What AVM Programs Cannot Do

Design and implementation limitations to be aware of with various Versions.

• Logic Signatures cannot lookup balances of ALGO or other assets. Standard transaction accounting will apply after the Logic Signature has authorized a transaction. A 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).

• Programs cannot access information in previous blocks.

• Programs cannot access information in other transactions in the current block, unless they are a part of the same transaction group.

• Logic Signatures cannot know exactly what round the current transaction will commit in (but it is somewhere in between transaction first and last valid round).

• Programs cannot know exactly what time its transaction is committed.

• Programs cannot loop prior to Version 4. In Versoin 3 and prior, the branch instructions bnz (“branch if not zero”), bz (“branch if zero”) and b (“branch”) can only branch forward.

• Until Version 4, the AVM had no notion of subroutines (and therefore no recursion). As of Version 4, they are available through the opcodes callsub and retsub.

• Programs 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 and retsub.