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07_NeoVM_NeoContract.md

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NeoVM and NeoContract

This section is part of the Community Yellow Paper ^[See Community Yellow Paper repository] initiative, a community-driven technical specification for Neo blockchain.

WARNING: this section is an unreleased draft. It is expected to be incomplete or even to carry incorrect information, so feel free to contribute with us!

Neo execution layers

Neo smart contract execution is performed in two different layers: NeoContract and NeoVM.

NeoVM is the lightweight low-level virtual machine that performs the most basic operations. NeoContract is an interoperable layer that includes blockchain services, contract storage and more complex operations.

NeoVM

NeoVM is a stack machine capable of processing items, called Stack Items. These StackItems are limited to the following types:

  • Array: an array of StackItems
  • Boolean
  • ByteArray
  • Integer
  • InteropInterface
  • Map
  • Struct
From Type Array Boolean ByteArray Integer Interop Map Struct
Array * True Exception
Boolean * Boolean ? {0x01} : {} Boolean ? BigInt(1) : BigInt(0)
ByteArray (any byte != 0)? True : False *
Integer (Integer == 0)? False : True ToByteArray *
Interop (Interop == null)? False : True Exception *
Map True Exception *
Struct True Exception *

Opcodes

Value pushing / constants (0-96)

Word Opcode Hex Input Output Parameter Description
PUSH0/PUSHF 0 00 none {0x''} none An empty array of bytes is pushed onto the stack
PUSHBYTES{1..75} 1..75 01..4B none data data (1..75 bytes) The next opcode bytes is data to be pushed onto the stack
PUSHDATA1 76 4C none data 1 byte + data The next byte contains the number of bytes to be pushed onto the stack
PUSHDATA2 77 4D none data 2 bytes + data The next two bytes contain the number of bytes to be pushed onto the stack
PUSHDATA4 78 4E none data 4 bytes + data The next four bytes contain the number of bytes to be pushed onto the stack
PUSHM1 79 4F none BigInt(-1) or {0xff} none The number -1 is pushed onto the stack.
PUSH1 / PUSHT 81 51 none BigInt(1) or {0x01} none The number 1 is pushed onto the stack.
PUSH{K=2..16} 82..96 52..60 none BigInt(K) or hex(K) none The number K is pushed onto the stack.

Control Flow

Word Opcode Hex Input Output Parameter Description
PUSH0/PUSHF 0 00 none {0x''} none An empty array of bytes is pushed onto the stack

Stack

Word Opcode Hex Input Output Parameter Description
PUSH0/PUSHF 0 00 none {0x''} none An empty array of bytes is pushed onto the stack

Splice

Word Opcode Hex Input Output Parameter Description
PUSH0/PUSHF 0 00 none {0x''} none An empty array of bytes is pushed onto the stack

Bitwise Logic

Word Opcode Hex Input Output Parameter Description
PUSH0/PUSHF 0 00 none {0x''} none An empty array of bytes is pushed onto the stack

Arithmetic

Word Opcode Hex Input Output Parameter Description
PUSH0/PUSHF 0 00 none {0x''} none An empty array of bytes is pushed onto the stack

Crypto

Word Opcode Hex Input Output Parameter Description
PUSH0/PUSHF 0 00 none {0x''} none An empty array of bytes is pushed onto the stack

Array / Map / Struct (192-)

Word Opcode Hex Input Output Parameter Description
ARRAYSIZE 192 C0 (1): array (1): length(array) none An array is removed from top of the main stack. Its size is put on top of the main stack.
PACK 193 C1 (n+1): n x1 x2 ... xn (1): [x1,x2,...,xn] none A value n is taken from top of main stack. The next n items on main stack are removed, put inside n-sized array and this array is put on top of the main stack.
UNPACK 194 C2 (1): [x1,x2,...,xn] (n): x1,x2,...,xn none An array is removed from top of the main stack. Its elements are put on top of the main stack (in reverse order) and the array size is also put on main stack.
PICKITEM 195 C3 (2): n [x1,...,xn,...] (1): xn none An input index n (or key) and an array (or map) are taken from main stack. Element array[n] (or map[n]) is put on top of the main stack.
SETITEM 196 C4 (3): v n x=[x1,...,xn,...] (0): x[n]=v none A value v, index n (or key) and an array (or map) are taken from main stack. Attribution array[n]=v (or map[n]=v) is performed.
NEWARRAY 197 C5 (1): n (1): [0: 00, ..., n: 00] none A value n is taken from top of main stack. A zero-filled array type with size n is put on top of the main stack.
NEWSTRUCT 198 C6 (1): n (1): {0: 00, ..., n: 00} none A value n is taken from top of main stack. A zero-filled struct type with size n is put on top of the main stack.
NEWMAP 199 C7 none (1): map{} none A Map is created and put on top of the main stack.

TODO: continue

Stack Isolation (224-228)

Word Opcode Hex Input Output Parameter Description
CALL_I 224 E0 none R items R(1B),P(1B),n(2B) Current context is copied to the invocation stack (similar to CALL). Read return count (1-byte), parameter count (1-byte) and a 2-byte value n for jump to be performed.
CALL_E 225 E1 none R items R(1B),P(1B),scripthash(20B) Reads a scripthash and executes the corresponding contract (similar to APPCALL). Read return count (1-byte), parameter count (1-byte) and a 20-byte scripthash.
CALL_ED 226 E2 scripthash (20B) R items R(1B),P(1B) Reads a scripthash and executes the corresponding contract (similar to dynamic invoke APPCALL). Read return count (1-byte) and parameter count (1-byte). The 20-byte scripthash is pop from main stack.
CALL_ET 227 E3 none R (?) R(1B),P(1B),scripthash(20B) Reads a scripthash and executes the corresponding contract (similar to TAILCALL). Read return count (1-byte), parameter count (1-byte) and a 20-byte scripthash. Disposes the top item on invocation stack.
CALL_EDT 228 E4 scripthash (20B) R (?) R(1B),P(1B) Reads a scripthash and executes the corresponding contract (similar to dynamic invoke TAILCALL). Read return count (1-byte) and parameter count (1-byte). The 20-byte scripthash is pop from main stack. Disposes the top item on invocation stack.

Exceptions (240-241)

Word Opcode Hex Input Output Parameter Description
THROW 240 F0 none none (or FAULT) none Halts the execution of the vm by setting VMState.FAULT
THROWIFNOT 241 F1 x none (or FAULT) none Removes top stack item x, and halts the execution of the vm by setting VMState.FAULT only if x is False.

NeoContract