1. Ethereum Virtual Machine
CS1951 L
Spring 2019
12 February 2019
Maurice Herlihy
Brown University

2. UTXO Model

3. A txn can have multiple inputs …
UTXO Model
A txn can have multiple inputs …

4. UTXO Model
… and multiple outputs

5. UTXO Model
pizza order
change

6. Can also place other conditions on transfer …
UTXO Model
Can also place other conditions on transfer …
“smart contracts”

7. Account Model

8. In the beginning …
state
time

9. On the first day …
Transaction
(This happened)
state
new state
time

10. Not long after … block Transaction (This happened) state new state
time

11. Chain of Blocks
Txn
Txn
Txn
time

12. Chain of States
state
state
state
time

13. Block-State Duality
Txn
Txn
Txn
state
state
state
time

14. My God, it’s full of accounts!
Ethereum State
My God, it’s full of accounts!

15. Ethereum State Account Address Account State Account Address

16. External Account Owned by person or organization
Controlled by private keys
Holds Ether balance
Active agent:
transfers Ether, calls contract code

17. External Account
Address
{balance}

18. Contract Account Holds Ether balance Associated with code
Passive Agent:
Code called by other accounts
Code can transfer Ether, call other contracts

19. Contract Account
Address
{balance,
code,
storage}

20. Transaction Creation
Submitted by external party

21. Contract Creation Transaction

22. Contract Creation Transaction
Hello World!

23. Contract Creation Transaction
Hello World!
code
storage

24. Message Call Transaction
Run your code with these args
Yes sir!

25. Message Call Transaction
Run your code with these args
Yes sir!

26. EOA to EOA Message
Here’s the money I owe you
thanks!
transfer money

27. EOA to Contract & Vice-Versa
function call
function return

28. Contract to Contract Message
function call
function return

29. Money, Honey
Native currency called Ether
not this
but this

30. Gas Caller pays fee for each transaction step
Denial of Service attacks expensive

31. Gas Each step has fixed “gas” fee But gas price in Ether up to caller!
Low price means low priority …
And vice-versa

32. Gas If a call runs out of gas … Effects discarded Gas not refunded
If a call has leftover gas …
Unused gas refunded

33. Block Gas Limit Bitcoin has limit on block size
Ethereum has limit on block gas
Block full when transactions’ gas costs reach limit
We will see how this can be exploited later

34. Transaction Fields
Gas price
Value
Gas limit
Data
Nonce
Et cetera To

35. Transaction Fields Gas price Value Gas limit Data Nonce Et cetera To
How much caller pays for gas,
In Ether
Gas limit
Data
Nonce
Et cetera
Miners collect gas fees,
prioritize higher prices To
Extra-low price may never run

36. Transaction Fields Gas price Value Gas limit Data Nonce Et cetera To
Max gas caller willing to spend
Gas limit
Data
Call aborts if exceeded
Nonce
Et cetera
No refunds! To

37. Transaction sequence number
Transaction Fields
Gas price
Value
Transaction sequence number
(from sender)
Gas limit
Data
Nonce
Et cetera To

38. (external or contract)
Transaction Fields
Gas price
Value
Gas limit
Data
Nonce
Et cetera
destination address
(external or contract) To

39. How much Ether to transfer
Transaction Fields
Gas price
Value
Gas limit
Data
How much Ether to transfer
Nonce
Et cetera To

40. function name, args ,etc. …
Transaction Fields
Gas price
Value
Gas limit
Data
Nonce
Et cetera
Payload:
function name, args ,etc. … To

41. Transaction Fields Gas price Value Gas limit Data Nonce Et cetera To
ECDSA signature args ….

42. Ethereum Virtual Machine
EVM Code
EVM engine

43. Ethereum Virtual Machine
EVM Code)
(immutable)
Program Counter
Gas Available
Stack
Memory
(volatile)
storage
(persistent)

44. Most operations act on stack
Types of Instructions
Stack operations
Load, store
No registers
Control Flow
Most operations act on stack
Block Info
PUSH1 0x1
PUSH2 0x2
ADD
SHA-3
Logging
Environment

45. Types of Instructions Stack operations Load, store Control Flow
Local control flow
Control Flow
Block Info
SHA-3
Call other contracts
Logging
Call system libraries
Environment

46. Types of Instructions Stack operations Load, store Control Flow
Various crypto hashes provided
Block Info
SHA-3
Logging
Environment
Gas costs too expensive to compute directly

47. Types of Instructions Stack operations Load, store Control Flow
Caller’s address
Load, store
Control Flow
Ether balance
Block Info
Gas costs
SHA-3
Logging
Lots more …
Environment

48. Types of Instructions Stack operations Load, store Control Flow
Block Info
SHA-3
Load and store
from non-stack memory
Logging
Environment

49. Types of Instructions Stack operations Load, store Control Flow
block number
Stack operations
Load, store
and more …
Control Flow
Block Info
SHA-3
Logging
256 latest block hashes
block timestamp
Environment

50. communicate with outside world
Types of Instructions
Stack operations
Load, store
communicate with outside world
Control Flow
Block Info
SHA-3
Logging
Environment
Write events to log
debug

51. EVM Stack … All operations act on stack 1024 elements PUSH, POP,
COPY, SWAP …
256 bits

52. EVM Memory … byte-addressable Grows dynamically Gas charged for growth
8 bits
256-bit load
256-bit or 8-bit store
byte-addressable
Grows dynamically
Gas charged for growth

53. Observation: Storage is Expensive
Users need incentive to free up unneeded storage
Ethereum give a gas refund for
Freeing storage (overwrite with 0s)
Destroying contracts
This has had unexpected consequences …

54. Costs 5000 to zero out location
Details:
Costs 5000 to zero out location
15000 gas added to refund counter
At transaction end, …
Refund counter added to unused gas
(up to half of total gas used)
Destroying contracts similar arithmetic

55. Gas Token GasToken managed by a contract
Create GasToken when gas is cheap
Token allocates memory
Or creates an empty contract
Tokens can be transferred, sold, traded

56. Gas Token When gas prices climb, …. Buy GasTokens
Send to contract for destruction
Destruction gets gas refund
Up to half of contract’s gas costs

58. Is That Ethical? Negative effects: Uses memory only for speculation
Storage, network, etc underpriced
GasToken might drive up prices
Miners may be worse off

59. Is That Ethical? Positive Effects Gas-banking service for users
Hedge against increases
Predictability in face of volatility

60. Proposal for Rent Storage is forever Cost to system: bytes X time
Cost to user: bytes
Partial fix: refund for deletion

61. Proposal for Rent Keep track of:
Last block to access data: last_accessed
Size in bytes: storage_bytes
On store, extra charge of:
O((block.number – last_accessed)
* storage_bytes)

62. Proposal for Rent Why would anyone want this?
May benefit dapps that use storage for short durations
Disk space for a full node may go down
This proposal still under discussion.

64. Ideas we covered in this lecture
UTXO versus Account Mode
Block-State duality
Accounts, External and Contract
Ether and Gas
Transaction state
Ethereum Virtual Machine