Variable packing in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - Variable packing

Declare multiple variables

↓

Check variable sizes

↓

Pack variables into storage slots

↓

Store packed variables efficiently

↓

Access variables by unpacking

↓

End

Variable packing groups smaller variables together to save storage space in blockchain smart contracts.

Execution Sample

Blockchain / Solidity

uint8 a = 1;
uint8 b = 2;
uint16 c = 300;
// Packed into one 256-bit slot

This code packs three variables of different sizes into one storage slot to save space.

Execution Table

Step	Action	Variable	Size (bits)	Slot Used	Slot Content (bits)
1	Declare variable	a	8	Slot 0	00000001 (a)
2	Declare variable	b	8	Slot 0	00000001 (a) + 00000010 (b)
3	Declare variable	c	16	Slot 0	00000001 (a) + 00000010 (b) + 0000000100101100 (c)
4	Slot 0 packed content	a,b,c	32 total	Slot 0	00000001 00000010 0000000100101100
5	No more variables, packing complete	-	-	-	-

💡 All variables fit into one 256-bit slot, packing complete.

Variable Tracker

Variable	Declared	Packed Slot	Value
a	Yes	Slot 0 (bits 0-7)	1
b	Yes	Slot 0 (bits 8-15)	2
c	Yes	Slot 0 (bits 16-31)	300

Key Moments - 2 Insights

Why do variables a, b, and c share the same storage slot?

Because their combined size (8 + 8 + 16 = 32 bits) is less than the 256-bit slot size, so they fit together to save space, as shown in execution_table row 4.

What happens if variables exceed 256 bits in total size?

They will be split into multiple storage slots. Packing only works when combined size fits within one slot, as implied by the exit_note.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the size in bits of variable c?

A 16 bits

B 8 bits

C 32 bits

D 256 bits

💡 Hint

Check the 'Size (bits)' column in execution_table row 3.

At which step does the packing of all variables complete?

A Step 2

B Step 5

C Step 3

D Step 4

💡 Hint

Look for the step where packing is declared complete in execution_table.

If variable c was uint256 instead of uint16, how would the packing change?

A Variables a, b, and c all move to slot 1

B All variables still fit in one slot

C Variables a and b pack in slot 0, c in slot 1

D Packing is not possible at all

💡 Hint

Consider the size of c and slot size from variable_tracker and execution_table.

Concept Snapshot

Variable packing groups small variables into one 256-bit storage slot.
Variables must fit combined size ≤ 256 bits.
Saves blockchain storage and gas costs.
Access requires unpacking bits.
If too large, variables use multiple slots.

Full Transcript

Variable packing in blockchain smart contracts means putting several small variables together into one storage slot to save space and gas. For example, three variables a, b, and c with sizes 8, 8, and 16 bits fit into one 256-bit slot. The execution table shows each step packing variables into slot 0. This saves storage because blockchain slots are expensive. If variables are too big to fit, they use multiple slots. Beginners often wonder why variables share slots; it's because their total size fits within one slot. If a variable is bigger, packing splits across slots. This method helps optimize smart contract storage efficiently.