Stack operations (PUSH, POP) in ARM Architecture - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the PUSH operation do in ARM architecture?

PUSH saves one or more registers onto the stack. It decreases the stack pointer and stores the data at the new top of the stack.

beginner

What is the purpose of the POP operation in ARM architecture?

POP retrieves one or more registers from the stack. It loads data from the top of the stack and increases the stack pointer to remove those items.

intermediate

Why is the stack pointer adjusted during PUSH and POP?

The stack pointer moves to keep track of the current top of the stack. PUSH decreases it to add data, POP increases it to remove data, ensuring correct stack order.

intermediate

In ARM assembly, which registers are commonly saved using PUSH before a function call?

Registers like R0 to R3, LR (link register), and sometimes others are saved to preserve their values during function calls.

advanced

What happens if you POP more registers than were PUSHed?

This causes stack corruption, leading to incorrect data retrieval and possible program crashes because the stack pointer points to wrong memory.

What does the PUSH instruction do in ARM architecture?

A Stores registers onto the stack and decreases the stack pointer

B Loads registers from the stack and increases the stack pointer

C Increases the stack pointer without storing data

D Deletes data from memory

PUSH saves registers by storing them on the stack and moves the stack pointer down.

Which register typically points to the top of the stack in ARM?

A LR (Link Register)

B R0

C SP (Stack Pointer)

D PC (Program Counter)

The SP register always points to the current top of the stack.

What happens to the stack pointer during a POP operation?

A It decreases

B It stays the same

C It resets to zero

D It increases

POP increases the stack pointer to remove data from the stack.

Why do functions use PUSH before calling another function?

A To save registers so their values are not lost

B To clear the stack

C To speed up execution

D To increase memory size

PUSH saves registers to preserve their values during function calls.

What is a risk of incorrect POP operations?

A Faster program execution

B Stack corruption and program errors

C Stack overflow

D No effect

Incorrect POP can corrupt the stack, causing errors or crashes.

Explain how PUSH and POP operations work together to manage data on the stack in ARM architecture.

Think about how data is added and removed from the stack.

Describe why it is important to balance PUSH and POP operations in ARM assembly programming.

Consider what happens if you POP more than you PUSH.

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