If-else implementation in assembly in ARM Architecture

Introduction

If-else lets a program choose between two paths based on a condition. In assembly, it helps control what the processor does next.

Deciding if a number is positive or negative to print a message.

Checking if a user input matches a password to allow access.

Choosing between two calculations based on a value.

Turning on a light if a sensor reads above a threshold, else turning it off.

Core Concept

ARM Architecture

CMP Rn, Rm
BEQ label_if_equal
BNE label_if_not_equal
; code for if part
B label_end
label_if_not_equal:
; code for else part
label_end:

CMP compares two registers by subtracting but does not store the result.

BEQ branches if the compared values are equal; BNE branches if not equal.

Key Points

This checks if R0 is zero. If yes, it runs the code at is_zero, else it runs the code after BEQ.

ARM Architecture

CMP R0, #0
BEQ is_zero
; code if not zero
B end
is_zero:
; code if zero
end:

This compares R1 and R2. If R1 is greater, it jumps to greater, else continues with the next code.

ARM Architecture

CMP R1, R2
BGT greater
; code if R1 <= R2
B end
greater:
; code if R1 > R2
end:

Detailed Explanation

This program compares the value in register R0 with zero. If R0 is greater than zero, it sets R1 to 1 (meaning positive). Otherwise, it sets R1 to 0 (not positive).

ARM Architecture

; Check if R0 is positive or not
CMP R0, #0
BGT positive
; else part
MOV R1, #0 ; R1 = 0 means not positive
B end
positive:
MOV R1, #1 ; R1 = 1 means positive
end:

Important Notes

Assembly uses labels and branch instructions to implement if-else logic.

Conditions like equal, not equal, greater than, less than are checked using flags set by CMP.

Always ensure branch labels are unique and placed correctly to avoid errors.

Summary

If-else in assembly uses CMP to compare values and branch instructions like BEQ, BNE, BGT to jump to code blocks.

Labels mark the start of if or else code sections.

This lets the processor choose which instructions to run based on conditions.

Practice

1. In ARM assembly, which instruction is primarily used to compare two values before an if-else branch?

easy

A. MOV

B. CMP

C. ADD

D. STR

Solution

1. Step 1: Understand the purpose of CMP

   The CMP instruction compares two registers by subtracting one from the other and setting condition flags without storing the result.

2. Step 2: Identify its role in if-else

   These condition flags are then used by branch instructions to decide which code path to take, implementing if-else logic.

3. Final Answer:

   CMP -> Option B

4. Quick Check:

   Compare = CMP [OK]

Hint: CMP sets flags for branching decisions in if-else [OK]

Common Mistakes:

• Confusing CMP with MOV which moves data
• Using ADD instead of CMP for comparison
• Thinking STR stores comparison results

2. Which of the following is the correct syntax for a conditional branch in ARM assembly that jumps if two values are equal?

easy

A. BNE label

B. BL label

C. BEQ label

D. BGT label

Solution

1. Step 1: Understand branch conditions

   BEQ means 'Branch if Equal', which jumps if the zero flag is set after a CMP instruction.

2. Step 2: Match condition to syntax

   BNE means 'Branch if Not Equal', BGT means 'Branch if Greater Than', and BL is 'Branch with Link' (function call), so only BEQ matches the condition for equality.

3. Final Answer:

   BEQ label -> Option C

4. Quick Check:

   Equal branch = BEQ [OK]

Hint: BEQ branches when values are equal after CMP [OK]

Common Mistakes:

• Using BNE for equality
• Confusing BL with conditional branch
• Using BGT for equality check

3. Given the ARM assembly snippet:

CMP R0, #10
BEQ equal_label
MOV R1, #0
B end_label
equal_label:
MOV R1, #1
end_label:

What value will R1 hold if R0 equals 10?

medium

A. 0

B. 10

C. Undefined

D. 1

Solution

1. Step 1: Analyze CMP and BEQ

   The CMP compares R0 with 10. If equal, BEQ jumps to equal_label.

2. Step 2: Follow the branch logic

   If R0 is 10, execution jumps to equal_label where R1 is set to 1. Otherwise, R1 is set to 0 before jumping to end_label.

3. Final Answer:

   1 -> Option D

4. Quick Check:

   R0=10 triggers BEQ, sets R1=1 [OK]

Hint: If equal, BEQ jumps to set R1=1 [OK]

Common Mistakes:

• Assuming R1 stays 0
• Ignoring the branch to equal_label
• Confusing MOV targets

4. Identify the error in this ARM assembly if-else snippet:

CMP R2, #5
BNE else_label
MOV R3, #1
else_label:
MOV R3, #0

medium

A. Missing branch to skip else after if block

B. Incorrect comparison instruction

C. Wrong register used in MOV

D. No labels defined

Solution

1. Step 1: Understand the flow

   If R2 equals 5, BNE is not taken, so MOV R3, #1 executes, then execution continues to else_label and MOV R3, #0 runs.

2. Step 2: Identify missing branch

   There is no branch to skip the else block after the if block, so both blocks run sequentially, overwriting R3.

3. Final Answer:

   Missing branch to skip else after if block -> Option A

4. Quick Check:

   Missing jump after if causes both blocks to run [OK]

Hint: Add branch after if block to skip else [OK]

Common Mistakes:

• Assuming else block is skipped automatically
• Using wrong branch instruction
• Ignoring label definitions

5. How would you implement a nested if-else in ARM assembly to check if R4 is greater than 10, then if R5 equals 20, setting R6 to 1; else set R6 to 0?

hard

A. Use CMP and BGT for R4, then CMP and BEQ for R5 with nested labels

B. Use only one CMP for both conditions combined

C. Use MOV instructions without branches

D. Use STR to store comparison results

Solution

1. Step 1: Implement outer if with CMP and BGT

   Compare R4 with 10 using CMP, then branch with BGT to inner if block if true.

2. Step 2: Implement inner if with CMP and BEQ

   Inside the inner block, compare R5 with 20 and branch with BEQ to set R6 to 1; else set R6 to 0.

3. Step 3: Use labels to separate blocks

   Use labels to mark inner if, else, and end blocks to control flow clearly.

4. Final Answer:

   Use CMP and BGT for R4, then CMP and BEQ for R5 with nested labels -> Option A

5. Quick Check:

   Nested if uses CMP + BGT then CMP + BEQ [OK]

Hint: Use nested CMP and conditional branches with labels [OK]

Common Mistakes:

• Trying to combine conditions in one CMP
• Skipping branches for nested logic
• Using STR instead of branch instructions