General purpose registers (R0-R12) in ARM Architecture - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are general purpose registers R0 to R12 used for in ARM architecture?

They are used to hold temporary data and intermediate values during program execution. These registers help the CPU perform calculations and store data quickly.

beginner

How many general purpose registers are there from R0 to R12 in ARM?

There are 13 general purpose registers, numbered R0 through R12.

intermediate

Which ARM registers are typically used to pass arguments to functions?

Registers R0 to R3 are usually used to pass the first four arguments to functions.

beginner

Can general purpose registers R0-R12 be used for any type of data?

Yes, they can hold integers, addresses, or any data the program needs temporarily. They are flexible and not limited to a specific data type.

intermediate

Why is it important to save the contents of some general purpose registers before calling a function?

Because functions may change the values in registers R4-R11, saving them ensures that important data is not lost and can be restored after the function finishes.

How many general purpose registers are there from R0 to R12 in ARM?

A 12

B 13

C 14

D 10

Registers R0 through R12 make a total of 13 general purpose registers.

Which registers are commonly used to pass the first four arguments to a function in ARM?

A R0 to R3

B R4 to R7

C R8 to R12

D R10 to R12

Registers R0 to R3 are used to pass the first four arguments to functions.

What type of data can general purpose registers hold?

A Only floating point numbers

B Only addresses

C Only integers

D Any temporary data like integers or addresses

General purpose registers can hold any temporary data such as integers, addresses, or other values.

Why should some registers be saved before calling a function?

A Because registers are read-only

B Because registers cannot be used inside functions

C Because functions may overwrite their values

D Because registers are slow

Functions may change the values in registers, so saving them preserves important data.

Which of the following is NOT a general purpose register in ARM?

A R13

B R12

C R0

D R5

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

Explain the role of general purpose registers R0 to R12 in ARM architecture.

Think about how the CPU uses these registers during program execution.

Why is it important to save and restore some general purpose registers when calling functions?

Consider what happens to register values inside a function.

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