Signed and unsigned integers in ARM Architecture - Mini Project: Build & Apply

Understanding Signed and Unsigned Integers in ARM Architecture

📖 Scenario: You are learning how ARM processors handle numbers. ARM uses signed and unsigned integers to represent positive and negative numbers differently. This helps the processor understand how to store and calculate values.Imagine you have a box that can hold numbers. Signed integers let the box hold both positive and negative numbers, while unsigned integers let the box hold only positive numbers but with a bigger maximum value.

🎯 Goal: Build a simple example showing how to define signed and unsigned integers in ARM assembly style notation and understand their value ranges.

📋 What You'll Learn

Create variables representing signed and unsigned 8-bit integers with exact values

Add a variable to hold the maximum value for unsigned 8-bit integers

Write a comment explaining the range of signed 8-bit integers

Add a final comment summarizing the difference between signed and unsigned integers

💡 Why This Matters

🌍 Real World

Understanding signed and unsigned integers is essential for low-level programming, embedded systems, and working directly with ARM processors in devices like smartphones and IoT gadgets.

💼 Career

Knowledge of signed and unsigned integers is critical for roles in embedded software development, firmware engineering, and systems programming where precise control over data representation is required.

1

Create signed and unsigned 8-bit integer variables

Create two variables named signed_val and unsigned_val. Set signed_val to -10 and unsigned_val to 250. Use 8-bit integer notation.

ARM Architecture

# Define signed and unsigned 8-bit integers
# Your code here

Hint

Use .byte to define 8-bit integers. Signed values can be negative; unsigned values are positive.

2

Add maximum value for unsigned 8-bit integer

Add a variable named max_unsigned_8bit and set it to the maximum value for an unsigned 8-bit integer, which is 255.

ARM Architecture

signed_val: .byte -10
unsigned_val: .byte 250
# Define max unsigned 8-bit value
# Your code here

Hint

The maximum value for unsigned 8-bit integers is 255 because 8 bits can represent 2⁸ = 256 values starting from 0.

3

Add comment explaining signed 8-bit integer range

Add a comment above signed_val explaining that signed 8-bit integers range from -128 to 127.

ARM Architecture

# Your comment here
signed_val: .byte -10
unsigned_val: .byte 250
max_unsigned_8bit: .byte 255

Hint

Comments start with # in ARM assembly.

4

Add final comment summarizing signed vs unsigned

Add a final comment at the end of the code explaining that signed integers can hold negative and positive numbers, while unsigned integers hold only positive numbers but have a higher maximum value.

ARM Architecture

# Signed 8-bit integers range from -128 to 127
signed_val: .byte -10
unsigned_val: .byte 250
max_unsigned_8bit: .byte 255
# Your summary comment here

Hint

Summarize the key difference between signed and unsigned integers in a comment.

Practice

1.

What is the main difference between signed and unsigned integers in ARM architecture?

easy

A. Signed integers use more memory than unsigned integers.

B. Signed integers can represent negative and positive numbers, unsigned only non-negative.

C. Unsigned integers can represent negative numbers, signed cannot.

D. Signed integers are faster to process than unsigned integers.

Solution

1. Step 1: Understand signed integers

   Signed integers use two's complement to represent both negative and positive numbers.

2. Step 2: Understand unsigned integers

   Unsigned integers represent only zero and positive numbers, allowing a larger positive range.

3. Final Answer:

   Signed integers can represent negative and positive numbers, unsigned only non-negative. -> Option B

4. Quick Check:

   Signed = negative and positive, Unsigned = only positive [OK]

Hint: Signed = negatives allowed, unsigned = only zero or positive [OK]

Common Mistakes:

• Thinking unsigned can represent negative numbers
• Assuming signed uses more memory
• Believing signed is always faster

2.

Which of the following is the correct way to declare an unsigned 8-bit integer in ARM assembly?

easy

A. mov r0, #255 (assuming r0 is unsigned 8-bit)

B. movs r0, #255

C. mov r0, #-1

D. mov r0, #256

Solution

1. Step 1: Understand unsigned 8-bit range

   Unsigned 8-bit integers range from 0 to 255, so 255 is valid.

2. Step 2: Check instructions

   mov r0, #255 moves the immediate value 255 into register r0 correctly.

3. Final Answer:

   mov r0, #255 (assuming r0 is unsigned 8-bit) -> Option A

4. Quick Check:

   Unsigned 8-bit max = 255, mov loads immediate [OK]

Hint: Unsigned 8-bit max is 255, use mov with that value [OK]

Common Mistakes:

• Using movs which sets flags unnecessarily
• Trying to move negative values to unsigned
• Using values above 255 for 8-bit

3.

Consider a 4-bit signed integer using two's complement. What decimal value does the binary 1101 represent?

medium

A. -3

B. 13

C. -5

D. 5

Solution

1. Step 1: Identify two's complement sign

   In 4-bit two's complement, if the leftmost bit is 1, the number is negative.

2. Step 2: Calculate decimal value

   Binary 1101: invert bits -> 0010, add 1 -> 0011 (3 decimal), so value is -3. But careful: 1101 is -3 or -5? Let's check carefully:
   1101 binary = (1x-8) + (1x4) + (0x2) + (1x1) = -8 + 4 + 0 + 1 = -3

3. Final Answer:

   -3 -> Option A

4. Quick Check:

   Two's complement 1101 = -3 decimal [OK]

Hint: If MSB is 1, subtract 2^bits from value [OK]

Common Mistakes:

• Treating binary as unsigned
• Forgetting two's complement negative calculation
• Mixing bit positions

4.

In ARM assembly, a programmer tries to store the value -1 in an unsigned 8-bit register. What is the likely result?

medium

A. The program crashes at runtime.

B. Syntax error occurs during assembly.

C. The register stores 0.

D. The register stores 255 due to wrap-around.

Solution

1. Step 1: Understand unsigned storage

   Unsigned 8-bit registers store values 0 to 255. Negative values wrap around using modulo 256.

2. Step 2: Calculate wrap-around for -1

   -1 modulo 256 equals 255, so the register stores 255.

3. Final Answer:

   The register stores 255 due to wrap-around. -> Option D

4. Quick Check:

   Unsigned wrap-around: -1 = 255 [OK]

Hint: Negative to unsigned wraps modulo max+1 [OK]

Common Mistakes:

• Expecting syntax error for negative immediate
• Assuming register stores zero
• Thinking program crashes

5.

You have a 16-bit unsigned integer register holding the value 65535. You subtract 1 from it. What is the result stored in the register?

hard

A. 65536

B. -1

C. 65534

D. 0

Solution

1. Step 1: Understand 16-bit unsigned range

   16-bit unsigned integers range from 0 to 65535.

2. Step 2: Perform subtraction

   65535 - 1 = 65534, which fits in 16 bits without wrap-around.

3. Final Answer:

   65534 -> Option C

4. Quick Check:

   65535 - 1 = 65534 within unsigned 16-bit [OK]

Hint: Subtract within range stays positive, no wrap [OK]

Common Mistakes:

• Thinking result becomes negative
• Assuming overflow to zero
• Confusing unsigned with signed behavior