Thumb and ARM instruction sets in ARM Architecture - Step-by-Step Execution

Concept Flow - Thumb and ARM instruction sets

Start: CPU fetches instruction

↓

Check instruction mode

↓

ARM mode

↓

32-bit instruction

↓

Execute full ARM instruction

↓

Next instruction fetch

↓

Repeat

The CPU fetches instructions and checks if it is in ARM or Thumb mode. ARM mode uses 32-bit instructions, Thumb mode uses 16-bit compressed instructions. Each executes accordingly before fetching the next instruction.

Execution Sample

ARM Architecture

Mode = ARM
Fetch instruction (32-bit)
Decode and execute
Mode = Thumb
Fetch instruction (16-bit)
Decode and execute

This shows switching between ARM and Thumb modes, fetching and executing instructions of different sizes.

Analysis Table

Step	Mode	Instruction Size	Action	Result
1	ARM	32-bit	Fetch instruction	Instruction fetched from memory
2	ARM	32-bit	Decode instruction	Instruction decoded as ARM
3	ARM	32-bit	Execute instruction	Operation performed
4	ARM	32-bit	Next instruction fetch	Prepare next instruction
5	Thumb	16-bit	Fetch instruction	Instruction fetched from memory
6	Thumb	16-bit	Decode instruction	Instruction decoded as Thumb
7	Thumb	16-bit	Execute instruction	Operation performed
8	Thumb	16-bit	Next instruction fetch	Prepare next instruction
9	Thumb	16-bit	Condition check	If mode switch requested
10	ARM	32-bit	Switch mode	CPU switches back to ARM mode
11	ARM	32-bit	Fetch instruction	Instruction fetched from memory
12	ARM	32-bit	Decode instruction	Instruction decoded as ARM
13	ARM	32-bit	Execute instruction	Operation performed
14	ARM	32-bit	Next instruction fetch	Prepare next instruction
15	-	-	End	No more instructions or stop condition

💡 Execution stops when no more instructions are fetched or a stop condition occurs.

State Tracker

Variable	Start	After Step 4	After Step 8	After Step 10	Final
Mode	ARM	ARM	Thumb	ARM	ARM
Instruction Size	32-bit	32-bit	16-bit	32-bit	32-bit
Instruction	None	ARM instruction	Thumb instruction	ARM instruction	ARM instruction

Key Insights - 3 Insights

Why does the CPU switch between 32-bit and 16-bit instructions?

The CPU switches modes to balance performance and code size. ARM mode uses 32-bit instructions for full power, Thumb mode uses 16-bit instructions to save memory. See execution_table steps 1-8.

How does the CPU know which instruction set to use?

The CPU checks the current mode flag before fetching instructions. This is shown in execution_table step 1 and step 5 where mode determines instruction size.

Can the CPU execute ARM and Thumb instructions mixed together?

Yes, but it switches modes explicitly. The CPU fetches and executes instructions in one mode at a time, switching as needed (see steps 9-10).

Visual Quiz - 3 Questions

Look at the execution_table at step 6, what is the instruction size?

A 16-bit

B 8-bit

C 32-bit

D 64-bit

💡 Hint

Check the 'Instruction Size' column at step 6 in the execution_table.

At which step does the CPU switch from Thumb mode back to ARM mode?

A Step 8

B Step 10

C Step 4

D Step 12

💡 Hint

Look for 'Switch mode' action in the execution_table.

If the CPU stayed in ARM mode only, how would the 'Instruction Size' column change in the execution_table?

A It would always be 16-bit

B It would alternate between 16-bit and 32-bit

C It would always be 32-bit

D It would be 8-bit

💡 Hint

Refer to the 'Mode' and 'Instruction Size' columns in the execution_table for ARM mode steps.

Concept Snapshot

Thumb and ARM instruction sets:
- ARM mode uses 32-bit instructions for full power.
- Thumb mode uses 16-bit instructions to save memory.
- CPU switches modes to balance speed and size.
- Instructions are fetched and executed according to current mode.
- Mode switching is explicit and controlled by CPU.

Full Transcript

This visual execution trace shows how a CPU using ARM architecture switches between ARM and Thumb instruction sets. The CPU starts in ARM mode, fetching and executing 32-bit instructions. When switching to Thumb mode, it fetches and executes 16-bit instructions, which are smaller and save memory. The CPU checks its mode before each instruction fetch to decide instruction size. Mode switching happens explicitly, allowing mixing of ARM and Thumb instructions but only one mode active at a time. The execution table traces each step, showing mode, instruction size, action, and result. Variable tracking shows how mode and instruction size change over time. Key moments clarify why mode switching happens and how the CPU knows which instruction set to use. The quiz tests understanding of instruction sizes and mode switching steps. The concept snapshot summarizes the key points about ARM and Thumb instruction sets and their usage in CPU execution.

Practice

1. What is the main difference between ARM and Thumb instruction sets?

easy

A. ARM instructions are only for graphics, Thumb is for audio processing

B. Thumb instructions are 64-bit wide, ARM instructions are 16-bit wide

C. ARM instructions are 32-bit wide, Thumb instructions are mostly 16-bit wide

D. ARM and Thumb instructions have the same size and purpose

Solution

1. Step 1: Understand instruction size

   ARM instructions are designed to be 32 bits wide, allowing more complex operations.

2. Step 2: Compare with Thumb instructions

   Thumb instructions are mostly 16 bits wide, which helps save memory space.

3. Final Answer:

   ARM instructions are 32-bit wide, Thumb instructions are mostly 16-bit wide -> Option C

4. Quick Check:

   Instruction size difference = ARM 32-bit, Thumb 16-bit [OK]

Hint: Remember: ARM = 32-bit, Thumb = 16-bit instructions [OK]

Common Mistakes:

• Confusing instruction sizes
• Thinking both have same width
• Mixing up ARM and Thumb purposes

2. Which of the following is a correct syntax feature of Thumb instructions compared to ARM instructions?

easy

A. Thumb instructions mostly use 16-bit encoding

B. Thumb instructions use 32-bit registers only

C. Thumb instructions cannot be mixed with ARM instructions

D. Thumb instructions require double the memory of ARM

Solution

1. Step 1: Recall Thumb instruction encoding

   Thumb instructions mostly use 16-bit encoding to save memory.

2. Step 2: Check other options

   Thumb can mix with ARM instructions and uses less memory, not more.

3. Final Answer:

   Thumb instructions mostly use 16-bit encoding -> Option A

4. Quick Check:

   Thumb = 16-bit encoding [OK]

Hint: Thumb = mostly 16-bit encoding, saves memory [OK]

Common Mistakes:

• Assuming Thumb uses 32-bit registers only
• Believing Thumb and ARM cannot mix
• Thinking Thumb uses more memory

3. Consider a processor switching from ARM mode to Thumb mode. What is the main benefit of this switch?

medium

A. Improves performance by using 64-bit instructions

B. Increases power consumption for better speed

C. Disables interrupts for faster execution

D. Saves memory space by using smaller 16-bit instructions

Solution

1. Step 1: Understand mode switching purpose

   Switching to Thumb mode allows the processor to use smaller 16-bit instructions.

2. Step 2: Identify benefit of smaller instructions

   Smaller instructions save memory space and can improve code density.

3. Final Answer:

   Saves memory space by using smaller 16-bit instructions -> Option D

4. Quick Check:

   Thumb mode saves memory with 16-bit instructions [OK]

Hint: Switch to Thumb to save memory with 16-bit instructions [OK]

Common Mistakes:

• Thinking Thumb uses 64-bit instructions
• Confusing power consumption effects
• Assuming interrupts are disabled

4. A developer writes ARM assembly code but tries to run it in Thumb mode without modification. What is the likely result?

medium

A. Syntax errors occur because instruction sizes differ

B. Code runs correctly with no issues

C. Processor automatically converts ARM to Thumb instructions

D. Code runs faster due to Thumb optimizations

Solution

1. Step 1: Understand instruction compatibility

   ARM and Thumb instructions have different sizes and encodings.

2. Step 2: Analyze running ARM code in Thumb mode

   Running ARM code directly in Thumb mode causes syntax errors due to size mismatch.

3. Final Answer:

   Syntax errors occur because instruction sizes differ -> Option A

4. Quick Check:

   ARM code in Thumb mode = syntax errors [OK]

Hint: ARM code won't run in Thumb mode without changes [OK]

Common Mistakes:

• Assuming automatic conversion happens
• Believing code runs faster without changes
• Ignoring instruction size differences

5. A programmer wants to optimize an embedded system for both speed and memory. Which strategy best uses ARM and Thumb instruction sets together?

hard

A. Use ARM instructions only to save memory

B. Use ARM instructions for critical speed parts and Thumb for less critical code

C. Use only Thumb instructions everywhere for maximum speed

D. Switch randomly between ARM and Thumb without planning

Solution

1. Step 1: Identify strengths of ARM and Thumb

   ARM instructions are powerful and fast but larger; Thumb instructions save memory but may be slower.

2. Step 2: Apply strategy for optimization

   Use ARM instructions in speed-critical parts and Thumb instructions in less critical parts to balance speed and memory.

3. Final Answer:

   Use ARM instructions for critical speed parts and Thumb for less critical code -> Option B

4. Quick Check:

   Balance speed and memory by mixing ARM and Thumb [OK]

Hint: Use ARM for speed, Thumb for memory in critical vs non-critical code [OK]

Common Mistakes:

• Using only Thumb for speed
• Using ARM only to save memory
• Switching modes without plan