Why Stack operations (PUSH, POP) in ARM Architecture? - Purpose & Use Cases

The Big Idea

What if you could manage tasks so simply that the last thing you did is always the first to undo?

The Scenario

Imagine you are trying to keep track of a pile of books you are stacking on a table. You add one book at a time on top, and when you want to remove a book, you can only take the top one off first.

The Problem

Without a clear method, you might try to remove books from the middle or bottom, causing the pile to fall or making it hard to remember the order. Manually tracking which book to remove next is slow and confusing.

The Solution

Stack operations PUSH and POP work like this pile of books. PUSH adds an item on top, and POP removes the top item. This simple rule keeps everything organized and easy to manage, especially in computer memory.

Before vs After

✗ Before

remove any book from anywhere in the pile

✓ After

PUSH book on top; POP book from top only

What It Enables

It enables computers to manage tasks and data in a neat, last-in-first-out order, making processes efficient and error-free.

Real Life Example

When you use the undo button in a text editor, the computer uses a stack to remember your last actions and reverses them one by one.

Key Takeaways

Stacks add and remove items only from the top.

PUSH adds an item; POP removes the last added item.

This keeps data organized in a last-in-first-out way.

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