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RUNVM

The RUNVM instruction creates an isolated VM instance, allowing code execution while safely retrieving data such as stack state, registers, and gas consumption, ensuring the caller's state remains unaffected and allowing arbitrary code to run safely. This is helpful for v4-style plugins and similar use cases.

Fift syntaxStackDescription
flags RUNVMx_1 ... x_n n code [r] [c4] [c7] [g_l] [g_m] - x'_1 ... x'_m exit_code [c4'] [c5'] [g_c]Executes a child VM with the given code and stack values x_1 ... x_n. Returns the modified stack x'_1 ... x'_m along with an exit code.
Flags determine other arguments and return values. See details below.
RUNVMXx_1 ... x_n n code [r] [c4] [c7] [g_l] [g_m] flags - x'_1 ... x'_m exit_code [c4'] [c5'] [g_c]Same as RUNVM, but retrieves flags from the stack.

Flags are the same as for Fift RUNVMX; RUNVMX reads flags from the stack:

  • +1: sets c3 to code.
  • +2: pushes an implicit 0 before executing the code. Note: it only works when the +1 flag is set.
  • +4: takes persistent data c4 from the stack and returns its final value.
  • +8: takes the gas limit g_l from the stack and returns the consumed gas g_c.
  • +16: takes c7 (smart contract context) from the stack.
  • +32: returns the final value of c5 (actions).
  • +64: pops the hard gas limit g_m enabled by ACCEPT from the stack.
  • +128: enables "isolated gas consumption", meaning the child VM maintains a separate set of visited cells and a chksgn counter.
  • +256: pops an integer r and ensures exactly r values are returned from the top of the stack:
    • If RUNVM call succeeds and r is set, it returns r elements. If r is not set, it returns all available elements.
    • If RUNVM is successful but lacks elements on the stack, meaning the stack depth is less than r, it is treated as an exception in the child VM. The exit_code is set to -3, and exit_arg is set to 0, so 0 is returned as the only stack element.
    • If RUNVM fails with an exception, only one element is returned, exit_arg, which should not be confused with exit_code.
    • In the case of running out of gas, exit_code is set to -14, and exit_arg contains the amount of gas.

Gas cost:

  • 66 gas.
  • 1 gas for each stack element passed to the child VM (the first 32 elements are free).
  • 1 gas for each stack element returned from the child VM (the first 32 elements are free).

See also​

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