Exception entry and exit sequence in ARM Architecture - Step-by-Step Execution

Concept Flow - Exception entry and exit sequence

Exception Occurs

↓

Save Context

↓

Switch to Exception Mode

↓

Jump to Exception Vector

↓

Execute Exception Handler

↓

Restore Context

↓

Return from Exception

↓

Resume Normal Execution

This flow shows how the ARM processor handles an exception by saving the current state, switching modes, running the handler, then restoring state to resume.

Execution Sample

ARM Architecture

Exception occurs
Save registers
Switch mode
Jump to vector
Handle exception
Restore registers
Return from exception

Steps the ARM CPU takes to handle an exception and return to normal execution.

Analysis Table

Step	Action	Processor State	Registers Saved	Next Step
1	Exception occurs	User mode	None	Save context
2	Save context	User mode	R14, SPSR	Switch to exception mode
3	Switch to exception mode	Exception mode	Context saved	Jump to vector
4	Jump to exception vector	Exception mode	Context saved	Execute handler
5	Execute exception handler	Exception mode	Context saved	Restore context
6	Restore context	Exception mode	Registers restored	Return from exception
7	Return from exception	User mode	Context restored	Resume normal execution
8	Resume normal execution	User mode	None	Normal program flow

💡 Exception handled and processor returns to user mode resuming normal execution

State Tracker

Variable	Start	After Step 2	After Step 6	Final
Processor Mode	User	Exception	Exception	User
Registers Saved	None	R14, SPSR	None	None
Program Counter	Normal PC	Saved PC	Saved PC	Normal PC

Key Insights - 3 Insights

Why does the processor switch mode during exception handling?

The processor switches to exception mode to isolate exception handling from normal code, as shown in execution_table steps 3 and 7.

What is saved during the exception entry?

Register R14 and the SPSR are saved to preserve the current state, as seen in execution_table step 2.

How does the processor know where to jump when an exception occurs?

It jumps to a fixed exception vector address, indicated in execution_table step 4.

Visual Quiz - 3 Questions

According to the execution_table, at which step does the processor switch back to user mode?

A Step 5

B Step 3

C Step 7

D Step 2

💡 Hint

Look at the 'Processor State' column in execution_table rows for steps 3 and 7.

What registers are saved during the exception entry as per the variable_tracker?

A R14 and SPSR

B Only CPSR

C Only R0-R7

D No registers are saved

💡 Hint

Check the 'Registers Saved' row in variable_tracker after Step 2.

If the processor did not save the SPSR during exception entry, what would happen?

A Exception handler would run normally

B Processor could not restore previous state correctly

C Processor would skip the exception handler

D Processor would stay in exception mode permanently

💡 Hint

Consider the importance of SPSR in restoring processor state from key_moments and execution_table.

Concept Snapshot

Exception entry saves R14 and SPSR,
switches processor to exception mode,
jumps to a fixed vector address,
executes handler,
restores saved context,
and returns to user mode resuming normal flow.

Full Transcript

When an exception occurs in ARM architecture, the processor first saves the current state including R14 and the SPSR. Then it switches from user mode to an exception mode to isolate the handler. Next, it jumps to a fixed exception vector address to execute the exception handler code. After handling the exception, the processor restores the saved registers and SPSR to return to the original state. Finally, it switches back to user mode and resumes normal program execution. This sequence ensures that the program can continue correctly after the exception is handled.

Practice

1. What is the main purpose of the exception entry sequence in ARM architecture?

easy

A. To increase the processor clock speed temporarily

B. To execute normal instructions without interruption

C. To save the current processor state and switch to exception mode

D. To reset the processor completely

Solution

1. Step 1: Understand what happens during exception entry

   When an exception occurs, the processor saves its current state to avoid losing information.

2. Step 2: Recognize the mode switch

   The processor switches to a special mode to handle the exception safely.

3. Final Answer:

   To save the current processor state and switch to exception mode -> Option C

4. Quick Check:

   Exception entry = Save state + switch mode [OK]

Hint: Exception entry always saves state and switches mode [OK]

Common Mistakes:

• Thinking exception entry resets the processor
• Confusing exception entry with normal instruction execution
• Assuming clock speed changes during exception

2. Which of the following correctly describes the first step in the exception exit sequence?

easy

A. Switch the processor to a special exception mode

B. Restore the saved processor state from the stack

C. Disable all interrupts permanently

D. Clear all registers to zero

Solution

1. Step 1: Identify the start of exception exit

   The processor begins exception exit by restoring the saved state to continue normal operation.

2. Step 2: Understand what is restored

   The saved registers and program counter are loaded back from the stack or special registers.

3. Final Answer:

   Restore the saved processor state from the stack -> Option B

4. Quick Check:

   Exception exit starts by restoring state [OK]

Hint: Exception exit begins by restoring saved state [OK]

Common Mistakes:

• Confusing mode switch with state restoration
• Thinking interrupts are disabled permanently
• Assuming registers are cleared during exit

3. Consider this simplified ARM exception entry sequence:

1. Save CPSR to SPSR
2. Save return address to LR
3. Switch to exception mode
4. Set PC to exception vector

What will be the value of PC after step 4 if the exception vector address is 0x00000010?

medium

A. Value of CPSR

B. 0x00000000

C. Value of LR

D. 0x00000010

Solution

1. Step 1: Understand the exception vector role

   The PC is set to the exception vector address to start executing the exception handler.

2. Step 2: Apply the given vector address

   The vector address given is 0x00000010, so PC will be set to this value.

3. Final Answer:

   0x00000010 -> Option D

4. Quick Check:

   PC after entry = exception vector address [OK]

Hint: PC points to exception vector after entry [OK]

Common Mistakes:

• Confusing PC with LR or CPSR values
• Assuming PC remains unchanged
• Mixing up saved registers with PC

4. A programmer notices that after an exception exit, the processor does not return to the original task but stays in exception mode. What is the most likely cause?

medium

A. The saved state was not restored correctly during exception exit

B. The exception entry sequence did not save the CPSR

C. The processor clock speed was changed during exception

D. The exception vector address was set incorrectly

Solution

1. Step 1: Analyze the problem

   If the processor stays in exception mode after exit, it means it did not restore the previous mode and state.

2. Step 2: Identify the cause

   Not restoring the saved state (including CPSR and PC) during exception exit causes the processor to remain in exception mode.

3. Final Answer:

   The saved state was not restored correctly during exception exit -> Option A

4. Quick Check:

   Incorrect state restore = stuck in exception mode [OK]

Hint: Restore saved state fully to exit exception mode [OK]

Common Mistakes:

• Blaming clock speed changes
• Confusing entry saving with exit restoring
• Assuming vector address affects exit mode

5. During an exception entry, the processor saves the CPSR to SPSR and the return address to LR. If the exception handler modifies the LR but forgets to restore it before exit, what will happen when the exception exit sequence runs?

hard

A. The processor will return to an incorrect address, causing unexpected behavior

B. The processor will reset automatically to fix the error

C. The exception handler will run again immediately

D. The processor will ignore the LR and continue normally

Solution

1. Step 1: Understand the role of LR in exception handling

   LR holds the return address to resume normal execution after the exception.

2. Step 2: Analyze the effect of modifying LR without restoring

   If LR is changed and not restored, the processor will jump to the wrong address on exit.

3. Step 3: Predict the outcome

   This causes unexpected behavior or crashes because the program flow is disrupted.

4. Final Answer:

   The processor will return to an incorrect address, causing unexpected behavior -> Option A

5. Quick Check:

   Unrestored LR = wrong return address [OK]

Hint: Always restore LR to return correctly after exception [OK]

Common Mistakes:

• Assuming processor resets automatically
• Thinking exception handler repeats immediately
• Believing LR can be ignored safely