Access permissions (privileged, unprivileged) in ARM Architecture - Step-by-Step Execution

Concept Flow - Access permissions (privileged, unprivileged)

Start: CPU executes instruction

↓

Check current mode

↓

Allow full access

↓

Execute instruction

↓

Access allowed?

↓

Execute instruction

The CPU checks if it is in privileged or unprivileged mode before executing instructions. Privileged mode allows full access, while unprivileged mode restricts access and may raise exceptions if unauthorized.

Execution Sample

ARM Architecture

if (mode == privileged) {
  access = full;
} else {
  access = restricted;
  if (try_access_forbidden_resource) {
    raise_exception();
  }
}

This code checks the CPU mode and sets access permissions accordingly, raising an exception if unprivileged code tries to access forbidden resources.

Analysis Table

Step	CPU Mode	Access Check	Action	Result
1	Privileged	Full access allowed	Execute instruction	Instruction runs successfully
2	Unprivileged	Restricted access	Try access allowed resource	Instruction runs successfully
3	Unprivileged	Restricted access	Try access forbidden resource	Exception raised
4	Privileged	Full access allowed	Execute instruction	Instruction runs successfully

💡 Execution stops or continues based on access permissions and exceptions raised.

State Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
mode	privileged	privileged	unprivileged	unprivileged	privileged
access	undefined	full	restricted	restricted	full
exception	none	none	none	raised	none

Key Insights - 2 Insights

Why does unprivileged mode restrict access to some resources?

Unprivileged mode limits access to protect system stability and security. As shown in execution_table step 3, trying to access forbidden resources raises an exception.

Can privileged mode code cause exceptions due to access permissions?

No, privileged mode has full access, so no access-related exceptions occur, as seen in steps 1 and 4 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the access level at step 2?

A Restricted access

B No access

C Full access

D Exception raised

💡 Hint

Check the 'Access Check' column at step 2 in the execution_table.

At which step does an exception get raised due to access violation?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Result' column in the execution_table for the step with 'Exception raised'.

If the CPU mode changes from unprivileged to privileged, what happens to the access variable?

A Changes from full to restricted

B Changes from restricted to full

C Remains restricted

D Remains full

💡 Hint

Refer to the variable_tracker for 'access' changes after step 3 and step 4.

Concept Snapshot

Access permissions control what CPU instructions can do.
Privileged mode has full access to all resources.
Unprivileged mode has limited access to protect the system.
Unauthorized access in unprivileged mode causes exceptions.
CPU checks mode before executing instructions.

Full Transcript

In ARM architecture, the CPU runs in either privileged or unprivileged mode. Privileged mode allows full access to system resources and instructions. Unprivileged mode restricts access to protect the system from unauthorized actions. When the CPU executes an instruction, it first checks the current mode. If privileged, the instruction runs with full access. If unprivileged, the CPU restricts access and checks if the instruction tries to use forbidden resources. If so, it raises an exception to prevent damage or security breaches. This mechanism ensures system stability and security by controlling what code can do based on its privilege level.

Practice

1. What is the main difference between privileged and unprivileged modes in ARM architecture?

easy

A. Unprivileged mode can access all system resources; privileged mode cannot.

B. Privileged mode is used only for user applications.

C. Privileged mode has full system access; unprivileged mode has limited access.

D. Unprivileged mode allows direct hardware control.

Solution

1. Step 1: Understand the roles of modes

   Privileged mode is designed for system control with full access, while unprivileged mode restricts access to protect the system.

2. Step 2: Compare access levels

   Privileged mode can access all system resources; unprivileged mode is limited to prevent accidental or malicious system changes.

3. Final Answer:

   Privileged mode has full system access; unprivileged mode has limited access. -> Option C

4. Quick Check:

   Access control = Privileged full, Unprivileged limited [OK]

Hint: Privileged = full access; unprivileged = limited access [OK]

Common Mistakes:

• Confusing which mode has full access
• Thinking unprivileged mode controls hardware directly
• Assuming privileged mode is for user apps only

2. Which of the following is the correct way to describe unprivileged mode in ARM architecture?

easy

A. It allows unrestricted access to all system registers.

B. It restricts access to certain system resources and instructions.

C. It is the mode where the operating system kernel runs.

D. It grants permission to modify memory management settings.

Solution

1. Step 1: Identify unprivileged mode characteristics

   Unprivileged mode limits access to protect system integrity by restricting certain instructions and resources.

2. Step 2: Eliminate incorrect options

   Unrestricted access and kernel operations belong to privileged mode, not unprivileged.

3. Final Answer:

   It restricts access to certain system resources and instructions. -> Option B

4. Quick Check:

   Unprivileged = restricted access [OK]

Hint: Unprivileged mode restricts system resource access [OK]

Common Mistakes:

• Mixing up privileged and unprivileged mode roles
• Assuming unprivileged mode can modify kernel settings
• Confusing kernel mode with unprivileged mode

3. Consider this scenario: A user application running in unprivileged mode tries to execute a system control instruction. What will happen?

medium

A. An exception or fault is triggered to prevent execution.

B. The system ignores the instruction and continues normally.

C. The instruction executes successfully without restrictions.

D. The instruction runs but with limited effect.

Solution

1. Step 1: Understand instruction permissions in unprivileged mode

   Unprivileged mode restricts execution of system control instructions to protect the system.

2. Step 2: Identify system response to unauthorized instruction

   Attempting such instructions triggers an exception or fault to prevent unauthorized access.

3. Final Answer:

   An exception or fault is triggered to prevent execution. -> Option A

4. Quick Check:

   Unauthorized instruction = Exception triggered [OK]

Hint: Unauthorized system instructions cause exceptions [OK]

Common Mistakes:

• Assuming instructions silently fail without error
• Thinking unprivileged mode allows all instructions
• Believing instructions partially execute

4. A developer writes code that runs in privileged mode but accidentally switches to unprivileged mode before accessing a protected register. What is the likely result?

medium

A. The system triggers a fault or exception due to access violation.

B. The register access succeeds without any issue.

C. The code automatically switches back to privileged mode.

D. The register access is ignored but no error occurs.

Solution

1. Step 1: Recognize access rules for protected registers

   Protected registers require privileged mode access; unprivileged mode cannot access them.

2. Step 2: Determine system behavior on violation

   Accessing protected registers in unprivileged mode causes a fault or exception to maintain security.

3. Final Answer:

   The system triggers a fault or exception due to access violation. -> Option A

4. Quick Check:

   Unprivileged access to protected register = Fault [OK]

Hint: Accessing protected registers unprivileged triggers fault [OK]

Common Mistakes:

• Assuming automatic mode switch back to privileged
• Thinking access silently fails without error
• Believing unprivileged mode can access all registers

5. In an ARM system, how does separating privileged and unprivileged modes improve system stability and security?

hard

A. By running all code in privileged mode for maximum control.

B. By allowing all applications to modify system settings freely.

C. By disabling hardware access completely for all software.

D. By preventing user applications from accidentally or maliciously changing critical system resources.

Solution

1. Step 1: Understand the purpose of mode separation

   Separating modes limits user applications from accessing critical system resources directly.

2. Step 2: Explain how this separation protects the system

   This prevents accidental or malicious changes that could cause crashes or security breaches.

3. Final Answer:

   By preventing user applications from accidentally or maliciously changing critical system resources. -> Option D

4. Quick Check:

   Mode separation = Protect system stability and security [OK]

Hint: Mode separation protects system from user errors [OK]

Common Mistakes:

• Thinking all code runs with full system control
• Assuming hardware access is disabled for all software
• Believing user apps can freely modify system settings