General purpose registers (R0-R12) in ARM Architecture - Practice Problems & Coding Challenges

Challenge - 5 Problems

📋 Factual

intermediate

Purpose of General Purpose Registers

What is the primary role of the general purpose registers R0 to R12 in ARM architecture?

A They store temporary data and intermediate results during program execution.

B They hold the program counter and status flags.

C They are used exclusively for stack management.

D They store floating-point numbers only.

💡 Hint

Think about what registers typically do in a CPU during calculations.

General purpose registers R0 to R12 are used to hold temporary data and intermediate results while the CPU executes instructions. They are versatile and not dedicated to special functions like the program counter or stack pointer.

🧠 Conceptual

intermediate

Register Usage in Function Calls

Which registers among R0 to R12 are typically used to pass arguments to functions in ARM calling conventions?

A R10 to R12

B R4 to R7

C R8 to R12

D R0 to R3

💡 Hint

Consider the first few registers used for passing data to functions.

In ARM calling conventions, registers R0 to R3 are used to pass the first four arguments to functions. This allows fast access without using memory.

🔍 Analysis

advanced

Effect of Using R7 in ARM Assembly

In ARM assembly, what is a common special use of register R7 in many operating systems?

A It is used to hold the system call number when making system calls.

B It stores the return address after a function call.

C It acts as the stack pointer.

D It is reserved for floating-point operations.

💡 Hint

Think about how system calls are made in ARM Linux.

In many ARM-based operating systems like Linux, R7 is used to hold the system call number before invoking a system call. This helps the kernel identify which service is requested.

❓ Comparison

advanced

Difference Between R12 and R13

Which statement correctly distinguishes the roles of R12 and R13 in ARM architecture?

A R12 is the program counter, and R13 is a general purpose register.

B R12 is a general purpose register, while R13 is typically used as the stack pointer.

C Both R12 and R13 are used exclusively for floating-point operations.

D R12 is used for system calls, and R13 holds the link register.

💡 Hint

Recall the special registers related to stack and program flow.

R12 is a general purpose register often called the Intra-Procedure-call scratch register, while R13 is reserved as the stack pointer (SP) in ARM architecture.

❓ Reasoning

expert

Register Preservation Across Function Calls

Which registers among R0 to R12 must a function preserve (save and restore) if it uses them, according to the ARM Procedure Call Standard (AAPCS)?

A R0 to R12

B R0 to R3

C R4 to R11

D R8 to R12 only

💡 Hint

Think about which registers are caller-saved and which are callee-saved.

According to the ARM Procedure Call Standard, registers R0 to R3 are caller-saved (used for passing arguments and can be overwritten), while R4 to R11 are callee-saved and must be preserved by the called function if used.

Practice

1. Which of the following registers are considered general purpose registers in ARM architecture?

easy

A. R0 to R12

B. R13 to R15

C. Program Counter and Status Register

D. Floating Point Registers

Solution

1. Step 1: Identify general purpose registers in ARM

   Registers R0 through R12 are used for general data storage and manipulation.

2. Step 2: Differentiate special registers

   Registers R13 to R15 and others like Program Counter are special purpose, not general.

3. Final Answer:

   R0 to R12 -> Option A

4. Quick Check:

   General purpose registers = R0-R12 [OK]

Hint: General purpose registers are R0 to R12 only [OK]

Common Mistakes:

• Confusing R13-R15 as general purpose
• Including special registers like Program Counter
• Thinking floating point registers are general purpose

2. Which of the following is the correct way to refer to the 5th general purpose register in ARM assembly?

easy

A. R6

B. R5

C. R4

D. R7

Solution

1. Step 1: Understand register numbering

   Registers start at R0, so the 1st is R0, 2nd is R1, and so on.

2. Step 2: Count to the 5th register

   Counting from R0: R0(1), R1(2), R2(3), R3(4), R4(5). So the 5th register is R4.

3. Step 3: Verify options

   R5 is R5, which is actually the 6th register, so the correct is R4 (R4).

4. Final Answer:

   R4 -> Option C

5. Quick Check:

   5th register = R4 [OK]

Hint: Count from R0 to find the register number [OK]

Common Mistakes:

• Starting count from R1 instead of R0
• Mixing register numbers with their positions
• Choosing R5 thinking it is the 5th register

3. Given the ARM assembly instructions:
MOV R0, #10
ADD R1, R0, #5
SUB R2, R1, R0
What is the final value stored in register R2?

medium

A. 5

B. 15

C. 10

D. 0

Solution

1. Step 1: Execute MOV and ADD instructions

   MOV R0, #10 sets R0 = 10. ADD R1, R0, #5 sets R1 = 10 + 5 = 15.

2. Step 2: Execute SUB instruction

   SUB R2, R1, R0 sets R2 = 15 - 10 = 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   R2 = R1 - R0 = 15 - 10 = 5 [OK]

Hint: Calculate step-by-step register values [OK]

Common Mistakes:

• Mixing up source and destination registers
• Forgetting immediate values in ADD
• Assuming R2 equals R1 or R0 directly

4. Identify the error in the following ARM assembly snippet:
MOV R13, #20
ADD R0, R13, #5
What is the issue?

medium

A. ADD instruction requires three registers, not two registers and an immediate

B. Immediate value #5 is invalid

C. MOV instruction cannot move immediate values

D. R13 is not a general purpose register and should not be used like this

Solution

1. Step 1: Check register usage

   R13 is the stack pointer, a special purpose register, not general purpose.

2. Step 2: Validate instruction correctness

   Using R13 like a general purpose register is incorrect; MOV and ADD with immediate values are valid.

3. Final Answer:

   R13 is not a general purpose register and should not be used like this -> Option D

4. Quick Check:

   R13 is special purpose, not general [OK]

Hint: Remember R13-R15 are special, not general registers [OK]

Common Mistakes:

• Assuming R13 is general purpose
• Thinking immediate values are invalid
• Believing ADD can't use immediate values

5. You want to swap the values of registers R2 and R3 using only R0 as a temporary register. Which sequence of instructions correctly achieves this?

hard

A. MOV R2, R3MOV R3, R2MOV R0, R2

B. MOV R0, R2MOV R2, R3MOV R3, R0

C. MOV R0, R3MOV R3, R2MOV R3, R0

D. MOV R0, R2MOV R3, R2MOV R2, R0

Solution

1. Step 1: Save R2 value temporarily

   MOV R0, R2 stores R2's value in R0 to keep it safe.

2. Step 2: Move R3 value to R2

   MOV R2, R3 copies R3's value into R2, overwriting original R2.

3. Step 3: Restore original R2 value to R3

   MOV R3, R0 moves saved R2 value from R0 into R3, completing the swap.

4. Final Answer:

   MOV R0, R2; MOV R2, R3; MOV R3, R0 -> Option B

5. Quick Check:

   Use temp register R0 to swap R2 and R3 [OK]

Hint: Use R0 as temp: save R2, copy R3 to R2, restore R2 to R3 [OK]

Common Mistakes:

• Overwriting R2 before saving it
• Swapping without a temporary register
• Mixing source and destination registers