Storage layout in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - Storage layout

Define variables

↓

Assign storage slots

↓

Store values in slots

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Read values from slots

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Use values in contract logic

Storage layout shows how variables are assigned to storage slots in a blockchain contract and how values are stored and read.

Execution Sample

Blockchain / Solidity

uint256 a = 5;
uint256 b = 10;

// a stored at slot 0
// b stored at slot 1

function getA() public view returns (uint256) {
  return a;
}

This code stores two numbers in separate storage slots and reads one back.

Execution Table

Step	Action	Storage Slot	Value Stored	Variable	Result/Output
1	Define variable a	slot 0	uninitialized	a	none
2	Define variable b	slot 1	uninitialized	b	none
3	Assign a = 5	slot 0	5	a	none
4	Assign b = 10	slot 1	10	b	none
5	Call getA()	slot 0	5	a	returns 5
6	End	-	-	-	Execution complete

💡 Execution stops after returning value of a from slot 0

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
a	uninitialized	5	5	5
b	uninitialized	uninitialized	10	10

Key Moments - 2 Insights

Why does variable 'a' use slot 0 and 'b' use slot 1?

Variables are assigned storage slots in the order they are declared. 'a' is first, so slot 0; 'b' is second, so slot 1, as shown in steps 1 and 2 of the execution_table.

What happens if we read a variable before assigning a value?

Reading before assignment returns default uninitialized value (usually 0). In the table, before step 3, 'a' is uninitialized.

Visual Quiz - 3 Questions

Look at the execution_table, what value is stored in slot 1 after step 4?

A 10

B 5

C uninitialized

D 0

💡 Hint

Check the 'Storage Slot' and 'Value Stored' columns at step 4 in execution_table.

At which step does variable 'a' get its value assigned?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Action' column in execution_table where 'Assign a = 5' happens.

If we add a new variable 'c' after 'b', which storage slot will it use?

A slot 0

B slot 1

C slot 2

D slot 3

💡 Hint

Variables are assigned slots in order; 'a' is slot 0, 'b' slot 1, so next is slot 2.

Concept Snapshot

Storage layout assigns each variable a unique storage slot.
Variables declared first get lower slot numbers.
Values are stored and read from these slots.
Uninitialized slots hold default values.
Reading a variable returns the value in its slot.

Full Transcript

Storage layout in blockchain contracts means each variable is stored in a specific slot in contract storage. When you declare variables, the first one gets slot 0, the second slot 1, and so on. Assigning a value stores it in that slot. Reading a variable fetches the value from its slot. If a variable is not assigned yet, its slot holds a default value, usually zero. This layout helps the contract know where to find each variable's data.