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Understanding the FuelVM Binary File ​

When you compile your Sway code using the forc build command, it generates a bytecode file. This binary file contains the compiled code that the Fuel Virtual Machine (FuelVM) will interpret and execute.

For example, consider the following smart contract:

ts
contract;

use std::b512::B512;

abi EchoValues {
    fn echo_u8(value: u8) -> u8;

    fn echo_str_8(value: str[8]) -> str[8];

    fn echo_str(value: str) -> str;

    fn echo_tuple(tuple: (u8, bool, u64)) -> (u8, bool, u64);

    fn echo_b512(input: B512) -> B512;

    fn echo_u64(value: u64) -> u64;
}

impl EchoValues for Contract {
    fn echo_u8(value: u8) -> u8 {
        value
    }

    // Avoid executing this function; it is currently unsupported.
    // Refer to: https://github.com/FuelLabs/sway/issues/5110 for more details.
    // The function is defined here to include the type 'str' in the ABI,
    // ensuring Typegen can accurately interpret it.
    // For further information, see: https://github.com/FuelLabs/fuels-ts/issues/1469
    fn echo_str(value: str) -> str {
        value
    }

    fn echo_str_8(value: str[8]) -> str[8] {
        value
    }

    fn echo_tuple(tuple: (u8, bool, u64)) -> (u8, bool, u64) {
        tuple
    }

    fn echo_b512(input: B512) -> B512 {
        input
    }
    fn echo_u64(value: u64) -> u64 {
        value
    }
}
See code in context

After running forc build, a binary file will be generated with the following content:

sh
$ cat out/debug/echo-values.bin
�GT]����]@`I]G�I@sH]G�I@sHr�{6�]D`J]C�%E]@`J$@Ͼ{RD�^�%

At first glance, the content appears unreadable. However, forc provides a helpful interpreter for this bytecode: the forc parse-bytecode command. This command takes the binary data and outputs the equivalent FuelVM assembly:

sh
$ forc parse-bytecode out/debug/echo-values.bin
half-word   byte   op                raw           notes
        0   0      JI(4)             90 00 00 04   jump to byte 16
        1   4      NOOP              47 00 00 00
        2   8      Undefined         00 00 00 00   data section offset lo (0)
        3   12     Undefined         00 00 00 34   data section offset hi (52)
        4   16     LW(63, 12, 1)     5d fc c0 01
        5   20     ADD(63, 63, 12)   10 ff f3 00
        6   24     LW(17, 6, 73)     5d 44 60 49
        7   28     LW(16, 63, 1)     5d 43 f0 01
        8   32     EQ(16, 17, 16)    13 41 14 00
        9   36     JNZI(16, 11)      73 40 00 0b   conditionally jump to byte 44
       10   40     RVRT(0)           36 00 00 00
       11   44     LW(16, 63, 0)     5d 43 f0 00
       12   48     RET(16)           24 40 00 00
       13   52     Undefined         00 00 00 00
       14   56     Undefined         00 00 00 01
       15   60     Undefined         00 00 00 00
       16   64     XOR(20, 27, 53)   21 51 bd 4b

When deploying your smart contract using the SDK, the binary file plays a crucial role. It is sent to the FuelVM in a transaction, allowing the FuelVM to interpret and execute your smart contract.