Transaction Execution Approval Language (TEAL)

TEAL is a bytecode based stack language that executes inside Algorand transactions. TEAL programs can be used to check the parameters of the transaction and approve the transaction as if by a signature. This use of TEAL is called a LogicSig. Starting with v2, TEAL programs may also execute as Applications which are invoked with explicit application call transactions. Programs have read-only access to the transaction they are attached to, transactions in their atomic transaction group, and a few global values. In addition, Application programs have access to limited state that is global to the application and per-account local state for each account that has opted-in to the application. Programs cannot modify or create transactions, only reject or approve them. For both types of program, approval is signaled by finishing with the stack containing a single non-zero uint64 value.

The Stack

The stack starts empty and contains values of either uint64 or bytes (bytes are implemented in Go as a []byte slice). Most operations act on the stack, popping arguments from it and pushing results to it.

The maximum stack depth is currently 1000.

Scratch Space

In addition to the stack there are 256 positions of scratch space, also uint64-bytes union values, accessed by the load and store ops moving data from or to scratch space, respectively.

Execution Modes

Starting from version 2 TEAL evaluator can run programs in two modes: 1. LogicSig (stateless) 2. Application run (stateful)

Differences between modes include: 1. Max program length (consensus parameters LogicSigMaxSize, MaxAppTotalProgramLen & MaxExtraAppProgramPages) 2. Max program cost (consensus parameters LogicSigMaxCost, MaxAppProgramCost) 3. Opcode availability. For example, all stateful operations are only available in stateful mode. Refer to opcodes document for details.

Execution Environment for LogicSigs

TEAL LogicSigs run in Algorand nodes as part of testing a proposed transaction to see if it is valid and authorized to be committed into a block.

If an authorized program executes and finishes with a single non-zero uint64 value on the stack then that program has validated the transaction it is attached to.

The TEAL program has access to data from the transaction it is attached to (txn op), any transactions in a transaction group it is part of (gtxn op), and a few global values like consensus parameters (global op). Some "Args" may be attached to a transaction being validated by a TEAL program. Args are an array of byte strings. A common pattern would be to have the key to unlock some contract as an Arg. Args are recorded on the blockchain and publicly visible when the transaction is submitted to the network.

A program can either authorize some delegated action on a normal private key signed or multisig account or be wholly in charge of a contract account.

• If the account has signed the program (an ed25519 signature on "Program" concatenated with the program bytes) then if the program returns true the transaction is authorized as if the account had signed it. This allows an account to hand out a signed program so that other users can carry out delegated actions which are approved by the program.
• If the SHA512_256 hash of the program (prefixed by "Program") is equal to the transaction Sender address then this is a contract account wholly controlled by the program. No other signature is necessary or possible. The only way to execute a transaction against the contract account is for the program to approve it.

The TEAL bytecode plus the length of any Args must add up to less than 1000 bytes (consensus parameter LogicSigMaxSize). Each TEAL op has an associated cost and the program cost must total less than 20000 (consensus parameter LogicSigMaxCost). Most ops have a cost of 1, but a few slow crypto ops are much higher. Prior to v4, the program's cost was estimated as the static sum of all the opcode costs in the program (whether they were actually executed or not). Beginning with v4, the program's cost is tracked dynamically, while being evaluated. If the program exceeds its budget, it fails.

Constants

Constants are loaded into the environment into storage separate from the stack. They can then be pushed onto the stack by referring to the type and index. This makes for efficient re-use of byte constants used for account addresses, etc. Constants that are not reused can be pushed with pushint or pushbytes.

The assembler will hide most of this, allowing simple use of int 1234 and byte 0xcafed00d. These constants will automatically get assembled into int and byte pages of constants, de-duplicated, and operations to load them from constant storage space inserted.

Constants are loaded into the environment by two opcodes, intcblock and bytecblock. Both of these use proto-buf style variable length unsigned int, reproduced here. The intcblock opcode is followed by a varuint specifying the length of the array and then that number of varuint. The bytecblock opcode is followed by a varuint array length then that number of pairs of (varuint, bytes) length prefixed byte strings. This should efficiently load 32 and 64 byte constants which will be common as addresses, hashes, and signatures.

Constants are pushed onto the stack by intc, intc_[0123], pushint, bytec, bytec_[0123], and pushbytes. The assembler will handle converting int N or byte N into the appropriate form of the instruction needed.

Named Integer Constants

OnComplete

An application transaction must indicate the action to be taken following the execution of its approvalProgram or clearStateProgram. The constants below describe the available actions.

Value	Constant name	Description
0	NoOp	Only execute the ApprovalProgram associated with this application ID, with no additional effects.
1	OptIn	Before executing the ApprovalProgram, allocate local state for this application into the sender's account data.
2	CloseOut	After executing the ApprovalProgram, clear any local state for this application out of the sender's account data.
3	ClearState	Don't execute the ApprovalProgram, and instead execute the ClearStateProgram (which may not reject this transaction). Additionally, clear any local state for this application out of the sender's account data as in CloseOutOC.
4	UpdateApplication	After executing the ApprovalProgram, replace the ApprovalProgram and ClearStateProgram associated with this application ID with the programs specified in this transaction.
5	DeleteApplication	After executing the ApprovalProgram, delete the application parameters from the account data of the application's creator.

TypeEnum constants

Value	Constant name	Description
0	unknown	Unknown type. Invalid transaction
1	pay	Payment
2	keyreg	KeyRegistration
3	acfg	AssetConfig
4	axfer	AssetTransfer
5	afrz	AssetFreeze
6	appl	ApplicationCall

Operations

Most operations work with only one type of argument, uint64 or bytes, and panic if the wrong type value is on the stack.

Many instructions accept values to designate Accounts, Assets, or Applications. Beginning with TEAL v4, these values may always be given as an offset in the corresponding Txn fields (Txn.Accounts, Txn.ForeignAssets, Txn.ForeignApps) or as the value itself (a bytes address for Accounts, or a uint64 ID). The values, however, must still be present in the Txn fields. Before TEAL v4, most opcodes required the use of an offset, except for reading account local values of assets or applications, which accepted the IDs directly and did not require the ID to be present in they corresponding Foreign array. (Note that beginning with TEAL v4, those ID are required to be present in their corresponding Foreign array.) See individual opcodes for details. In the case of account offsets or application offsets, 0 is specially defined to Txn.Sender or the ID of the current application, respectively.

Many programs need only a few dozen instructions. The instruction set has some optimization built in. intc, bytec, and arg take an immediate value byte, making a 2-byte op to load a value onto the stack, but they also have single byte versions for loading the most common constant values. Any program will benefit from having a few common values loaded with a smaller one byte opcode. Cryptographic hashes and ed25519verify are single byte opcodes with powerful libraries behind them. These operations still take more time than other ops (and this is reflected in the cost of each op and the cost limit of a program) but are efficient in compiled code space.

This summary is supplemented by more detail in the opcodes document.

Some operations 'panic' and immediately end execution of the program. A transaction checked by a program that panics is not valid. A contract account governed by a buggy program might not have a way to get assets back out of it. Code carefully.

Arithmetic, Logic, and Cryptographic Operations

For one-argument ops, X is the last element on the stack, which is typically replaced by a new value.

For two-argument ops, A is the penultimate element on the stack and B is the top of the stack. These typically result in popping A and B from the stack and pushing the result.

For three-argument ops, A is the element two below the top, B is the penultimate stack element and C is the top of the stack. These operatiosn typically pop A, B, and C from the stack and push the result.

Op	Description
sha256	SHA256 hash of value X, yields [32]byte
keccak256	Keccak256 hash of value X, yields [32]byte
sha512_256	SHA512_256 hash of value X, yields [32]byte
ed25519verify	for (data A, signature B, pubkey C) verify the signature of ("ProgData" || program_hash || data) against the pubkey => {0 or 1}
+	A plus B. Panic on overflow.
-	A minus B. Panic if B > A.
/	A divided by B. Panic if B == 0.
*	A times B. Panic on overflow.
<	A less than B => {0 or 1}
>	A greater than B => {0 or 1}
<=	A less than or equal to B => {0 or 1}
>=	A greater than or equal to B => {0 or 1}
&&	A is not zero and B is not zero => {0 or 1}
\|\|	A is not zero or B is not zero => {0 or 1}
shl	A times 2^B, modulo 2^64
shr	A divided by 2^B
sqrt	The largest integer B such that B^2 <= X
bitlen	The highest set bit in X. If X is a byte-array, it is interpreted as a big-endian unsigned integer. bitlen of 0 is 0, bitlen of 8 is 4
exp	A raised to the Bth power. Panic if A == B == 0 and on overflow
==	A is equal to B => {0 or 1}
!=	A is not equal to B => {0 or 1}
!	X == 0 yields 1; else 0
len	yields length of byte value X
itob	converts uint64 X to big endian bytes
btoi	converts bytes X as big endian to uint64
%	A modulo B. Panic if B == 0.
\|	A bitwise-or B
&	A bitwise-and B
^	A bitwise-xor B
~	bitwise invert value X
mulw	A times B out to 128-bit long result as low (top) and high uint64 values on the stack
addw	A plus B out to 128-bit long result as sum (top) and carry-bit uint64 values on the stack
divmodw	Pop four uint64 values. The deepest two are interpreted as a uint128 dividend (deepest value is high word), the top two are interpreted as a uint128 divisor. Four uint64 values are pushed to the stack. The deepest two are the quotient (deeper value is the high uint64). The top two are the remainder, low bits on top.
expw	A raised to the Bth power as a 128-bit long result as low (top) and high uint64 values on the stack. Panic if A == B == 0 or if the results exceeds 2^128-1
getbit	pop a target A (integer or byte-array), and index B. Push the Bth bit of A.
setbit	pop a target A, index B, and bit C. Set the Bth bit of A to C, and push the result
getbyte	pop a byte-array A and integer B. Extract the Bth byte of A and push it as an integer
setbyte	pop a byte-array A, integer B, and small integer C (between 0..255). Set the Bth byte of A to C, and push the result
concat	pop two byte-arrays A and B and join them, push the result
substring s e	pop a byte-array A. For immediate values in 0..255 S and E: extract a range of bytes from A starting at S up to but not including E, push the substring result. If E < S, or either is larger than the array length, the program fails
substring3	pop a byte-array A and two integers B and C. Extract a range of bytes from A starting at B up to but not including C, push the substring result. If C < B, or either is larger than the array length, the program fails

These opcodes take byte-array values that are interpreted as big-endian unsigned integers. For mathematical operators, the returned values are the shortest byte-array that can represent the returned value. For example, the zero value is the empty byte-array. For comparison operators, the returned value is a uint64

Input lengths are limited to a maximum length 64 bytes, which represents a 512 bit unsigned integer. Output lengths are not explicitly restricted, though only b* and b+ can produce a larger output than their inputs, so there is an implicit length limit of 128 bytes on outputs.

Op	Description
b+	A plus B, where A and B are byte-arrays interpreted as big-endian unsigned integers
b-	A minus B, where A and B are byte-arrays interpreted as big-endian unsigned integers. Panic on underflow.
b/	A divided by B, where A and B are byte-arrays interpreted as big-endian unsigned integers. Panic if B is zero.
b*	A times B, where A and B are byte-arrays interpreted as big-endian unsigned integers.
b<	A is less than B, where A and B are byte-arrays interpreted as big-endian unsigned integers => { 0 or 1}
b>	A is greater than B, where A and B are byte-arrays interpreted as big-endian unsigned integers => { 0 or 1}
b<=	A is less than or equal to B, where A and B are byte-arrays interpreted as big-endian unsigned integers => { 0 or 1}
b>=	A is greater than or equal to B, where A and B are byte-arrays interpreted as big-endian unsigned integers => { 0 or 1}
b==	A is equals to B, where A and B are byte-arrays interpreted as big-endian unsigned integers => { 0 or 1}
b!=	A is not equal to B, where A and B are byte-arrays interpreted as big-endian unsigned integers => { 0 or 1}
b%	A modulo B, where A and B are byte-arrays interpreted as big-endian unsigned integers. Panic if B is zero.

These opcodes operate on the bits of byte-array values. The shorter array is interpeted as though left padded with zeros until it is the same length as the other input. The returned values are the same length as the longest input. Therefore, unlike array arithmetic, these results may contain leading zero bytes.

Op	Description
b\|	A bitwise-or B, where A and B are byte-arrays, zero-left extended to the greater of their lengths
b&	A bitwise-and B, where A and B are byte-arrays, zero-left extended to the greater of their lengths
b^	A bitwise-xor B, where A and B are byte-arrays, zero-left extended to the greater of their lengths
b~	X with all bits inverted

Loading Values

Opcodes for getting data onto the stack.

Some of these have immediate data in the byte or bytes after the opcode.

Op	Description
intcblock uint ...	prepare block of uint64 constants for use by intc
intc i	push Ith constant from intcblock to stack
intc_0	push constant 0 from intcblock to stack
intc_1	push constant 1 from intcblock to stack
intc_2	push constant 2 from intcblock to stack
intc_3	push constant 3 from intcblock to stack
pushint uint	push immediate UINT to the stack as an integer
bytecblock bytes ...	prepare block of byte-array constants for use by bytec
bytec i	push Ith constant from bytecblock to stack
bytec_0	push constant 0 from bytecblock to stack
bytec_1	push constant 1 from bytecblock to stack
bytec_2	push constant 2 from bytecblock to stack
bytec_3	push constant 3 from bytecblock to stack
pushbytes bytes	push the following program bytes to the stack
bzero	push a byte-array of length X, containing all zero bytes
arg n	push Nth LogicSig argument to stack
arg_0	push LogicSig argument 0 to stack
arg_1	push LogicSig argument 1 to stack
arg_2	push LogicSig argument 2 to stack
arg_3	push LogicSig argument 3 to stack
txn f	push field F of current transaction to stack
gtxn t f	push field F of the Tth transaction in the current group
txna f i	push Ith value of the array field F of the current transaction
gtxna t f i	push Ith value of the array field F from the Tth transaction in the current group
gtxns f	push field F of the Xth transaction in the current group
gtxnsa f i	push Ith value of the array field F from the Xth transaction in the current group
global f	push value from globals to stack
load i	copy a value from scratch space to the stack
store i	pop a value from the stack and store to scratch space
gload t i	push Ith scratch space index of the Tth transaction in the current group
gloads i	push Ith scratch space index of the Xth transaction in the current group
gaid t	push the ID of the asset or application created in the Tth transaction of the current group
gaids	push the ID of the asset or application created in the Xth transaction of the current group

Transaction Fields

Index	Name	Type	Notes
0	Sender	[]byte	32 byte address
1	Fee	uint64	micro-Algos
2	FirstValid	uint64	round number
3	FirstValidTime	uint64	Causes program to fail; reserved for future use
4	LastValid	uint64	round number
5	Note	[]byte
6	Lease	[]byte
7	Receiver	[]byte	32 byte address
8	Amount	uint64	micro-Algos
9	CloseRemainderTo	[]byte	32 byte address
10	VotePK	[]byte	32 byte address
11	SelectionPK	[]byte	32 byte address
12	VoteFirst	uint64
13	VoteLast	uint64
14	VoteKeyDilution	uint64
15	Type	[]byte
16	TypeEnum	uint64	See table below
17	XferAsset	uint64	Asset ID
18	AssetAmount	uint64	value in Asset's units
19	AssetSender	[]byte	32 byte address. Causes clawback of all value of asset from AssetSender if Sender is the Clawback address of the asset.
20	AssetReceiver	[]byte	32 byte address
21	AssetCloseTo	[]byte	32 byte address
22	GroupIndex	uint64	Position of this transaction within an atomic transaction group. A stand-alone transaction is implicitly element 0 in a group of 1
23	TxID	[]byte	The computed ID for this transaction. 32 bytes.
24	ApplicationID	uint64	ApplicationID from ApplicationCall transaction. LogicSigVersion >= 2.
25	OnCompletion	uint64	ApplicationCall transaction on completion action. LogicSigVersion >= 2.
26	ApplicationArgs	[]byte	Arguments passed to the application in the ApplicationCall transaction. LogicSigVersion >= 2.
27	NumAppArgs	uint64	Number of ApplicationArgs. LogicSigVersion >= 2.
28	Accounts	[]byte	Accounts listed in the ApplicationCall transaction. LogicSigVersion >= 2.
29	NumAccounts	uint64	Number of Accounts. LogicSigVersion >= 2.
30	ApprovalProgram	[]byte	Approval program. LogicSigVersion >= 2.
31	ClearStateProgram	[]byte	Clear state program. LogicSigVersion >= 2.
32	RekeyTo	[]byte	32 byte Sender's new AuthAddr. LogicSigVersion >= 2.
33	ConfigAsset	uint64	Asset ID in asset config transaction. LogicSigVersion >= 2.
34	ConfigAssetTotal	uint64	Total number of units of this asset created. LogicSigVersion >= 2.
35	ConfigAssetDecimals	uint64	Number of digits to display after the decimal place when displaying the asset. LogicSigVersion >= 2.
36	ConfigAssetDefaultFrozen	uint64	Whether the asset's slots are frozen by default or not, 0 or 1. LogicSigVersion >= 2.
37	ConfigAssetUnitName	[]byte	Unit name of the asset. LogicSigVersion >= 2.
38	ConfigAssetName	[]byte	The asset name. LogicSigVersion >= 2.
39	ConfigAssetURL	[]byte	URL. LogicSigVersion >= 2.
40	ConfigAssetMetadataHash	[]byte	32 byte commitment to some unspecified asset metadata. LogicSigVersion >= 2.
41	ConfigAssetManager	[]byte	32 byte address. LogicSigVersion >= 2.
42	ConfigAssetReserve	[]byte	32 byte address. LogicSigVersion >= 2.
43	ConfigAssetFreeze	[]byte	32 byte address. LogicSigVersion >= 2.
44	ConfigAssetClawback	[]byte	32 byte address. LogicSigVersion >= 2.
45	FreezeAsset	uint64	Asset ID being frozen or un-frozen. LogicSigVersion >= 2.
46	FreezeAssetAccount	[]byte	32 byte address of the account whose asset slot is being frozen or un-frozen. LogicSigVersion >= 2.
47	FreezeAssetFrozen	uint64	The new frozen value, 0 or 1. LogicSigVersion >= 2.
48	Assets	uint64	Foreign Assets listed in the ApplicationCall transaction. LogicSigVersion >= 3.
49	NumAssets	uint64	Number of Assets. LogicSigVersion >= 3.
50	Applications	uint64	Foreign Apps listed in the ApplicationCall transaction. LogicSigVersion >= 3.
51	NumApplications	uint64	Number of Applications. LogicSigVersion >= 3.
52	GlobalNumUint	uint64	Number of global state integers in ApplicationCall. LogicSigVersion >= 3.
53	GlobalNumByteSlice	uint64	Number of global state byteslices in ApplicationCall. LogicSigVersion >= 3.
54	LocalNumUint	uint64	Number of local state integers in ApplicationCall. LogicSigVersion >= 3.
55	LocalNumByteSlice	uint64	Number of local state byteslices in ApplicationCall. LogicSigVersion >= 3.
56	ExtraProgramPages	uint64	Number of additional pages for each of the application's approval and clear state programs. An ExtraProgramPages of 1 means 2048 more total bytes, or 1024 for each program. LogicSigVersion >= 4.

Additional details in the opcodes document on the txn op.

Global Fields

Global fields are fields that are common to all the transactions in the group. In particular it includes consensus parameters.

Index	Name	Type	Notes
0	MinTxnFee	uint64	micro Algos
1	MinBalance	uint64	micro Algos
2	MaxTxnLife	uint64	rounds
3	ZeroAddress	[]byte	32 byte address of all zero bytes
4	GroupSize	uint64	Number of transactions in this atomic transaction group. At least 1
5	LogicSigVersion	uint64	Maximum supported TEAL version. LogicSigVersion >= 2.
6	Round	uint64	Current round number. LogicSigVersion >= 2.
7	LatestTimestamp	uint64	Last confirmed block UNIX timestamp. Fails if negative. LogicSigVersion >= 2.
8	CurrentApplicationID	uint64	ID of current application executing. Fails if no such application is executing. LogicSigVersion >= 2.
9	CreatorAddress	[]byte	Address of the creator of the current application. Fails if no such application is executing. LogicSigVersion >= 3.

Asset Fields

Asset fields include AssetHolding and AssetParam fields that are used in asset_read_* opcodes

Index	Name	Type	Notes
0	AssetBalance	uint64	Amount of the asset unit held by this account
1	AssetFrozen	uint64	Is the asset frozen or not

Index	Name	Type	Notes
0	AssetTotal	uint64	Total number of units of this asset
1	AssetDecimals	uint64	See AssetParams.Decimals
2	AssetDefaultFrozen	uint64	Frozen by default or not
3	AssetUnitName	[]byte	Asset unit name
4	AssetName	[]byte	Asset name
5	AssetURL	[]byte	URL with additional info about the asset
6	AssetMetadataHash	[]byte	Arbitrary commitment
7	AssetManager	[]byte	Manager commitment
8	AssetReserve	[]byte	Reserve address
9	AssetFreeze	[]byte	Freeze address
10	AssetClawback	[]byte	Clawback address

Flow Control

Op	Description
err	Error. Panic immediately. This is primarily a fencepost against accidental zero bytes getting compiled into programs.
bnz target	branch to TARGET if value X is not zero
bz target	branch to TARGET if value X is zero
b target	branch unconditionally to TARGET
return	use last value on stack as success value; end
pop	discard value X from stack
dup	duplicate last value on stack
dup2	duplicate two last values on stack: A, B -> A, B, A, B
dig n	push the Nth value from the top of the stack. dig 0 is equivalent to dup
swap	swaps two last values on stack: A, B -> B, A
select	selects one of two values based on top-of-stack: A, B, C -> (if C != 0 then B else A)
assert	immediately fail unless value X is a non-zero number
callsub target	branch unconditionally to TARGET, saving the next instruction on the call stack
retsub	pop the top instruction from the call stack and branch to it

State Access

Op	Description
balance	get balance for account A, in microalgos. The balance is observed after the effects of previous transactions in the group, and after the fee for the current transaction is deducted.
min_balance	get minimum required balance for account A, in microalgos. Required balance is affected by ASA and App usage. When creating or opting into an app, the minimum balance grows before the app code runs, therefore the increase is visible there. When deleting or closing out, the minimum balance decreases after the app executes.
app_opted_in	check if account A opted in for the application B => {0 or 1}
app_local_get	read from account A from local state of the current application key B => value
app_local_get_ex	read from account A from local state of the application B key C => [... stack, value, 0 or 1]
app_global_get	read key A from global state of a current application => value
app_global_get_ex	read from application A global state key B => [... stack, value, 0 or 1]
app_local_put	write to account specified by A to local state of a current application key B with value C
app_global_put	write key A and value B to global state of the current application
app_local_del	delete from account A local state key B of the current application
app_global_del	delete key A from a global state of the current application
asset_holding_get i	read from account A and asset B holding field X (imm arg) => {0 or 1 (top), value}
asset_params_get i	read from asset A params field X (imm arg) => {0 or 1 (top), value}

Assembler Syntax

The assembler parses line by line. Ops that just use the stack appear on a line by themselves. Ops that take arguments are the op and then whitespace and then any argument or arguments.

The first line may contain a special version pragma #pragma version X, which directs the assembler to generate TEAL bytecode targeting a certain version. For instance, #pragma version 2 produces bytecode targeting TEAL v2. By default, the assembler targets TEAL v1.

Subsequent lines may contain other pragma declarations (i.e., #pragma <some-specification>), pertaining to checks that the assembler should perform before agreeing to emit the program bytes, specific optimizations, etc. Those declarations are optional and cannot alter the semantics as described in this document.

"//" prefixes a line comment.

Constants and Pseudo-Ops

A few pseudo-ops simplify writing code. int and byte and addr followed by a constant record the constant to a intcblock or bytecblock at the beginning of code and insert an intc or bytec reference where the instruction appears to load that value. addr parses an Algorand account address base32 and converts it to a regular bytes constant.

byte constants are:

byte base64 AAAA...
byte b64 AAAA...
byte base64(AAAA...)
byte b64(AAAA...)
byte base32 AAAA...
byte b32 AAAA...
byte base32(AAAA...)
byte b32(AAAA...)
byte 0x0123456789abcdef...
byte "\x01\x02"
byte "string literal"

int constants may be 0x prefixed for hex, 0 prefixed for octal, or decimal numbers.

intcblock may be explictly assembled. It will conflict with the assembler gathering int pseudo-ops into a intcblock program prefix, but may be used if code only has explicit intc references. intcblock should be followed by space separated int constants all on one line.

bytecblock may be explicitly assembled. It will conflict with the assembler if there are any byte pseudo-ops but may be used if only explicit bytec references are used. bytecblock should be followed with byte constants all on one line, either 'encoding value' pairs (b64 AAA...) or 0x prefix or function-style values (base64(...)) or string literal values.

Labels and Branches

A label is defined by any string not some other op or keyword and ending in ':'. A label can be an argument (without the trailing ':') to a branch instruction.

Example:

int 1
bnz safe
err
safe:
pop

Encoding and Versioning

A program starts with a varuint declaring the version of the compiled code. Any addition, removal, or change of opcode behavior increments the version. For the most part opcode behavior should not change, addition will be infrequent (not likely more often than every three months and less often as the language matures), and removal should be very rare.

For version 1, subsequent bytes after the varuint are program opcode bytes. Future versions could put other metadata following the version identifier.

It is important to prevent newly-introduced transaction fields from breaking assumptions made by older versions of TEAL. If one of the transactions in a group will execute a TEAL program whose version predates a given field, that field must not be set anywhere in the transaction group, or the group will be rejected. For example, executing a TEAL version 1 program on a transaction with RekeyTo set to a nonzero address will cause the program to fail, regardless of the other contents of the program itself.

This requirement is enforced as follows:

• For every transaction, compute the earliest TEAL version that supports all the fields and and values in this transaction. For example, a transaction with a nonzero RekeyTo field will have version (at least) 2.

• Compute the largest version number across all the transactions in a group (of size 1 or more), call it maxVerNo. If any transaction in this group has a TEAL program with a version smaller than maxVerNo, then that TEAL program will fail.

Varuint

A 'proto-buf style variable length unsigned int' is encoded with 7 data bits per byte and the high bit is 1 if there is a following byte and 0 for the last byte. The lowest order 7 bits are in the first byte, followed by successively higher groups of 7 bits.

What TEAL Cannot Do

Design and implementation limitations to be aware of with various versions of TEAL.

• TEAL cannot create or change a transaction, only approve or reject.
• Stateless TEAL cannot lookup balances of Algos or other assets. (Standard transaction accounting will apply after TEAL has run and authorized a transaction. A TEAL-approved transaction could still be invalid by other accounting rules just as a standard signed transaction could be invalid. e.g. I can't give away money I don't have.)
• TEAL cannot access information in previous blocks. TEAL cannot access most information in other transactions in the current block. (TEAL can access fields of the transaction it is attached to and the transactions in an atomic transaction group.)
• TEAL cannot know exactly what round the current transaction will commit in (but it is somewhere in FirstValid through LastValid).
• TEAL cannot know exactly what time its transaction is committed.
• TEAL cannot loop prior to v4. In v3 and prior, the branch instructions bnz "branch if not zero", bz "branch if zero" and b "branch" can only branch forward so as to skip some code.
• Until v4, TEAL had no notion of subroutines (and therefore no recursion). As of v4, use callsub and retsub.
• TEAL cannot make indirect jumps. b, bz, bnz, and callsub jump to an immediately specified address, and retsub jumps to the address currently on the top of the call stack, which is manipulated only by previous calls to callsub.