Return value in R0 in ARM Architecture - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to move the return value 5 into register R0.

ARM Architecture

MOV R0, [1]

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A#5

BR1

C5

DR0

Attempts:

3 left

💡 Hint

Common Mistakes

Using MOV R0, 5 without the '#' causes an error.

Using MOV R0, R1 moves the value from R1, not the number 5.

In ARM assembly, immediate values are prefixed with '#'. So to move the value 5 into R0, use MOV R0, #5.

2fill in blank

medium

Complete the code to return the value stored in register R2 by moving it into R0.

ARM Architecture

MOV [1], R2

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AR0

BR1

CR3

DR2

Attempts:

3 left

💡 Hint

Common Mistakes

Moving R2 into R1 does not return the value.

Moving R2 into itself does not change the return register.

The ARM calling convention uses R0 to hold return values. To return the value in R2, move it into R0 with MOV R0, R2.

3fill in blank

hard

Fix the error in the code to correctly return the value 10 in R0.

ARM Architecture

MOV R0, [1]

Drag options to blanks, or click blank then click option'

A10

B#10

CR10

DR1

Attempts:

3 left

💡 Hint

Common Mistakes

Writing MOV R0, 10 without '#' causes an error.

Using MOV R0, R10 moves from register R10, not the number 10.

Immediate values must be prefixed with '#'. So to move 10 into R0, write MOV R0, #10.

4fill in blank

hard

Fill both blanks to move the value from R3 into R0 and then branch to the function return.

ARM Architecture

MOV [1], R3
B [2]

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AR0

BLR

CR1

DPC

Attempts:

3 left

💡 Hint

Common Mistakes

Branching to PC causes an infinite loop.

Moving value into R1 does not set the return value.

To return a value, move it into R0. Then branch to the address in LR (link register) to return from the function.

5fill in blank

hard

Fill all three blanks to load immediate 7 into R4, move it to R0 for return, and branch back to the caller.

ARM Architecture

MOV [1], [2]
MOV [3], R4
B LR

Drag options to blanks, or click blank then click option'

AR4

B#7

CR0

DR7

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting '#' before 7.

Moving value into wrong register before return.

First, load immediate 7 into R4 with MOV R4, #7. Then move R4 into R0 to set the return value. Finally, branch to LR to return.

Practice

1. In ARM assembly, which register is used to hold the return value of a function?

easy

A. LR

B. R1

C. R0

D. SP

Solution

1. Step 1: Understand ARM calling convention

   In ARM, the first register R0 is used to pass the return value from a function to the caller.

2. Step 2: Identify the return value register

   Since R0 holds the return value, the caller reads the result from R0 after the function returns.

3. Final Answer:

   R0 -> Option C

4. Quick Check:

   Return value register = R0 [OK]

Hint: Return values are always in R0 register in ARM [OK]

Common Mistakes:

• Confusing LR with return value
• Using SP as return value
• Thinking R1 holds return value

2. Which ARM assembly instruction is commonly used to return from a function after setting R0?

easy

A. MOV R0, #0

B. ADD R0, R0, #1

C. STR R0, [SP]

D. BX LR

Solution

1. Step 1: Identify return instruction in ARM

   The instruction BX LR branches to the address stored in the link register (LR), effectively returning from the function.

2. Step 2: Confirm usage after setting R0

   After placing the return value in R0, BX LR is used to return control to the caller.

3. Final Answer:

   BX LR -> Option D

4. Quick Check:

   Return instruction = BX LR [OK]

Hint: Return with BX LR after setting R0 [OK]

Common Mistakes:

• Using MOV instead of BX LR to return
• Storing R0 to stack before return
• Using ADD as return instruction

3. Consider this ARM assembly snippet:

MOV R0, #5
BX LR

What value will the caller receive after this function returns?

medium

A. 5

B. 0

C. LR value

D. Undefined

Solution

1. Step 1: Analyze MOV instruction

   The instruction MOV R0, #5 sets the R0 register to 5, which is the return value.

2. Step 2: Analyze return instruction

   BX LR returns control to the caller, with R0 holding the return value 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   R0 value after return = 5 [OK]

Hint: Value in R0 before BX LR is the return value [OK]

Common Mistakes:

• Thinking LR value is returned
• Assuming R0 resets to 0
• Ignoring MOV sets return value

4. Identify the error in this ARM function that intends to return 10:

ADD R0, R0, #10
BX LR

medium

A. R0 is not initialized before ADD

B. BX LR should be MOV PC, LR

C. ADD instruction syntax is incorrect

D. Function should use R1 for return value

Solution

1. Step 1: Check initial value of R0

   The instruction ADD R0, R0, #10 adds 10 to R0, but R0 is not initialized, so the result is unpredictable.

2. Step 2: Confirm correct return method

   BX LR is correct to return; the problem is the uninitialized R0.

3. Final Answer:

   R0 is not initialized before ADD -> Option A

4. Quick Check:

   Uninitialized R0 causes wrong return value [OK]

Hint: Initialize R0 before adding to it [OK]

Common Mistakes:

• Assuming R0 starts at zero
• Confusing BX LR with MOV PC, LR
• Using wrong register for return value

5. You want to write an ARM function that returns the sum of two input values passed in R0 and R1. Which code correctly sets the return value in R0 before returning?

hard

A. MOV R0, R1
BX LR

B. ADD R0, R0, R1
BX LR

C. ADD R1, R0, R1
BX LR

D. SUB R0, R0, R1
BX LR

Solution

1. Step 1: Understand input and output registers

   Inputs are in R0 and R1; the sum should be stored in R0 as the return value.

2. Step 2: Check each option

   ADD R0, R0, R1 BX LR adds R0 and R1, stores result in R0, then returns with BX LR, which is correct.

3. Final Answer:

   ADD R0, R0, R1 BX LR -> Option B

4. Quick Check:

   Sum in R0 before BX LR returns correct value [OK]

Hint: Sum inputs into R0, then BX LR to return [OK]

Common Mistakes:

• Storing sum in R1 instead of R0
• Using SUB instead of ADD
• Not returning with BX LR