Gas calculation basics
Specification Overview
The entire state of TVM consists of the five components:
stack
control registers
current continuation
current codepage
gas limits
Collectively these are called SCCCG.
The Gas component limits gas usage and сontains four signed 64-bit integers:
the remaining gas: gr
the current gas limit: gl
the maximal gas limit: gm
the gas credit: gc.
The following is always true:
gc is initialized by zero for internal messages, gr is initialized by gl + gc and gradually decreases, as the TVM runs. When gr becomes negative or if contract terminates with gc > 0, an out of gas exception is triggered.
For most primitives gas is calculated according to the following formula:
where b is the instruction length in bits. The same is true for Acki Nacki implementation.
Apart from integer constants, the following expressions may appear:
The total price of loading cells. Currently it is 100 gas units per cell.
The total price of creating new Cells from Builders. Currently it is 500 gas units.
Exception throwing. 50 gas units per exception.
Tuple gas price. 1 gas unit for every tuple element.
The usage of these additional integers remains unclear now. Research is underway.
Global gas limits
Global gas limits are values stored in the masterchain configuration contract. Global values are standard and do not change at contract deployment.
The following values are now used for shardchains:
Global_gas_price = 1000
Global_gas_limit = 1000000
Global_gas_credit = 10000
Global_block_gas_limit = 10000000
Gas-related TVM primitives
These is the list of official TVM primitives used for gas-related operations:
F800 — ACCEPT, sets current gas limit gl to its maximal allowed value gm, and resets the gas credit gc to zero, decreasing the value of gr by gc in the process. In other words, the current smart contract agrees to buy some gas to finish the current transaction. This action is required to process external messages, which bring no value (hence no gas) with themselves.
F801 — SETGASLIMIT (g – ), sets current gas limit gl to the minimum of g and gm, and resets the gas credit gc to zero. If the gas consumed so far (including the present instruction) exceeds the resulting value of gl , an (unhandled) out of gas exception is thrown before setting new gas limits. Notice that SETGASLIMIT with an argument g ≥ 2 63 − 1 is equivalent to ACCEPT.
F802 — BUYGAS (x – ), computes the amount of gas that can be bought for x nanograms, and sets gl accordingly in the same way as SETGASLIMIT.
F804 — GRAMTOGAS (x – g), computes the amount of gas that can be bought for x nanograms. If x is negative, returns 0. If g exceeds 2 63−1, it is replaced with this value.
F805 — GASTOGRAM (g – x), computes the price of g gas in nanograms.
F806–F80F — Reserved for gas-related primitives. These are yet to be released.
The general gas formula is the same as specified by TON specifications. Overall, Acki Nacki nodes operate in compliance with the specification.
For every executed primitive, the amount of gas is added to the virtual machine according to the specification formula. Gas value for every primitive is based on gr.
Gas initialization types
1. Calling contract from another contract:
An internal message with a balance value is received. In this case, the following formulas are applied to determine limits:
gm = min(account balance / gas price, global_gas_limit)
gl = min(message value / gas price, global_gas_limit)
gc = 0
gr = gc + gl
By default gas costs are allocated to the caller contract that triggers the transaction with a message. Accepting is also available for internal contracts. If ACCEPT is not called, gas is taken from the caller contract according to the message value. In other words, the message value defines the current limit. The message value determines the starting TVM gas limit.
So, to put it plain, if ACCEPT is not called, the message pays, if ACCEPT is used, additional gas can be bought by the target contract. This approach enables flexible contract design where either total gas is paid by the caller contract (but in this case it has to have enough gas at any moment of time) or the target contract also incurs costs.
2. Offchain contract call:
External messages do not carry balance values. In this case, the values are calculated according to the following formulas:
gm = min(account balance / gas price, global_gas_limit)
gl = 0
gc = min(gm, global_gas_credit)
gr = gc + gl
As external messages have no gas value, gas is credited to execute it. Target contracts have to cover costs by calling Accept to buy gas. If a contract returns an exception before the credit is given, no gas fee applies
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