Instructions

Most instructions are contained within a single, parameterless machine word.

Some instructions take a machine word as argument and are so considered double-word instructions. They are recognized by the form “instr + arg”.

Regarding Opcodes

An instruction's operation code, or opcode, is the machine word uniquely identifying the instruction. For reasons of efficient arithmetization, certain properties of the instruction are encoded in the opcode. Concretely, interpreting the field element in standard representation:

for all double-word instructions, the least significant bit is 1.
for all instructions shrinking the operational stack, the second-to-least significant bit is 1.
for all u32 instructions , the third-to-least significant bit is 1.

The first property is used by instruction skiz. The second property helps with proving consistency of the Op Stack. The third property allows efficient arithmetization of the running product for the Permutation Argument between Processor Table and U32 Table.

Op Stack Manipulation

Instruction	Opcode	old op stack	new op stack	Description
`pop` + `n`	3	`_ c b a` for `n=3`	`_`	Pops the `n` top elements from the stack. 1 ⩽ `n` ⩽ 5
`push` + `a`	1	`_`	`_ a`	Pushes `a` onto the stack.
`divine` + `n`	9	`_` for `n=2`	`_ b a`	Pushes `n` non-deterministic elements `a` to the stack. Interface for secret input. 1 ⩽ `n` ⩽ 5
`pick` + `i`	17	`_ d x c b a` for `i=3`	`_ d c b a x`	Moves the element indicated by `i` to the top of the stack. 0 ⩽ `i` < 16
`place` + `i`	25	`_ d c b a x` for `i=3`	`_ d x c b a`	Moves the top of the stack to the indicated position `i`. 0 ⩽ `i` < 16
`dup` + `i`	33	`_ e d c b a` for `i=3`	`_ e d c b a d`	Duplicates the element `i` positions away from the top. 0 ⩽ `i` < 16
`swap` + `i`	41	`_ e d c b a` for `i=3`	`_ e a c b d`	Swaps the `i`th stack element with the top of the stack. 0 ⩽ `i` < 16

Instruction divine n (together with merkle_step) make Triton a virtual machine that can execute non-deterministic programs. As programs go, this concept is somewhat unusual and benefits from additional explanation. The name of the instruction is the verb (not the adjective) meaning “to discover by intuition or insight.”

From the perspective of the program, the instruction divine n makes some n elements magically appear on the stack. It is not at all specified what those elements are, but generally speaking, they have to be exactly correct, else execution fails. Hence, from the perspective of the program, it just non-deterministically guesses the correct values in a moment of divine clarity.

Looking at the entire system, consisting of the VM, the program, and all inputs – both public and secret – execution is deterministic: the divined values were supplied as and are read from secret input.

Control Flow

Instruction	Opcode	old op stack	new op stack	old `ip`	new `ip`	old jump stack	new jump stack	Description
`halt`	0	`_`	`_`	`ip`	`ip+1`	`_`	`_`	Solves the halting problem (if the instruction is reached). Indicates graceful shutdown of the VM.
`nop`	8	`_`	`_`	`ip`	`ip+1`	`_`	`_`	Do nothing
`skiz`	2	`_ a`	`_`	`ip`	`ip+s`	`_`	`_`	Skip next instruction if `a` is zero. `s` ∈ {1, 2, 3} depends on `a` and whether the next instruction takes an argument.
`call` + `d`	49	`_`	`_`	`ip`	`d`	`_`	`_ (ip+2, d)`	Push `(ip+2,d)` to the jump stack, and jump to absolute address `d`
`return`	16	`_`	`_`	`ip`	`o`	`_ (o, d)`	`_`	Pop one pair off the jump stack and jump to that pair's return address (which is the first element).
`recurse`	24	`_`	`_`	`ip`	`d`	`_ (o, d)`	`_ (o, d)`	Peek at the top pair of the jump stack and jump to that pair's destination address (which is the second element).
`recurse_or_return`	32	`_ b a .....`	`_ b a .....`	`ip`	`d` or `o`	`_ (o, d)`	`_ (o, d)` or `_`	Like `recurse` if `st5 = a != b = st6`, like `return` if `a == b`. See also extended description below.
`assert`	10	`_ a`	`_`	`ip`	`ip+1`	`_`	`_`	Pops `a` if `a == 1`, else crashes the virtual machine.

The instructions return, recurse, and recurse_or_return require a non-empty jump stack. Should the jump stack be empty, executing any of these instruction causes Triton VM to crash.

Instruction recurse_or_return behaves – surprise! – either like instruction recurse or like instruction return. The (deterministic) decision which behavior to exhibit is made at runtime and depends on stack elements st5 and st6. If st5 != st6, then recurse_or_return acts like instruction recurse, else like return. The instruction is designed to facilitate loops using pointer equality as termination condition and to play nicely with instructions merkle_step and merkle_step_mem.

Memory Access

Instruction	Opcode	old op stack	new op stack	old RAM	new RAM	Description
`read_mem` + `n`	57	`_ p+2` for `n=3`	`_ v2 v1 v0 p-1`	[p: v0, p+1, v1, …]	[p: v0, p+1, v1, …]	Reads consecutive values `vi` from RAM at address `p` and puts them onto the op stack. Decrements RAM pointer (`st0`) by `n`. 1 ⩽ `n` ⩽ 5
`write_mem` + `n`	11	`_ v2 v1 v0 p` for `n=3`	`_ p+3`	[]	[p: v0, p+1, v1, …]	Writes op stack's `n` top-most values `vi` to RAM at the address `p+i`, popping the `vi`. Increments RAM pointer (`st0`) by `n`. 1 ⩽ `n` ⩽ 5

For the benefit of clarity, the effect of every possible argument is given below.

instruction	old op stack	new op stack	old RAM	new RAM
`read_mem 1`	`_ p`	`_ a p-1`	[p: a]	[p: a]
`read_mem 2`	`_ p+1`	`_ b a p-1`	[p: a, p+1: b]	[p: a, p+1: b]
`read_mem 3`	`_ p+2`	`_ c b a p-1`	[p: a, p+1: b, p+2: c]	[p: a, p+1: b, p+2: c]
`read_mem 4`	`_ p+3`	`_ d c b a p-1`	[p: a, p+1: b, p+2: c, p+3: d]	[p: a, p+1: b, p+2: c, p+3: d]
`read_mem 5`	`_ p+4`	`_ e d c b a p-1`	[p: a, p+1: b, p+2: c, p+3: d, p+4: e]	[p: a, p+1: b, p+2: c, p+3: d, p+4: e]
`write_mem 1`	`_ a p`	`_ p+1`	[]	[p: a]
`write_mem 2`	`_ b a p`	`_ p+2`	[]	[p: a, p+1: b]
`write_mem 3`	`_ c b a p`	`_ p+3`	[]	[p: a, p+1: b, p+2: c]
`write_mem 4`	`_ d c b a p`	`_ p+4`	[]	[p: a, p+1: b, p+2: c, p+3: d]
`write_mem 5`	`_ e d c b a p`	`_ p+5`	[]	[p: a, p+1: b, p+2: c, p+3: d, p+4: e]

Hashing

Instruction	Opcode	old op stack	new op stack	Description
`hash`	18	`_ jihgfedcba`	`_ yxwvu`	Hashes the stack's 10 top-most elements and puts their digest onto the stack, shrinking the stack by 5.
`assert_vector`	26	`_ edcba edcba`	`_ edcba`	Assert equality of `st(i)` to `st(i+5)` for `0 <= i < 4`. Crashes the VM if any pair is unequal. Pops the 5 top-most elements.
`sponge_init`	40	`_`	`_`	Initializes (resets) the Sponge's state. Must be the first Sponge instruction executed.
`sponge_absorb`	34	`_ jihgfedcba`	`_`	Absorbs the stack's ten top-most elements into the Sponge state.
`sponge_absorb_mem`	48	`_ dcba p`	`_ hgfe (p+10)`	Absorbs the ten RAM elements at addresses `p`, `p+1`, … into the Sponge state. Overwrites stack elements `st1` through `st4` with the first four absorbed elements.
`sponge_squeeze`	56	`_`	`_ zyxwvutsrq`	Squeezes the Sponge and pushes the 10 squeezed elements onto the stack.

The instruction hash works as follows. The stack's 10 top-most elements (jihgfedcba) are popped from the stack, reversed, and concatenated with six zeros, resulting in abcdefghij000000. The Tip5 permutation is applied to abcdefghij000000, resulting in αβγδεζηθικuvwxyz. The first five elements of this result, i.e., αβγδε, are reversed and pushed to the stack. For example, the old stack was _ jihgfedcba and the new stack is _ εδγβα.

The instructions sponge_init, sponge_absorb, sponge_absorb_mem, and sponge_squeeze are the interface for using the Tip5 permutation in a Sponge construction. The capacity is never accessible to the program that's being executed by Triton VM. At any given time, at most one Sponge state exists. Only instruction sponge_init resets the state of the Sponge, and only the three Sponge instructions influence the Sponge's state. Notably, executing instruction hash does not modify the Sponge's state. When using the Sponge instructions, it is the programmer's responsibility to take care of proper input padding: Triton VM cannot know the number of elements that will be absorbed.

Base Field Arithmetic on Stack

Instruction	Opcode	old op stack	new op stack	Description
`add`	42	`_ b a`	`_ c`	Computes the sum (`c`) of the top two elements of the stack (`b` and `a`) over the field.
`addi` + `a`	65	`_ b`	`_ c`	Computes the sum (`c`) of the top element of the stack (`b`) and the immediate argument (`a`).
`mul`	50	`_ b a`	`_ c`	Computes the product (`c`) of the top two elements of the stack (`b` and `a`) over the field.
`invert`	64	`_ a`	`_ b`	Computes the multiplicative inverse (over the field) of the top of the stack. Crashes the VM if the top of the stack is 0.
`eq`	58	`_ b a`	`_ (a == b)`	Tests the top two stack elements for equality.

Bitwise Arithmetic on Stack

Instruction	Opcode	old op stack	new op stack	Description
`split`	4	`_ a`	`_ hi lo`	Decomposes the top of the stack into the lower 32 bits and the upper 32 bits.
`lt`	6	`_ b a`	`_ a<b`	“Less than” of the stack's two top-most elements. Crashes the VM if `a` or `b` is not u32.
`and`	14	`_ b a`	`_ a&b`	Bitwise and of the stack's two top-most elements. Crashes the VM if `a` or `b` is not u32.
`xor`	22	`_ b a`	`_ a^b`	Bitwise exclusive or of the stack's two top-most elements. Crashes the VM if `a` or `b` is not u32.
`log_2_floor`	12	`_ a`	`_ ⌊log₂(a)⌋`	The number of bits in `a` minus 1, i.e., $⌊ lo g_{2} a ⌋$ . Crashes the VM if `a` is 0 or not u32.
`pow`	30	`_ e b`	`_ b**e`	The top of the stack to the power of the stack's runner up. Crashes the VM if exponent `e` is not u32.
`div_mod`	20	`_ d n`	`_ q r`	Division with remainder of numerator `n` by denominator `d`. Guarantees the properties `n == q·d + r` and `r < d`. Crashes the VM if `n` or `d` is not u32 or if `d` is 0.
`pop_count`	28	`_ a`	`_ w`	Computes the hamming weight or “population count” of `a`. Crashes the VM if `a` is not u32.

Extension Field Arithmetic on Stack

Instruction	Opcode	old op stack	new op stack	Description
`xx_add`	66	`_ z y x b c a`	`_ w v u`	Adds the two extension field elements encoded by field elements `z y x` and `b c a`.
`xx_mul`	74	`_ z y x b c a`	`_ w v u`	Multiplies the two extension field elements encoded by field elements `z y x` and `b c a`.
`x_invert`	72	`_ z y x`	`_ w v u`	Inverts the extension field element encoded by field elements `z y x` in-place. Crashes the VM if the extension field element is 0.
`xb_mul`	82	`_ z y x a`	`_ w v u`	Scalar multiplication of the extension field element encoded by field elements `z y x` with field element `a`. Overwrites `z y x` with the result.

Input/Output

Instruction	Opcode	old op stack	new op stack	Description
`read_io` + `n`	73	`_` for `n=3`	`_ c b a`	Reads `n` B-Field elements from standard input and pushes them to the stack. 1 ⩽ `n` ⩽ 5
`write_io` + `n`	19	`_ c b a` for `n=3`	`_`	Pops `n` elements from the stack and writes them to standard output. 1 ⩽ `n` ⩽ 5

Many-In-One

Instruction	Opcode	old op stack	new op stack	Description
`merkle_step`	36	`_ i edcba`	`_ (i div 2) zyxwv`	Helps traversing a Merkle tree during authentication path verification. Crashes the VM if `i` is not u32. See extended description below.
`merkle_step_mem`	44	`_ p f i edcba`	`_ p+5 f (i div 2) zyxwv`	Helps traversing a Merkle tree during authentication path verification with the authentication path being supplied in RAM. Crashes the VM if `i` is not u32. See extended description below.
`xx_dot_step`	80	`_ z y x b a`	`_ z+p2 y+p1 x+p0 b+3 a+3`	Reads two extension field elements from RAM located at the addresses corresponding to the two top stack elements, multiplies the extension field elements, and adds the product `(p0, p1, p2)` to an accumulator located on stack immediately below the two pointers. Also, increase the pointers by the number of words read.
`xb_dot_step`	88	`_ z y x b a`	`_ z+p2 y+p1 x+p0 b+3 a+1`	Reads one base field element from RAM located at the addresses corresponding to the top of the stack, one extension field element from RAM located at the address of the second stack element, multiplies the field elements, and adds the product `(p0, p1, p2)` to an accumulator located on stack immediately below the two pointers. Also, increase the pointers by the number of words read.

The instruction merkle_step works as follows. The 6th element of the stack i is taken as the node index for a Merkle tree that is claimed to include data whose digest is the content of stack registers st4 through st0, i.e., edcba. The sibling digest of edcba is εδγβα and is read from the input interface of secret data. The least-significant bit of i indicates whether edcba is the digest of a left leaf or a right leaf of the Merkle tree's current level. Depending on this least significant bit of i, merkle_step either

(i = 0 mod 2) interprets edcba as the left digest, εδγβα as the right digest, or
(i = 1 mod 2) interprets εδγβα as the left digest, edcba as the right digest.

In either case,

the left and right digests are hashed, and the resulting digest zyxwv replaces the top of the stack, and
6th register i is shifted by 1 bit to the right, i.e., the least-significant bit is dropped.

Instruction merkle_step_mem works very similarly to instruction merkle_step. The main difference, as the name suggests, is the source of the sibling digest: Instead of reading it from the input interface of secret data, it is supplied via RAM. Stack element st7 is taken as the RAM pointer, holding the memory address at which the next sibling digest is located in RAM. Executing instruction merkle_step_mem increments the memory pointer by the length of one digest, anticipating an authentication path that is laid out continuously. Stack element st6 does not change when executing instruction merkle_step_mem in order to facilitate instruction recurse_or_return.

In conjunction with instructions recurse_or_return and assert_vector, the instructions merkle_step and merkle_step_mem allow efficient verification of a Merkle authentication path. Furthermore, instruction merkle_step_mem allows verifiable re-use of an authentication path. This is necessary, for example, when verifiably updating a Merkle tree: first, the authentication path is used to confirm inclusion of some old leaf, and then to compute the tree's new root from the new leaf.