#assembly #assembler


A layer of syntactic sugar between Rust and inline assembly

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0.1.1 Nov 1, 2018
0.1.0 Nov 1, 2018

#10 in #assembler

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A layer of syntactic sugar between Rust and inline assembly

Rust currently has the asm! macro for writing inline ASM within a function defined in Rust. It uses the same basic format as GCC uses for its own inline ASM--and that format isn't the most ergonomic. Here's a small example, taken from the OSDev wiki and translated into Rust:

// Retrieves a value from memory in a different segment than the one currently being used (x86[-64])
unsafe fn farpeekl(segment_selector: u16, offset: *const u32) -> u32 {
    let ret: u32;
        push %fs
        mov $1, %fs
        mov %fs:($2), $0
        pop %fs
    "   : "=r"(ret) : "r"(segment_selector), "r"(offset)

(This example actually looks a little cleaner in my opinion than it does when written for GCC, but it could still use some work.)

The asm! macro is currently an unstable, nightly-only feature. From what I've seen, there are several reasons, but one of them is the syntax. It's too easy to forget the precise order of things (which come first: inputs or outputs?), and parts of it are needlessly redundant. Using "=r", "r", or "~r" means the register is, respectively, an output, an input, or clobbered, but the different types also have to be separated by colons. So using asm!, the programmer has to remember both ways to tell the compiler what it should expect to happen to each register.

This crate attempts to improve the syntax surrounding inline ASM so that it's both easier to read and easier to write without looking up the required format every time. It works by using a procedural macro (1) to overload Rust's let statements, making variables capable of storing information about how they'll be used in upcoming inline ASM blocks, and (2) to parse asm blocks that allow variables defined with the new syntax to be used directly in the ASM code.

Change Log

  • 0.1 - Initial release
  • 0.2 - Inner blocks are now supported.


To use this crate, add the following to Cargo.toml:

rusty-asm = "0.2.1"

Then reference the crate in your main source file and activate the features you'll need:

extern crate rusty_asm;
use rusty_asm::rusty_asm; // Because who wants to write `rusty_asm::rusty_asm!`?

Note that you'll still need a nightly compiler for this. rusty_asm doesn't make inline ASM stable.

Supported Features

The following features are available:

  • proc-macro: Causes proc-macro2 to act as a thin wrapper over proc_macro, including the parts that are still unstable. The benefit of this feature is that it allows rusty-asm to provide its own warnings, which should make debugging your own code easier.

Basic Syntax

In the place where you want to add some inline ASM, call rusty_asm! like so:

rusty_asm! {
    // (arbitrary Rust statements go here)

    asm (/* maybe some options in here */) {
        // (insert your ASM code here, in quotes)

    // (possibly some cleanup code here)

The contents of the asm block need to be a string literal to make sure that Rust's parser doesn't mess up the formatting. (Macros currently don't have access to whitespace information.) See the examples below for more specifics about how it should look.

Also, it's possible to have multiple asm blocks in the same rusty_asm! block, in case you want to reuse your bridge variables (see below).

Bridge Variables

A bridge variable is a variable that bridges the gap between Rust and ASM by incorporating the input/ouput/clobber information in its definition. They can only be defined inside rusty_asm! blocks, and because the macro makes a new scope, they are dropped when execution leaves those blocks (along with any other variables that are defined in the same scope). In order to define a bridge variable, you'll need to use one of three keywords that are only valid inside rusty_asm! blocks:

  • in
  • out
  • inout

Each of these keywords is used in a "let" statement to define a bridge variable. The exact syntax is as follows:

let [mut] : [:] in() [= ];
let [mut] : [:] out() [= ];
let [mut] : [:] inout() [= ];

The optional <type> is any Rust type, as far as the macro knows, but it should be something that makes sense to put in the appropriate register (e.g. usize, i8, etc. for a general-purpose integer register).

In addition, you can specify that you'll clobber a particular register (or that you'll clobber memory) with this syntax:


where <constraint> is either the name of a register (like "eax") or "memory".

These statements correspond to LLVM constraints in the following way:

// in, out, or inout:
// clobber

In each case, <new_constraint> is equivalent to <constraint> except that for the out and clobber keywords, the '=' or '~' is prepended to satisfy asm! and LLVM. So, for instance, if you write the constraint as "r", it will be automatically translated to "=r" or "~r" as needed before being given to the compiler. The inout keyword results in two new constraints: (1) the equivalent constraint for the out keyword (e.g. "=r") and (2) an input constraint that's tied to it (e.g. "0").

In order to let Rust know how to work with the bridge variables, rusty_asm! removes the new keywords and constraints during macro expansion, so as far as Rust knows, they're just ordinary variables.

The asm Block

When an asm block is encountered, it is converted directly into an asm! invocation, using all of the constraints that have been created thus far. The asm block's syntax is as follows:

asm [()] {

<options> is an optional comma-separated list of the options that would be after the 4th colon if asm! were being used, such as "volatile". <asm-code> is pure ASM code, enclosed in quotes, except that it can (and should) use the bridge variables that have been defined above the asm block.

In order to reference a bridge variable from inside an asm block, insert $<ident> into the code, where <ident> is the variable's identifier. As with the asm! macro, $$ encodes a literal dollar sign.

The rusty_asm! Block and Scope

The new macro puts its entire contents inside a new scope, so that any variables defined therein are dropped at the end. Their values can be moved to variables outside the macro's scope before it ends, using regular Rust code, if they need to be preserved. In addition, just like any of Rust's code blocks, this one has a return value that can be used by ending the block with an expression.

Also, as of version 0.2, the macro also correctly handles inner blocks, shadowing and dropping bridge variables just like Rust shadows and drops regular variables. That means you can now write code like this:

// Sends 1, 2, or 4 bytes at once to an ISA address (x86/x64).
unsafe fn poke_isa(port: u16, value: usize, bytes: u8) {
    rusty_asm! {
        let port: in("{dx}") = port;
        if bytes == 1 {
            let value: in("{al}") = value as u8;
            asm("volatile", "intel") {
                "out $port, $value"
        } else if bytes == 2 {
            let value: in("{ax}") = value as u16;
            asm("volatile", "intel") {
                "out $port, $value"
        } else {
            assert_eq!(bytes, 4);
            let value: in("{eax}") = value as u32;
            asm("volatile", "intel") {
                "out $port, $value"

Defining bridge variables in if let and while let constructions is still not supported, since Rust doesn't support explicit type annotations in them either, and I imagine the syntax would become overly complex.

Further Reading

There are too many platform-specific constraints and options that you can specify to list them all here. Follow these links for more information.

Usage Examples

Note that while all of these examples use x86 assembly, rusty_asm! should work with any assembly dialect that Rust supports (which probably means any dialect that LLVM supports).

// Disables interrupts on an x86 CPU.
unsafe fn disable_interrupts() {
    rusty_asm! {
        asm("volatile") { // This block has to be marked "volatile" to make sure the compiler, seeing
           "cli"          // no outputs and no clobbers, doesn't assume it does nothing and
        }                 // decide to "optimize" it away.
// Shifts the hexadecimal digits of `existing` up and puts `digit` in the resulting gap.
fn append_hex_digit(existing: usize, digit: u8) -> usize {
    assert!(digit < 0x10);
    unsafe {
        rusty_asm! {
            let mut big: inout("r") = existing;
            let little: in("r") = digit as usize;

            asm {"
                shll %4, $big
                orl $little, $big


assert_eq!(append_hex_digit(0, 0), 0);
assert_eq!(append_hex_digit(0, 0xf), 0xf);
assert_eq!(append_hex_digit(4, 2), 0x42);


The bridge variable declaration syntax is slightly more restrictive than that of general let statements in that it only allows an identifier after the let keyword, not an arbitrary pattern. So, for instance, this statement would not work:

rusty_asm! {
    let (a, b): (in("r"), out("r")) = (12, 14);
    /* ... */


~20K SLoC