Stack operations (PUSH, POP) in ARM Architecture - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push register R0 onto the stack.

ARM Architecture

PUSH [1]

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

A{R0}

BR0

C[R0]

D(R0)

Attempts:

3 left

💡 Hint

Common Mistakes

Writing PUSH R0 without braces causes a syntax error.

Using square or round brackets instead of curly braces.

In ARM assembly, the PUSH instruction requires the register(s) to be enclosed in curly braces {} to specify the register list.

2fill in blank

medium

Complete the code to pop register R1 from the stack.

ARM Architecture

POP [1]

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

AR1

B[R1]

C{R1}

D(R1)

Attempts:

3 left

💡 Hint

Common Mistakes

Using POP R1 without braces causes an error.

Using square or round brackets instead of curly braces.

The POP instruction in ARM assembly requires the register list in curly braces, so {R1} is correct.

3fill in blank

hard

Fix the error in the code to push registers R0 and R2 onto the stack.

ARM Architecture

PUSH [1]

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

A(R0, R2)

B[R0, R2]

CR0, R2

D{R0, R2}

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses or square brackets instead of curly braces.

Listing registers without any brackets.

Multiple registers must be enclosed in curly braces separated by commas for PUSH.

4fill in blank

hard

Fill both blanks to push registers R4 and R5, then pop R4.

ARM Architecture

PUSH [1]
POP [2]

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

A{R4, R5}

BR4

C{R4}

DR5

Attempts:

3 left

💡 Hint

Common Mistakes

Using no braces for POP single register.

Using wrong brackets or no brackets for PUSH multiple registers.

PUSH requires a register list in braces, so {R4, R5} is correct. POP also requires braces even for a single register, so {R4} is correct.

5fill in blank

hard

Fill all three blanks to push R7, pop R6, and then push R5 and R4.

ARM Architecture

PUSH [1]
POP [2]
PUSH [3]

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

A{R7}

B{R6}

C{R5, R4}

DR7

Attempts:

3 left

💡 Hint

Common Mistakes

Using no braces or wrong brackets for register lists.

Forgetting commas between multiple registers.

All PUSH and POP instructions require register lists in curly braces. So {R7}, {R6}, and {R5, R4} are correct.

Practice

1. What does the PUSH instruction do in ARM architecture stack operations?

easy

A. It adds registers to the stack by decreasing the stack pointer.

B. It removes registers from the stack by increasing the stack pointer.

C. It clears the stack completely.

D. It duplicates the top value of the stack.

Solution

1. Step 1: Understand the role of PUSH

   PUSH adds data to the stack, which means it stores registers by moving the stack pointer downwards.

2. Step 2: Compare with POP

   POP removes data by moving the stack pointer upwards, opposite to PUSH.

3. Final Answer:

   It adds registers to the stack by decreasing the stack pointer. -> Option A

4. Quick Check:

   PUSH = add to stack [OK]

Hint: PUSH means add data, stack pointer moves down [OK]

Common Mistakes:

• Confusing PUSH with POP
• Thinking PUSH increases stack pointer
• Assuming PUSH clears the stack

2. Which of the following is the correct syntax to push registers R0 and R1 onto the stack in ARM assembly?

easy

A. PUSH [R0, R1]

B. PUSH (R0, R1)

C. PUSH {R0, R1}

D. PUSH R0 R1

Solution

1. Step 1: Recall ARM PUSH syntax

   ARM uses curly braces to list registers for PUSH, like PUSH {R0, R1}.

2. Step 2: Identify incorrect syntax

   Parentheses, spaces without braces, or square brackets are not valid for PUSH in ARM.

3. Final Answer:

   PUSH {R0, R1} -> Option C

4. Quick Check:

   Curly braces list registers in PUSH [OK]

Hint: Use curly braces {} to list registers in PUSH [OK]

Common Mistakes:

• Using parentheses instead of braces
• Listing registers without any brackets
• Using square brackets which are invalid

3. Given the following ARM instructions:
SUB SP, SP, #8
PUSH {R4, R5}
POP {R6, R7}
What happens to the stack pointer (SP) after these operations?

medium

A. SP decreases by 8, then decreases by 8, then increases by 8

B. SP decreases by 8, then increases by 8, then decreases by 8

C. SP decreases by 8, then decreases by 8, then increases by 16

D. SP decreases by 8, then decreases again by 8, then increases by 8

Solution

1. Step 1: Analyze SUB SP, SP, #8

   This subtracts 8 from SP, moving it down by 8 bytes.

2. Step 2: Understand PUSH {R4, R5}

   PUSH stores two registers, so SP decreases by another 8 bytes (4 bytes each register).

3. Step 3: Understand POP {R6, R7}

   POP removes two registers, so SP increases by 8 bytes.

4. Final Answer:

   SP decreases by 8, then decreases by 8, then increases by 8 -> Option A

5. Quick Check:

   SP moves down for SUB and PUSH, up for POP [OK]

Hint: PUSH decreases SP, POP increases SP by register size times count [OK]

Common Mistakes:

• Forgetting SUB changes SP before PUSH
• Assuming POP decreases SP
• Miscounting bytes per register

4. Identify the error in this ARM assembly snippet:
PUSH R0, R1
POP {R2, R3}

medium

A. PUSH cannot push more than one register at a time.

B. PUSH syntax is incorrect; registers must be in braces.

C. POP syntax is incorrect; registers must be in parentheses.

D. POP cannot pop into different registers.

Solution

1. Step 1: Check PUSH syntax

   PUSH requires registers to be enclosed in curly braces, so PUSH R0, R1 is invalid.

2. Step 2: Check POP syntax

   POP syntax with curly braces is correct, so POP {R2, R3} is valid.

3. Final Answer:

   PUSH syntax is incorrect; registers must be in braces. -> Option B

4. Quick Check:

   PUSH needs braces { } [OK]

Hint: Always use braces {} for multiple registers in PUSH/POP [OK]

Common Mistakes:

• Using commas without braces in PUSH
• Confusing parentheses with braces
• Thinking POP cannot pop multiple registers

5. You want to save registers R0, R1, and R2 on the stack, then restore them later. Which sequence correctly uses PUSH and POP to do this without corrupting the stack?

hard

A. PUSH {R2, R1, R0}...POP {R0, R1, R2}

B. PUSH {R0, R1, R2}...POP {R0, R1, R2}

C. PUSH {R0, R1, R2}...POP {R1, R2, R0}

D. PUSH {R0, R1, R2}...POP {R2, R1, R0}

Solution

1. Step 1: Understand stack order with PUSH

   PUSH stores registers in order: R0, then R1, then R2, with R2 at the lowest address.

2. Step 2: POP must restore in reverse order

   POP restores registers in reverse order of PUSH to correctly restore the original values.

3. Step 3: Check options

   PUSH {R0, R1, R2}
   ...
   POP {R2, R1, R0} pops in reverse order as pushed, preserving correct values.

4. Final Answer:

   PUSH {R0, R1, R2} ... POP {R2, R1, R0} -> Option D

5. Quick Check:

   POP order is reverse of PUSH order to restore correctly [OK]

Hint: POP registers in reverse order of PUSH to restore correctly [OK]

Common Mistakes:

• Reversing POP order incorrectly
• Changing register order between PUSH and POP
• Assuming POP order does not matter