Move instruction (MOV) in PLC Programming - Time & Space Complexity

Time Complexity: Move instruction (MOV)

O(n)

Understanding Time Complexity

We want to understand how the time to run a move instruction changes as the amount of data changes.

Specifically, how does copying values from one place to another scale with input size?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

MOV Source, Destination
    

This code copies the value from the Source to the Destination register or memory location.

Identify Repeating Operations

• Primary operation: Single move operation copying one value.
• How many times: Exactly once per instruction execution.

How Execution Grows With Input

Since this instruction moves one value at a time, the time grows directly with how many move instructions run.

Input Size (n)	Approx. Operations
10	10 moves
100	100 moves
1000	1000 moves

Pattern observation: Time grows linearly with the number of move instructions executed.

Final Time Complexity

Time Complexity: O(n)

This means the time to move data grows directly in proportion to how many values you move.

Common Mistake

[X] Wrong: "A single MOV instruction takes the same time no matter how much data I move."

[OK] Correct: Each MOV copies only one value. To move many values, you need many MOV instructions, so time grows with data size.

Interview Connect

Understanding how simple instructions like MOV scale helps you reason about program speed and efficiency in automation tasks.

Self-Check

"What if we used a block move instruction that moves multiple values at once? How would the time complexity change?"

Practice

1. What does the MOV instruction do in PLC programming?

easy

A. Compares two values and sets a flag

B. Copies a value from a source to a destination without changing the source

C. Deletes a value from memory

D. Adds two values and stores the result

Solution

1. Step 1: Understand the MOV instruction purpose

   The MOV instruction copies data from one place to another without altering the original data.

2. Step 2: Compare with other options

   Other options describe different instructions like addition, deletion, or comparison, which are not MOV's function.

3. Final Answer:

   Copies a value from a source to a destination without changing the source -> Option B

4. Quick Check:

   MOV copies value unchanged [OK]

Hint: MOV copies data without changing source [OK]

Common Mistakes:

• Thinking MOV adds or deletes data
• Confusing MOV with comparison instructions
• Assuming MOV changes the source value

2. Which of the following is the correct syntax for the MOV instruction in PLC programming?

easy

A. MOV source, destination

B. MOV(destination, source)

C. MOV source -> destination

D. MOV = source to destination

Solution

1. Step 1: Recall standard MOV syntax

   The MOV instruction syntax is typically written as MOV followed by source and destination separated by a comma.

2. Step 2: Evaluate other options

   Options A, B, and D use incorrect syntax formats not recognized in PLC MOV instructions.

3. Final Answer:

   MOV source, destination -> Option A

4. Quick Check:

   Correct MOV syntax uses comma [OK]

Hint: MOV uses source, destination format [OK]

Common Mistakes:

• Reversing source and destination order
• Using parentheses or arrows incorrectly
• Adding extra symbols like '='

3. Given the following PLC code snippet:

MOV 100, D0
MOV D0, D1
MOV D1, D2

What is the final value stored in register D2?

medium

A. D0

B. 0

C. 100

D. D1

Solution

1. Step 1: Trace the first MOV instruction

   MOV 100, D0 sets D0 to 100.

2. Step 2: Trace the second and third MOV instructions

   MOV D0, D1 copies 100 to D1; MOV D1, D2 copies 100 to D2.

3. Final Answer:

   100 -> Option C

4. Quick Check:

   Value 100 moves through D0 to D2 [OK]

Hint: Follow value moves step-by-step [OK]

Common Mistakes:

• Assuming D2 remains 0
• Confusing register names with values
• Ignoring the order of MOV instructions

4. Identify the error in this MOV instruction:

MOV D1 D2

What is the problem?

medium

A. MOV requires parentheses

B. Source and destination are reversed

C. MOV cannot move between registers

D. Missing comma between source and destination

Solution

1. Step 1: Check syntax of MOV instruction

   MOV requires a comma between source and destination to separate them.

2. Step 2: Verify other options

   Source and destination order is correct; MOV can move between registers; parentheses are not required.

3. Final Answer:

   Missing comma between source and destination -> Option D

4. Quick Check:

   Comma separates source and destination [OK]

Hint: Always separate source and destination with a comma [OK]

Common Mistakes:

• Omitting the comma
• Swapping source and destination
• Adding unnecessary parentheses

5. You want to copy the value from input register X10 to output register Y20 only if X10 is greater than 50. Which PLC code snippet correctly uses the MOV instruction with this condition?

hard

A. LD X10
GT 50
MOV X10, Y20

B. LD X10 > 50
MOV X10, Y20

C. CMP X10, 50
MOV X10, Y20

D. LD X10
MOV X10, Y20
IF X10 > 50

Solution

1. Step 1: Understand conditional MOV usage

   To move only if X10 > 50, first load X10, then check if greater than 50, then move.

2. Step 2: Analyze each option

   LD X10 GT 50 MOV X10, Y20 correctly loads X10, tests GT 50, then moves X10 to Y20. Others lack proper condition or order.

3. Final Answer:

   LD X10 GT 50 MOV X10, Y20 -> Option A

4. Quick Check:

   Condition before MOV ensures correct transfer [OK]

Hint: Check condition before MOV to control data transfer [OK]

Common Mistakes:

• Placing MOV before condition check
• Using incorrect comparison instructions
• Missing condition entirely