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.