Inline Assembly in Sway

While many users will never have to touch assembly language while writing sway code, it is a powerful tool that enables many advanced use-cases (e.g., optimizations, building libraries, etc).

ASM Block

In Sway, the way we use assembly inline is to declare an asm block like this:

asm() {...}

Declaring an asm block is similar to declaring a function. We can specify register names to operate on as arguments, we can perform operations within the block, and we can return a value. Here's an example showing what this might look like:

pub fn add_1(num: u32) -> u32 {
    asm(r1: num, r2) {
        add r2 r1 one;
        r2: u32

An asm block can only return a single register. If you really need to return more than one value, you can modify a tuple. Here's an example showing how you can implement this (u64, u64):


fn adder(a: u64, b: u64, c: u64) -> (u64, u64) {
    let empty_tuple = (0u64, 0u64);
    asm(output: empty_tuple, r1: a, r2: b, r3: c, r4, r5) {
        add r4 r1 r2; // add a & b and put the result in r4
        add r5 r2 r3; // add b & c and put the result in r5
        sw output r4 i0; // store the word in r4 in output + 0 words
        sw output r5 i1; // store the word in r5 in output + 1 word
        output: (u64, u64) // return both values

fn main() -> bool {
    let (first, second) = adder(1, 2, 3);
    assert(first == 3);
    assert(second == 5);

Note that this is contrived example meant to demonstrate the syntax; there's absolutely no need to use assembly to add integers!

Note that in the above example:

  • we initialized the register r1 with the value of num.
  • we declared a second register r2 (you may choose any register names you want).
  • we use the add opcode to add one to the value of r1 and store it in r2.
  • one is an example of a "reserved register", of which there are 16 in total. Further reading on this is linked below under "Semantics".
  • we return r2 & specify the return type as being u32 (the return type is u64 by default).

An important note is that the ji and jnei opcodes are not available within an asm block. For those looking to introduce control flow to asm blocks, it is recommended to surround smaller chunks of asm with control flow (if, else, and while).

For examples of assembly in action, check out the Sway standard library.

For a complete list of all instructions supported in the FuelVM: Instructions.

And to learn more about the FuelVM semantics: Semantics.