PC from stack, execution resumes at next instruction after exception.

Exception Example (Divide by Zero)

MOVE.W #0,D0 MOVE.L #500000, D1 DIVU D0,D1

PC = $1040 SR =

500,000 / 0 = ?* Divide by Zero Exception

T S I I I X N Z V C

0 0 0 0 0 0 0 0 0 0 1 0 1 1 00

0 0 1 6

Step 1: Adjust SR

Store SR internally. Set S = 1 (SV Mode). Set T = 0 (Disable Tracing). If interrupt, update Interrupt Mask Bits.

Step 1: Adjust SR

T S I I I X N Z V C

0 0 1 0 0 0 0 0 0 0 1 0 1 1 00

T = 0 S = 1 Not interrupt = unchanged

Step 2: Get Vector Number

Divide by zero exception VN is VN = 5 VN x 4 used to find Vector Address (VA).

VA = VN x 4 = 20 = $000014

Vector Address (VA)

Special memory location that stores addresses of exception handlers.

VA obtained by multiplying VN with 4. VN x 4 = VA.

VA contains a long-word value containing address of exception handler.

Contents inside VA loaded into PC, execution continued at this location.

Vector Table

Value

$00400500

$00400600

$00400700

$00400800

$00400900

$00400A00

$00400B00

Address

$0000

$0060

$0024

$0020

$0014

$000C

$0008

Step 3: Save Processor Information

Saves processor context into stack. SSP is used already in SV mode.

PC = $001040SSP/A7 = $004000SR = $0016

$0016 (SR)

$0000 (PC)

$1040 (PC)

Old SSP $4000

New SSP $3FFA

Step 4: Fetch New PC

Loads PC from vector table in Step 2. Processing resumes at exception handler.

Value

$00400500

$00400600

$00400700

$00400800

$00400900

$00400A00

$00400B00

Address

$0000

$0060

$0024

$0020

$0014

$000C

$0008

New PC = $00400800

Exception Handler Contents

Save D0-D7, A0–A7

Handler Code

Reload D0-D7, A0-A7

RTE (Return to Exception)

Save Registers into Stack

Code to handle exception

Reload Registers from Stack

Reload PC, SR from stack, execution resumes at next instruction after DIVU.

$00400800

Handling Multiple Exceptions

Handling Multiple Exceptions

It is possible for another exception to happen when the current exception is being handled.An bus error occurs when M68k is handling

an interrupt.A trace exception generated during a divide

by zero exception.

How does M68k handle multiple exceptions? In M68k, each exception has priority level:

Reset exception has higher priority than Trace exception.

An interrupt exception is more important than a Divide-by-Zero exception.

When multiple exceptions happen, the more important one executed first.

Exception Grouping & Priority

Group Exception Priority

Reset

Bus Error

Address Error

Trace

Interrupt

Illegal

Privilege

TRAP

TRAPV

CHK

Zero Divide

0

(Highest)

(Lowest)

1

(Highest)

(Lowest)

2All have

same priority.

Impo

rtan

ce

Handling Multiple Exceptions

Exceptions divided into 3 priority levels:The higher-up, the more important.Group 0 > Group 1 > Group 2.Reset > Bus Error > Address Error > …..

What happens during multiple exceptions? The new exception compared to the current

exception being handled: If higher, creates an exception inside an exception:

Saves processor context into stack. Handle new exception. Restore context from stack, return to old exception handle.

If lower, wait, then handle new exception. Save new exception request. Wait until current exception finishes. Handle new exception.

New < Old Exception - Example

During a Trace exception (T = 1), an interrupt request has occurred. How would M68k handle this situation?

Exception Grouping & Priority

Group Exception Priority

Reset

Bus Error

Address Error

Trace

Interrupt

Illegal

Privilege

TRAP

TRAPV

CHK

Zero Divide

0

(Highest)

(Lowest)

1

(Highest)

(Lowest)

2All have

same priority.

Trace exception higher than interrupt. Trace will be handled first.

New < Old Exception

EXG (D0,D1)

Trace Handler

Save contextInterrupt Handler

Restore Context

*Trace Handler executed first, then Interrupt Handler executed.

New > Old Exception - Example

During an Interrupt exception, an Address Error Exception has occurred. How would M68k handle this situation?

Exception Grouping & Priority

Group Exception Priority

Reset

Bus Error

Address Error

Trace

Interrupt

Illegal

Privilege

TRAP

TRAPV

CHK

Zero Divide

0

(Highest)

(Lowest)

1

(Highest)

(Lowest)

2All have

same priority.

Address Error exception higher than Interrupt. Address Error will be handled first.

New > Old Exception

EXG (D0,D1)

Interrupt Handler

Save context

AddressError

Handler

Restore Context

Ins. in Handler

*Address Error Handler executed first, then Interrupt Handler executed.

Conclusion

Conclusion

Exceptions are extraordinary situations that effect normal M68k operation.

Exception Handling help M68k to:Recover from errors.Prioritize CPU tasks.

M68k recognizes exceptions using VN.

Conclusion

Exception handling involves: Saving processor context. Handling the exception. Restore processor context. Return to normal execution once exception is

handled. Handles are instructions used to manage

exceptions. Located in SV memory location.

Exception Handler Operation

TRAPV

Save return address and

flags on stack.

Read handler address from Vector Table.

Read return address and

flags from stack.

Save Registers

Handler Code

Reload Registers

RTE

The End

Please read:

Antonakos, pg. 103-133.