Gas: The EVM's Resource Model

Gas is the EVM's mechanism for bounding computation. Every opcode has a cost, and every transaction starts with a finite gas budget. What I found surprising is how abruptly execution halts when gas runs out — there's no graceful winding down.

Why gas exists

Without gas, a contract could loop forever, and every Ethereum node would be stuck executing it. I think of gas as a pragmatic solution to the halting problem: you can run any computation you want, as long as you can pay for it.

Static vs. dynamic costs

Static costs are fixed per opcode:

Tier	Cost	Examples
Zero	0	STOP, JUMPDEST
Base	2	PUSH0, PC, MSIZE, GAS, POP
Very Low	3	ADD, SUB, LT, GT, EQ, AND, OR, NOT, BYTE, SHL, SHR, SAR, PUSH1-32, DUP, SWAP
Low	5	MUL, DIV, MOD, SIGNEXTEND
Mid	8	ADDMOD, MULMOD, JUMP
High	10	JUMPI

Dynamic costs depend on operand values:

Memory expansion: grows quadratically — $C_{m e m} (a) = 3 a + ⌊ a^{2} /512 ⌋$
EXP: base cost + 50 per byte in the exponent
SLOAD/SSTORE: cold vs. warm access (Module 3)

The memory expansion formula

$C_{m e m} (a) = 3 a + ⌊ \frac{a ^{2}}{512} ⌋$

At small sizes, this is essentially linear (3 gas per 32-byte word). But the quadratic term kicks in at scale — I plotted a few values to feel it:

Words	Bytes	Gas cost
1	32	3
10	320	30
100	3,200	319
1000	32,000	4,953
10000	320,000	225,312

Large memory allocations end up prohibitively expensive — and that's entirely by design.

Out-of-gas vs. REVERT

Both halt execution and discard state changes, but I had to work through how they differ in gas refunds:

	Out-of-gas	REVERT
State changes	Discarded	Discarded
Remaining gas	All consumed	Refunded to caller
Return data	Empty	Can include error message

This distinction matters for smart contract design: I now appreciate why a well-written contract should REVERT with an error message rather than running out of gas silently.

Rust Pattern: Result<T, E>

We model execution outcomes as Result<ExecutionResult, EvmError>. A successful REVERT returns Ok(ExecutionResult { reverted: true, .. }) — it's not an Err, because the interpreter completed successfully; it's the contract that chose to revert.

Exercises

2.1 — Decode known opcodes and reject unknown bytes
2.2 — Verify PUSH immediate sizes
2.3 — Execute ADD and verify the result
2.4 — Verify exact gas accounting for a sequence of opcodes
2.5 — MUL, SUB, DIV correctness
2.6 — Division by zero returns 0
2.7 — LT, GT, EQ produce 0 or 1
2.8 — BYTE extracts individual bytes from a word
2.9 — SHL and SHR shift operations
2.10 — SAR preserves the sign bit
2.11 — Out-of-gas detection
2.12 — STOP returns empty output
2.13 — RETURN copies a memory range to output

Run: cargo test opcode and cargo test interpreter