Functions: Function Definition and Invocation Operations

Summary

This CHIP introduces OP_DEFINE (0x89) and OP_INVOKE (0x8a), “enabling Bitcoin Cash contract bytecode to be factored into reusable functions.”

Motivation

The CHIP names three benefits:

• Smaller transactions. Removing duplicated bytecode makes room for finite field arithmetic, pairing-based cryptography, zero-knowledge proofs, homomorphic encryption and post-quantum cryptography.
• Privacy and operational security. Contracts can be designed to leak less about their security measures, assets and history.
• Auditability. Shared logic lives in one place, so contracts are shorter and easier to review.

What it specifies

• The VM gains a function table: a map from a function identifier to an immutable byte vector (the function body).
• OP_DEFINE pops an identifier (0 to 7 bytes) and then a body, and stores the body in the table. Redefining an existing identifier is an error, so functions are immutable.
• OP_INVOKE pops an identifier, saves the current execution position on the control stack, runs the stored body, then resumes after the OP_INVOKE.
• The number of functions and their size are bounded by existing VM limits (memory slots and the 10,000-byte stack element limit). Execution is charged by the VM Limits cost rules.

History

This CHIP replaced CHIP-2024-12 OP_EVAL, which proposed a single “evaluate this bytecode” opcode. Jason Dreyzehner withdrew OP_EVAL on 28 May 2025 and opened this topic with the new design. The Bitcoin Cash Node upgrade spec links the discussion thread “CHIP-2025-08 Functions (Takes 2 & 3)”, reflecting later revisions.

Current status

Activated on 15 May 2026 (version 2.0.2 was frozen for lock-in). The stakeholder table records 88 approvals, 1 disapproval (Coin Wallet) and 423 neutral; non-responses were counted as neutral.