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Versioning and Encoding

Versioning

To preserve existing semantics for previously written programs, AVM code is versioned.

When new opcodes are added or existing behavior is changed, a new Version is introduced. Programs carrying old versions are executed with their original semantics.

In the AVM bytecode, the Version is an incrementing integer, denoted in the program as vX (where X is the version number, see Pragma directive section).

The AVM current Version is: \( 12 \).

For further details about the available opcodes per version, refer to the AVM Opcodes Specification.

A compiled program starts with a varuint declaring the Version of the compiled code.

Any addition, removal, or change of opcode behavior increments the Version.

Existing opcode behavior SHOULD NOT change. Opcode additions will be infrequent, and opcode removals SHOULD be very rare.

For Version \( 1 \), subsequent bytes after the varuint are program opcode bytes.

Future versions MAY put other metadata following the version identifier.

Newly introduced transaction types and fields MUST NOT break assumptions made by programs written before they existed.

If one of the transactions in a group executes a program whose Version predates a transaction type or field that can violate expectations, that transaction type or field MUST NOT be used anywhere in the transaction group.

📎 EXAMPLE

A Version \( 1 \) program included in a transaction group that includes an application call transaction or a non-zero rekey-to field will fail regardless of the program itself.

This requirement is enforced as follows:

  1. For every transaction in the group, compute the earliest Version that supports all the fields and values in this transaction.

  2. Compute maxVerNo, the largest Version number across all the transactions in a group (of size 1 or more).

  3. If any transaction in this group has a program with a version smaller than maxVerNo, then that program will fail.

In addition, Applications MUST be Version 4 or greater to be called in an inner transaction.

Encoding

Trailing versioning information on each program, as well as intcblock and bytecblock instructions, are handled using proto-buf style variable length unsigned ints.

These are encoded with packets of \( 7 \) data bits per byte. For every packet, the high data bit is a \( 1 \) if there is a byte after the current one, and \( 0 \) for the last byte of the sequence. The lowest order \( 7 \) bits are in the first byte, followed by successively higher groups of 7 bits.