Month: November 2023

Microvium Async – Part 4
The job queue

Microvium Async – Part 4
The job queue

TL;DR: the job queue in Microvium is used just for executing async continuations. It uses only 2 bytes of idle memory and doesn’t require allocating any threads or special support from the host of any kind.

This is the last of a 4-part series on async-await in the Microvium JavaScript engine:

In the previous posts, I talked about how async-await is useful in embedded systems, how it uses continuation-passing style (CPS) at its core to achieve a tiny memory footprint, and how it creates promises when needed to interface between its native CPS protocol and JavaScript’s standard promise protocol. In this post, I’ll talk about the design choices behind the job queue in Microvium.

What is a job queue?

JavaScript is fundamentally single-threaded1, but job queues allow some level of multi-tasking without multithreading, by allowing work to be broken up into small chunks that get executed sequentially.

If you’re an embedded programmer, you may be familiar with the so-called “super-loop” architecture, where a single main function has an infinite loop and will check various things to see what work needs to be done on each cycle of the loop. Often these checks are hard-coded, but an alternative design that’s quite scalable is to have a queue of work that needs to be done, and the main loop just pull work off the queue and performs it.

In Microvium, the job queue is used solely to execute async continuations (which includes all promise subscribers). If foo is awaiting bar, then when bar completes, a job will be scheduled to continue foo where it left off2.

This behavior is required for Microvium to be ECMAScript spec compliant, but it also has the advantage of using less peak stack memory, since a cascade of continuations will each be executed in a fresh job when the call stack is empty, rather than executing on top of each other like a cascade of traditional callbacks.

The Microvium job queue is invisible to the host

An important factor in the design of Microvium is to make it as easy as possible to integrate into host firmware. One thing I was worried about with the introduction of a job queue is that it would complicate the C integration API, because it could require some way to pump the queue (to execute enqueued jobs). Either this might appear like an explicit API call to Microvium, like mvm_processJobs(), or it might require additional hooks in the porting layer to allow Microvium to hook into the operating system to set up a job thread, or hook into a global super-loop, etc.

None of these options are acceptable to me. I want the “getting started” process to be as seamless as possible for newcomers. Forcing newcomers to do this integration work is just too complicated in my opinion. It would require conveying new concepts like “what is a job queue?” and “when should or shouldn’t I pump the job queue?”, just to get started with “hello world”.

The solution I went with in the end, is that Microvium automatically pumps the job queue at the end of mvm_call. That is, when the host (C code) calls a JavaScript function (using mvm_call), if the JavaScript function creates any new jobs, those jobs will be executed before control returns to the host. The VM waits until the guest (JavaScript code) call-stack is empty, but before returning to the host, and then pumps the job queue until all jobs have been completed.

This approach makes it similar to explicit callback-based asynchrony, where callbacks would be executed inline and therefore before control returns to the host. The difference is that the continuations are executed a little bit later before returning to the host. The amount of time that mvm_call blocks the host for doesn’t change between using async-await and using explicit callbacks.

To put it another way, the program may be sitting idle, waiting for something to happen, such as a timer, I/O event, or user interaction. When something happens, the host might mvm_call the guest to let it know about the event. The way that Microvium is designed, the guest will then fully process the event and reach its next equilibrium state before mvm_call returns to the host.

Job queue structure

How does the job queue look under the hood?

I’m very hesitant to add things to Microvium that use more memory permanently. So the job queue is implemented as a single VM register, which is just a 2-byte slot. It has multiple states:

  1. No jobs: the register is empty.
  2. Single job: the register points to a closure.
  3. Multiple jobs: the register points to a linked list.

Single job

When there’s only a single job enqueued, the jobs register simply points to the job. A job is any function, but in particular it’s almost always going to be a closure since a closure is a function type that can encapsulate some dynamic state (async continuations being a special case of closures). The job queue mechanism doesn’t care what the state is, but since this is a job queue for async continuations, the job closure is almost always going to capture the following state:

  1. The continuation to execute.
  2. Whether the awaited operation passed or failed (isSuccess).
  3. The result or error value (result).

This is all the state required for the job to invoke the continuation with (isSuccess, result), as required by the CPS protocol I defined in the previous post.

Multiple jobs

The vast majority of the time, there will only be zero or one jobs in the job queue. This is because it’s relatively rare for multiple async functions to be awaiting the same promise. But Microvium does need to cater for the case of multiple jobs. When this happens, the jobs register won’t point to a single job, but to a linked list of jobs.

The job pointed to by the register is the first job to execute, but there is an interesting design choice here where the prev pointer of the first node points to the last node in the list (the back of the list), making the linked list into a cycle.

This is simply to facilitate appending to the job queue in O(1) time when adding new jobs, because we can access the last node directly through the first node.

Normally you can append to a doubly-linked list in O(1) time by maintaining a separate pointer to the last node, but I didn’t want to permanently dedicate a whole VM register for the relatively-rare and short-lived case where multiple jobs are waiting. Representing the jobs in a cycle is a neat way to get the best of both.

Note that jobs are removed from the cycle before they’re executed, so the cycle does not imply that jobs are executed multiple times or that we hold onto jobs after they’re executed. It’s merely an optimization that allows Microvium to access both the beginning and end of the queue via a single register in O(1) time.

A quick note about GC overhead. The diagram above with 12 jobs in the queue has 24 memory allocations. All 12 jobs are guaranteed to be executed before control returns to the host3, so they can be considered to be short-lived (allocated and freed within a single call to the VM). In a language like C, doing a malloc for every job would be a lot of overhead, especially when jobs are as small as “run the next piece of this async function”. But in Microvium, this overhead isn’t too bad because of the characteristics of the Microvium garbage-collected heap:

  1. Allocation headers are small — only 2 bytes per allocation.
  2. Unlike with malloc, there is no memory fragmentation because the heap is compacted.
  3. There is no overhead for collecting garbage memory4. Rather, Microvium occurs collection overhead on everything that isn’t garbage. Even hundreds of tiny temporary allocations can be swept away in one fell swoop.
  4. Allocation time is O(1) because the Microvium heap is compacted. Creating new allocations is roughly analogous to just bumping a “free pointer” forward by N bytes5.


The job queue is one of the less complicated parts of the implementation of async/await, but still important. In the end, I’m quite happy with how the design turned out, requiring only 2 bytes of memory (the register itself) when the feature isn’t being used, being lean in the hot path where only one job is enqueued and executed at a time, but still allowing an unlimited number of jobs while keeping O(1) insertion and removal. And this was achieved without impacting the interface between the host and the VM at all, so users of Microvium don’t have any additional integration work.

  1. There are some multi-threaded capabilities in modern JavaScript, but it’s still single threaded at heart and Microvium doesn’t support any multithreading primitives. 

  2. In the previous posts, I may have used the shorthand of saying that the bar “calls” foo’s continuation. But now you know that it’s more accurate to say that bar *schedules* foo’s continuation to be called on the job queue. 

  3. In the case of reentrant calls, the all jobs will be executed before control returns to the host from the outer-most mvm_call. 

  4. Garbage memory does create memory pressure which indirectly has overhead by increasing the chance of a collection cycle. But during a collection cycle, the GC moves all living allocations to a new region and then frees the old region in one go, so the individual dead allocations do not contribute at all to the amount of time it takes to perform a GC collection. 

  5. It’s not quite as efficient as just bumping a free pointer, since it needs to check if the heap actually has enough space, and potentially expand the heap if there isn’t. 

Microvium async – Part 3
Making promises

Microvium async – Part 3
Making promises

TL;DR: Microvium’s async/await uses continuation-passing style (CPS) at its core for efficiency, but automatically creates promises as well when required. It does so by defining a handshake protocol between the caller and callee to establish when promises are required. Promises in Microvium also pretty compact, but not as compact as raw CPS.

This is part 3 of a 4-part series on async-await in the Microvium JavaScript engine:

In the previous posts, I talked about why async-await is useful, and how a suspended function in Microvium can be as small as 6 bytes by using continuation-passing style (CPS) rather than promises at its core. In this post, I’ll talk more about promises and how they interact with Microvium’s CPS core.

What is a Promise?

Async functions in JavaScript return promises, which are objects that represent a future return value. This post will mostly assume that you’re already familiar with the concept of a Promise in JavaScript. Take a look at MDN for a more detailed explanation.

Microvium doesn’t support the full ECMAScript spec of Promise objects. Like everything in Microvium, it supports a useful subset of the spec:

  • Promises are objects which are instances of the Promise class and inherit from Promise.prototype.
  • You can manually construct promises with new Promise(...).
  • You can await promises.
  • Async functions return promises, if you observe the result (more on that later).
  • Async host1 functions will also automatically return promises (more on that later).

Notably, Microvium promises don’t have a then or catch method, but you could implement these yourself in user code by adding a function to Promise.prototype which is an async function that awaits the given promise and calls the handlers. Microvium also doesn’t have built-in functions like Promise.all, but these can similarly be implemented in user code if you need them. The general philosophy of Microvium has been to keep it small by omitting things that can be added in user code, since that gives the user control over the space trade-off.

It’s interesting to note then that Microvium doesn’t support thenables. Firstly, promises do not have a then method out of the box. Secondly, you cannot await something that isn’t a promise (e.g. a different object which happens to have a then method).

Memory structure

The memory structure of a promise object in Microvium is as follows, with 4 slots (a slot in Microvium is 2 bytes):

The next and __proto__ slots are common to all objects, and I discuss these more in Microvium has Classes.

The status slot is an enumeration indicating whether the promise is pending, resolved, or rejected.

To squeeze the size down as small as possible, the out slot is overloaded and can be any of the following:

  • A single subscriber (the slot points to a closure)
  • A list of subscribers (the slot points to an array)
  • No subscribers (the slot is empty)
  • The resolved value (if the promise is resolved)
  • The error value (if the promise is rejected)

With this design, a promise requires exactly 10 bytes of memory (4 slots plus the allocation header), which isn’t too bad. To put this in context by comparison, a single slot (e.g. a single variable) in the XS JavaScript engine is already 16 bytes.

An interesting thing to note is that there is no separate resolve and reject list of subscribers, and instead just one list of subscribers. My early designs of promises had separate resolve and reject subscribers, because this seemed natural given that JavaScript naturally has separate then and catch handlers. But after several iterations, I realized that it’s significantly more memory-efficient to combine these. So now, a subscriber is defined as a function which is called with arguments (isSuccess, result) so it’s able to handle both the resolve and reject cases. You may notice this is exactly the same signature as a CPS continuation function as I defined in the previous post, meaning a continuation can be directly subscribed to a promise.


So, we’ve discussed how Microvium’s async-await uses CPS under the hood (the previous post) and how promises look, but there’s a missing piece of the puzzle: how do promises interact with CPS? Before getting into the details, I need to lay some groundwork.

The most common way that you use an async function in JavaScript is to call it from another async function and await the result. For example:

const x = await myAsyncFunction(someArgs);

This syntactic form, where the result of a function call is immediately awaited, is what I call an await-call, and I’ll use this terminology in the rest of the post. Await-calling is the most efficient way of calling an async function in Microvium because the resulting Promise is not observable to the user code and is completely elided in favor of using the CPS protocol entirely.

CPS Protocol

As covered in the previous post, Microvium uses CPS under the hood as the foundation for async-await (see wikipedia’s Continuation-passing style). I’ve created what I call the “Microvium CPS protocol” as a handshake between a caller and callee to try negotiate the passing of a continuation callback. The handshake works as follows.

An async caller’s side of the the handshake:

  1. If a caller await-calls a callee, it will pass the caller’s continuation to the callee in a special callback VM register, to say “hey, I support CPS, so please call this callback when you’re done, if you support CPS as well”.
  2. When control returns back to the caller, the returned value is either a Promise or an elided promise2. The latter is a special sentinel value representing the absence of a promise. If it’s a promise, the caller will subscribe its continuation to the promise. If the promise is elided, it signals that the callee accepted the callback already and will call it when it’s finished, so there’s nothing else to do.

An async callee’s side of the handshake:

  1. An async callee is either called with a CPS callback or it isn’t (depending on how it was called). If there is a callback, the callee will remember it and invoke it later when it’s finished the async operation. The synchronous return value to the caller will be an elided promise to say “thanks for calling me; just to let you know that I also support CPS so I’ll call your callback when I’m done”.
  2. If no callback was passed, then the caller doesn’t support CPS, so the engine synthesizes a new Promise which it returns to the caller. When the async callee finishes running, it will invoke all the promise’s subscribers.

These callbacks are defined such that you call them with (isSuccess, result) when the callee is finished the async operation. For example callback(true, 42) to resolve to 42, or callback(false, new Error(...)) to reject to an error.

If both the caller and callee support CPS, this handshake completely elides the construction of any promises. This is the case covered in the previous post.

But this post is about promises! So let’s work through some of the cases where the promises aren’t elided.

Observing the result of an async function

Let’s take the code example from the previous post but say that instead of foo directly awaiting the call to bar, it stores the result in a promise, and then awaits the promise, as follows:

async function foo() {
  const promise = bar();
  await promise;

async function bar() {
  let x = 42;
  await baz();

Note: Like last time, the variable x here isn’t used but is just to show where variables would go in memory.

The key thing here is that we’re intentionally breaking CPS by making the promise observable, so we can see what happens.

The memory structure while bar is awaiting will look like this:

The memory structure looks quite similar to that showed in the previous post, but now with a promise sitting between foo continuation and bar continuation. Foo’s continuation is subscribed to the promise (foo will continue when the promise settles), and bar‘s “callback” is the promise. A promise is not a callable object, so the term “callback” is not quite correct here, but when bar completes, the engine will call of the subscribers of the promise. (Or more accurately, it will enqueue all of the subscribers in the job queue, which is the topic of the next post.) So the promise here is treated like a container of callbacks.

This structure comes about because when bar is called, it will notice that it wasn’t provided with a callback (because the call was not an await-call) and so it will create the promise. The await promise statement also isn’t an await-call (it’s not a call at all), but since the awaitee is a promise, foo will subscribe its continuation to that promise.

The end result here is that we’ve introduced another 10 bytes of memory overhead and inefficiency by making the promise observable, but we’ve gained some level of flexibility because we could pass the promise around and potentially have multiple subscribers if multiple async functions await it.

A host async function

We can gain some more insight into what’s happening here if we consider the case where bar is actually a host function implemented in C rather than JavaScript. I gave an example of this in the first post in this series. Since you’ve made it this far in the series, let’s also make this example a little more complete, using an mvm_Handle to correctly anchor the global callback variable to the GC.

mvm_Handle globalCallback;

mvm_TeError bar(
  mvm_VM* vm,
  mvm_HostFunctionID hostFunctionID,
  mvm_Value* pResult, // Synchronous return value
  mvm_Value* pArgs,
  uint8_t argCount
) {
  // Get a callback to call when the async operation is complete
  mvm_Value callback = mvm_asyncStart(vm, pResult);

  // Let's save the callback for later
  mvm_handleSet(&globalCallback, callback);

  /* ... */

  return MVM_E_SUCCESS;

An async host function is just a normal host function but which calls mvm_asyncStart. That is, there is no special way of registering or declaring a C function as “async”. The built-in function mvm_asyncStart encapsulates all the logic required for the callee side of the CPS handshake:

  1. If the caller await-called bar, it will have passed a callback, which mvm_asyncStart will return as the callback variable in this example. If so, it will also set *pResult to be an elided promise, so that the caller knows we accepted the callback.
  2. Otherwise, mvm_asyncStart will set *pResult to be a new Promise, and will return a callback closure which settles that Promise (resolves or rejects it) when called.

In our example, foo didn’t await-call bar, so this promise will be created and the returned callback will be a closure that encapsulates the logic to resolve or reject the promise:

I think there’s a beautiful elegance here in that mvm_asyncStart accepts as an argument a writeable reference to the synchronous return value (as a pointer) and returns essentially a writable reference to the asynchronous return value (as a callback).

One of the big design goals of the Microvium C API is to be easy to use, and I think this design of mvm_asyncStart really achieves this. In other JavaScript engines, an async host function would typically have to explicitly create a promise object and return it, and then later explicitly resolve or reject the promise, which is not easy. Microvium not only makes it easy, but also allows the engine to elide the promise altogether.

Side note: if foo did directly await-call bar, the promise would not be created, but the aforementioned callback closure would still exist as a safety and convenience layer between the host function and foo‘s continuation. It serves as a convenient, unified way for the host to tell the engine when the async operation is complete, and it encapsulates the complexity of scheduling the continuation on the job queue, as well as providing a safety layer in case the host accidentally calls the callback multiple times or with the wrong arguments.

Using the Promise constructor

The last, and least memory-efficient way to use async-await in Microvium, is to manually create promises using the Promise constructor, as in the following example:

async function foo() {
  const promise = bar();
  await promise;

function bar() {
  return new Promise((resolve, reject) => {
    let x = 42;
    // ...

Syntactically, this example looks pretty simple. But there are a lot of implicit objects created here:

  • The Promise object itself.
  • The resolve closure to resolve the promise.
  • The reject closure to reject the promise.
  • The executor closure (the arrow function passed to the Promise constructor in the above code) which captures both the resolve and reject closures.

So, while this looks syntactically like the lowest-level use of promises, it’s actually the most complicated behind the scenes. The suspended form of the “async” operation bar here has ballooned from the 8 bytes shown in the previous post to now 32 bytes!


Microvium async at its core uses a CPS protocol to maximize memory efficiency, requiring as little as 6 bytes for a suspended async function (and 2 additional bytes per variable), but at the boundaries between pure CPS and promise-based async, the introduction of promises and closures as protocol adapters brings additional overhead, with the worst case being where you create a promise manually.

The CPS handshake allows Microvium to dynamically decide when promises are required. The careful design of mvm_asyncStart allows even native host functions to participate in this handshake without having to worry about the details. This is important because async JS code needs to await something, and at some point the stack of awaits will ideally bottom-out at a natively-async host API. Microvium’s unique design allows the whole await stack to be defined in terms of pure CPS at least some of the time, without a single promise — turtles all the way down.

Even in the worst case, async/await in Microvium is still much more memory-efficient than other JavaScript engines. Engines like Elk, mJS, and Espruino, don’t support async-await at all — I’m not aware of any engine that even comes close to the size of Microvium which supports async-await. And in larger engines that do support it, they’re much more memory-hungry. Even a single closure in XS may take as much as 112 bytes, and I’ve measured a suspended async function to take about 950 bytes. In V8 on x64, I measure a suspended async function to take about 420 bytes.

Of course, be aware that Microvium doesn’t support the full spec, so it’s not an apples-to-apples comparison. But however you look at it, Microvium’s design of async-await makes it feasible to use it on a completely new class of embedded devices where it was not possible before.

Next up: Part 4: The Job Queue

  1. The term “host” refers the outer C program, such as the firmware, and the term “guest” refers to the inner JavaScript program. 

  2. Microvium doesn’t support using await on a value isn’t a promise or elided promise. It will just produce an error code in that case. This is part of the general philosophy of Microvium to support a useful subset of the spec. Here, awaiting a non-promise is likely a mistake. 

Microvium Async – Part 2
Callback-based async-await

Microvium Async – Part 2
Callback-based async-await

TL;DR: A suspended async function in Microvium can take as little as 6 bytes of RAM by using continuation-passing style (CPS) instead of promises, using dramatically less memory than other engines.

This is part 2 of a 4-part series on async-await in the Microvium JavaScript engine:

In the previous post, I introduced the concept of async-await for embedded systems and mentioned how I measured a minimal async function in Node.js to take about 420 bytes of RAM in its suspended state (that’s a lot!). In this post, I’ll cover some of the design decisions that lead to async functions taking as little as 6 bytes of RAM in Microvium, making them much more practical to use within the tiny memory constraints that the Microvium engine is targeting.

Closures in Microvium

Microvium async-await is built on top of closures, so let’s first recap how closures work in Microvium.

Let’s say we have the following JavaScript code:

function makeCounter() {
  let x = 0;

  function incCounter() {
    return ++x;

  return incCounter;

const myCounter = makeCounter();
myCounter(); // returns 1
myCounter(); // returns 2
myCounter(); // returns 3

Here, the myCounter function is a closure, meaning that it’s a heap-allocated function value that embeds within it the state of the variable x. This example looks like this in memory:

In this diagram, the arrows represent pointers.

When you call the myCounter closure, the engine executes the function bytecode that the closure points to, but the bytecode also has the ability to read and write the variables in the closure such as x.

The key takeaway to remember is this: a closure in Microvium is a heap-allocated type which is callable and embeds within it the state of the captured local variables as well as a pointer to some bytecode to run. When a closure is called, the engine in some sense “mounts” the closure itself as the current scope, so that bytecode instructions can read and write to the slots of the closure, including the variables and even the bytecode pointer itself (which is always the first slot of the closure).

For a more detailed explanation of closures in Microvium, see Microvium closures are tinyMicrovium closures can use less RAM than in C, and Microvium closure variable indexing.

Async-await on closures

In the previous post, we saw that a suspended async function in Microvium can be resumed simply by calling its continuation. Such a continuation is a function that you can call with arguments (isSuccess, result) that will resume the corresponding suspended async function with the result argument being used as the value of the currently-blocked await expression (or the exception to throw). As pointed out in the previous post, you can get fairly direct access to these continuations through the Microvium C API1, so that a C host2 function can call the async continuation when it finishes its asynchronous work.

Under the hood, a continuation is actually just represented in memory as a normal closure, but the bytecode pointer of the closure doesn’t point to the start of the function but somewhere in the middle (it points to the next instruction after the await). So, when called, the closure won’t execute from the beginning of the function but just continue where it left off. The continuation closure contains within it all the local state of the async function, so it also doubles as a kind of heap-allocated “stack frame”.

Let’s take a look at an example. Let’s say we have three async functions, foobar, and baz, each calling the next and awaiting it:

async function foo() {
  await bar();
  console.log('after bar');

async function bar() {
  await baz();
  console.log('after baz');

async function baz() {
  await qux();
  console.log('after qux');

When all 3 async functions are suspended (waiting for qux to finish), the heap memory structure in the Microvium VM looks like this, with 3 closures (the arrows here represent pointers):

Each continuation closure has a resume slot that points to the await point where control is currently suspended (in the bytecode). By definition, the first slot of a closure is the code that gets invoked when the closure is called.

Each continuation also has a callback slot that points to the caller’s continuation, which it will invoke when the async operation is done. For example, when bar is finished, it will automatically call foo continuation.

It’s worth emphasizing that foo continuation in the above diagram is not the same as the function foo. Each time foo is called, the engine allocates a new foo continuation, which doubles as both its continuation closure and a kind of heap-allocated stack frame since it embeds all the local variables of foo.

Each of the continuations here are 6 bytes in this example, because a memory slot in Microvium is 2 bytes and every heap allocation also has a 2-byte allocation header (header not shown in the diagram). An additional 2 bytes would be required for every local variable. For example, let’s say that bar has a local variable x:

Here, I haven’t shown the implementation of qux, but it could be another async function or it could be implemented in the host as a C function. In the latter case, the C qux function would call mvm_asyncStart which would essentially return the baz continuation continuation as a JavaScript function. I showed an example like this in the previous post and go into more detail in the next post.

Continuation-passing style (CPS)

This form of asynchrony, where callbacks are passed to a callee to invoke when it completes, is called continuation-passing style (CPS). See Wikipedia. Even though the source code doesn’t explicitly pass any callbacks, the engine is implicitly creating these callbacks and passing them behind the scenes using a hidden VM register. When a host function calls mvm_asyncStart, it’s essentially requesting explicit access to the automatically-generated continuation of the caller. This will be discussed in more detail in the next post.

The resume point is mutable

In a normal closure that represents a local JavaScript function like an arrow function, the bytecode pointer slot (the first slot) will never change. But in an async continuation, the bytecode pointer (resume point) is considered to be mutable. It points to wherever the async function is currently awaiting, so that the same continuation can be reused to continue the async function at different places throughout its execution.

For interest, if you’re following the analogy between between async continuations and traditional stack frames, you can think of the resume slot as being analogous to the program counter that is pushed to stack to remember where to continue executing when the frame is reactivated. The callback slot in this case is analogous to the pushed stack base pointer register (like EBP in x86), because it saves information about the location of the caller’s frame.

It’s not that simple

In this post, I’ve tried to express the design as simply as possible by eliding some of the things that make async-await complicated to implement. Here are some things I’ll just mention to give you a sense of it, but won’t go into detail:

  • Calculating the continuation size – a lot of static analysis goes into calculating the size of each async function closure, so that it’s big enough to include all the required state at any await point.
  • Exception handling – exceptions need to be propagated up the async call chain, and user-defined catch blocks need to be properly suspended and restored over await points.
  • Temporaries – in an expression like await foo() + await bar(), the engine needs to keep the result of the first await while it is waiting for the second await, so that it can add them together. This state also needs to be kept in the continuation.
  • Nested closures – if you have nested arrow functions inside an async function, they need to be able to access the variables in the continuation closure. When combined with async temporaries and Microvium’s waterfall closure indexing, the static analysis for this turns out to be quite a complicated problem.
  • The job queue – in JavaScript, async functions are resumed on the promise job queue, so there is some work behind the scenes to schedule the continuation on the job queue when you call it, rather than executing it directly (the topic of a later post).
  • Multiple entry points – the bytecode of async functions in Microvium is the first structure to allow pointers into the middle of the structure instead of just the beginning like most allocations, which comes with its own issues since now the same allocation in bytecode memory can have multiple distinct pointers referencing it. Pointer alignment constraints also get in the way here.

But, isn’t JavaScript Promise-based?

If you’re already familiar with JavaScript’s async-await, you might be surprised by the fact that I haven’t mentioned Promises at all in this post so far! Async functions in JavaScript are defined to return Promise objects, so how can I show a memory footprint for 3 awaiting async functions that doesn’t include a Promise of any sort?

Microvium indeed doesn’t create any promises for the the examples shown in this post. In these examples, the promises are immediately awaited or discarded and so not observable to the user code, so Microvium can elide them without breaking ECMA-262 spec compliance.

One might think of this as optimizing a special case, but there’s a better way of looking at it. In Microvium, CPS is the core of async-await: every time you have an async function, you’re creating one of these continuations. When promises are created, they’re merely wrappers or mediators for the CPS continuations. In the next post, I’ll go into more detail about how promises work.

As far as I’m aware, this approach is completely novel and sets Microvium apart from all other JavaScript engines today. This approach allows Microvium async functions to be more than an order of magnitude more memory-efficient than most other JavaScript engines today.

More importantly, this level of memory consumption is acceptable to me when it comes to small embedded devices. If you only have 16 kB of RAM, using 100 bytes or more for a single suspended async function is ridiculous, so as a firmware engineer I would almost never use the feature and instead might spend the extra time hand-crafting state machines. Microvium’s memory efficiency tips the scales completely, making async-await preferable to many other forms of expressing the same logic in either JavaScript or C, provided you’re not dealing with a CPU-bound problem.

P.S. I don’t see a fundamental reason why this approach can’t be used in any JavaScript engine. If you’re reading this and you’re on the dev team of another JS engine and you want to know more about what I’ve done here, feel free to reach out. And if you use this idea, please credit me for the idea or inspiration.

Next: Part 3: Making promises

  1. I use the term “fairly direct” because C actually has access to a wrapper function that provides some safety and guarantees, such as enforcing that the continuation cannot be invoked multiple times. There’ll be more detail on this in the next post. 

  2. The term “host” refers the outer C program, such as the firmware, and the term “guest” refers to the inner JavaScript program. 

Microvium Async – Part 1
Introduction to async-await for embedded systems

Microvium Async – Part 1
Introduction to async-await for embedded systems

TL;DR: The async/await feature of JavaScript is an alternative to multithreading, state machines, or callback-based code for managing long-running operations. Microvium’s design of async-await makes it one of the most memory-efficient and ergonomic ways of writing such code for small devices.

This is the first of a series of posts on the design of async-await in the Microvium JavaScript engine:

The target audience of this post is primarily embedded programmers who may or may not be very familiar with async/await and might be interested in how it could help with embedded software development. In this post, I’ll talk about what it is, how to use it in Microvium, and how it compares to some of the other techniques that solve a similar problem. In later posts, I’ll dive deeper into some of the inner workings.

What is Async/Await?

In the world of programming, there are concepts that fundamentally shift how we write and reason about code. Async/await is one of these game-changing concepts. Primarily seen in languages like JavaScript and C#, the async/await pattern has revolutionized how we deal with asynchronous operations and manage flow control.

Async/await in JavaScript allows individual functions to be suspended and resumed, similar to suspending and resuming a thread but much safer, more ergonomic and memory-efficient.

When you declare a JavaScript function as async, its variables and state will be stored on the heap instead of the stack, so that the function can be suspended mid-execution without blocking other functions on the stack. It gets suspended at await points in the code.

Here’s a hypothetical example async function that describes the sequence of operations one might use to connect to an HTTP server and send a message1:

async function sendMessageToServer(message) {
  console.log('Turning on the modem...');
  await powerOnModem();

  console.log('Connecting to server...');
  const connection = await connectToServer('');

  console.log('Sending message...');
  await connection.sendMessage(message);

  console.log('Message sent');
  await connection.disconnect();
  await powerOffModem();

At each await, the function sendMessageToServer is suspended. The engine transforms the remainder of the function into a callback that can be later invoked to resume execution, such as when powerOnModem completes. This callback is known as a continuation because when called, it will continue the async function where it left off.

Awaiting a C Function

If you’re a C programmer, the concept of async/await may be easiest to understand by looking at how you might implement powerOnModem in C, leveraging the builtin Microvium C function mvm_asyncStart which returns the aforementioned callback:

mvm_Value callback; // Global variable to hold the callback

mvm_TeError powerOnModem(mvm_VM* vm, ...) {
  callback = mvm_asyncStart(vm, ...);
  // ...

// Later, invoke the callback when the modem is powered on.
// This will continue `sendMessageToServer`.
mvm_call(vm, callback, ...);

Side note: this is not a complete example. It doesn’t show the glue code to make powerOnModem accessible to the JS code or how to prevent the callback from being freed by the garbage collector. Refer to the Microvium documentation for a more detailed example.

When Microvium encounters the statement await powerOnModem(), it suspends sendMessageToServer on the heap as a continuation callback. The call to mvm_asyncStart in powerOnModem returns this continuation callback so that the C code can later call it2.

The arguments that the callback expects are (isSuccess, result), where isSuccess must be true or false depending on whether the asynchronous operation logically succeeded or failed, and the result must be whatever you want the result of the await to be. In our example, the result of await powerOnModem() isn’t used, so it doesn’t matter. But if you were implementing connectToServer, you would want the result to be the connection, whatever that might be, since we can see that the async function uses the result as the connection:

const connection = await connectToServer('');

Awaiting a JavaScript Function

If instead of C, you wanted to implement powerOnModem in JavaScript, you don’t need to call mvm_asyncStart — you instead just declare the function as async, similar to before:

async function powerOnModem() {
  await modemPowerOnDetected();

By declaring powerOnModem to be async, Microvium automatically calls the continuation callback of the caller (sendMessageToServer in this case) when powerOnModem finishes executing completely. So the caller resumes automatically when the callee finishes.

In a sense, the async-await feature in Microvium transforms your async functions to callback-based functions under the hood. The return from one async function automatically calls the callback of the calling async function.

Await Anywhere

You can use await expressions pretty much anywhere in an async function. Examples include loops, conditionals, function call arguments — await can appear anywhere in an async function where an expression can appear, allowing the function to pause at that point and transforming the remainder of the function into a continuation.

This makes async-await a really convenient alternative to the complicated state machines you might otherwise use for this kind of logic in C. Managing counters, conditional paths, and nesting in a state machine can become a nightmare, but async-await can make these things trivial.

What if you don’t await?

So far, we’ve seen examples where an async caller is suspended while waiting for an async callee. But that’s not much different to the behavior you get with normal function calls: when you call a function normally, the caller is suspended on the stack until the callee completes. What makes async functions different?

The difference becomes most apparent when consider the alternative: not awaiting. For example, let’s say we call sendMessageToServer without awaiting:

function processMessage(message) {

In this hypothetical example, we’re not awaiting sendMessageToServer, so control will move on to saveMessageToFlash before sendMessageToServer has run to completion.

There’s no multithreading here. sendMessageToServer does not continue in the background on a thread. Rather, the call to sendMessageToServer will just return early, when sendMessageToServer reaches its first await point, which in this case is when it’s waiting for the modem to power on. So in this example, once the program is waiting for the modem to power on, it will start to save the message to flash.

This allows you to do event-driven multi-tasking without multithreading and without manually creating state machines or callbacks. Unlike with multithreading, you don’t need to worry about locking and synchronization, which makes the code much simpler and safer.

Memory efficiency

Microvium async functions are very lightweight compared to most other ways of achieving the same objective. Async functions like those shown in this post take just 6 bytes of RAM while they’re awaiting something3, plus an additional 2 bytes for each local variable.

6 bytes is tiny! If you wanted to use multithreading for this instead, the stack you dedicate to it might be hundreds of bytes. For example, on a Cortex-M MCU, this article says that a context switch uses at least 17 to 51 words (68 to 204 bytes) on the stack.

In an RTOS environment, such stacks are typically permanently allocated, whether the corresponding task is busy doing work or not. For example, a task for managing the modem might have a dedicated stack which is nearly empty most of the time while the modem is disconnected or idle, but requires enough space to handle bursts of activity for connecting and sending messages.

Microvium async-await gives the syntactic convenience of a dedicated thread while only using the minimum amount of memory required at any one time. Async frames that are no longer in use are freed by the garbage collector so the memory can be reused by other parts of the program.

I’ll note that the 6-byte figure in Microvium is not a characteristic of JavaScript but of the particular implementation and tradeoffs that Microvium makes. For comparison, async-await in node.js uses about 420 bytes per suspended async function, as measured on my desktop machine4. In Moddable’s XS JavaScript engine, which is designed for resource constrained environments like embedded devices, I measure a suspended async function to take about 950 bytes!

Microvium closures are also tiny, so you’ll get similar memory efficiency if you implement your asynchronous code in Microvium using callbacks, but async/await gives you much better syntactic convenience.

It would even be hard to beat this level of memory efficiency using hand-crafted state machines in C, not to mention being substantially more complicated to implement.


In the world of embedded systems, managing asynchronous operations can be a complex and often cumbersome task. The introduction of the async/await pattern to the Microvium JavaScript engine not only simplifies these asynchronous operations but also brings efficiency and elegance to the code. By allowing functions to be suspended and resumed at specific points, asynchronous code becomes more legible and maintainable.

Microvium’s implementation of async/await takes memory efficiency to a new level, allowing such code to be targeted to much smaller devices than was possible before. The next smallest JavaScript engine that supports async-await is an order of magnitude larger than Microvium5. Even Espruino, which is quite large and well used, doesn’t support async/await.

In the next episode I’ll pop the hood and show you how Microvium achieves this 6-byte target size. Part 2: Callback-based async-await

  1. All the functions in these examples are made up. Microvium as an engine doesn’t give you any of these functions out-of-the-box. 

  2. This is a simplification. The mvm_asyncStart function actually returns a wrapper for either the continuation or a promise value, and the wrapper provides extra safety as well as executing the continuation or promise subscribers from the job queue rather than synchronously. mvm_asyncStart also manages the wrapping of the async C function in a promise if required. More on that in later posts. 

  3. That’s 6 bytes including a 2-byte allocation header. 

  4. That’s measured on a 64-bit Windows machine 

  5. I’m referring to QuickJS and XS, which both require at least 10x the flash and RAM space of Microvium.