MicroW8 loads WebAssembly modules with a maximum size of 256kb. Your module needs to export a function fn upd() which will be called once per frame. After calling upd MicroW8 will display the 320x240 8bpp framebuffer located at offset 120 in memory with the 32bpp palette located at 0x13000.

The memory has to be imported as env memory and has a maximum size of 256kb (4 pages).

If the module exports a function called start, it will be called once after the module is loaded.

Memory map

00000-00040: user memory
00040-00044: time since module start in ms
00044-0004c: gamepad state
0004c-00050: number of frames since module start
00050-00070: sound data (synced to sound thread)
00070-00078: reserved
00078-12c78: frame buffer
12c78-12c7c: sound registers/work area base address (for sndGes function)
12c7c-13000: reserved
13000-13400: palette
13400-13c00: font
13c00-14000: reserved
14000-40000: user memory


All API functions are found in the env module.


These all do what you'd expect them to. All angles are in radians.

fn asin(x: f32) -> f32

Returns the arcsine of x.

fn acos(x: f32) -> f32

Returns the arccosine of x.

fn atan(f32) -> f32

Returns the arctangent of x.

fn atan2(y: f32, x: f32) -> f32

Returns the angle between the point (x, y) and the positive x-axis.

fn sin(angle: f32) -> f32

Returns the sine of angle.

fn tan(angle: f32) -> f32

Returns the tangent of angle.

fn cos(angle: f32) -> f32

Returns the cosine of angle.

fn exp(x: f32) -> f32

Returns e^x.

fn log(x: f32) -> f32

Returns the natural logarithmus of x. Ie. e^log(x) == x.

fn pow(x: f32, y: f32) -> f32

Returns x^y.

fn fmod(x: f32, y: f32) -> f32

Returns x modulo y, ie. x - floor(x / y) * y. This means the sign of the result of fmod is the same as y.


MicroW8 provides a pretty good PRNG, namely xorshift64*. It is initialized to a constant seed at each startup, so if you want to vary the random sequence you'll need to provide a seed yourself.

fn random() -> i32

Returns a (pseudo-)random 32bit integer.

fn randomf() -> f32

Returns a (pseudo-)random float equally distributed in [0,1).

fn randomSeed(seed: i32)

Seeds the PRNG with the given seed. The seed function is reasonably strong so that you can use


as a cheap random-access PRNG (aka noise function).


The default palette can be seen here. (Press Z on the keyboard to switch to palette.)

The palette can be changed by writing 32bit rgba colors to addresses 0x13000-0x13400.

The drawing functions are sub-pixel accurate where applicable (line, circle). Pixel centers lie halfway between integer coordinates. Ie. the top-left pixel covers the area 0,0 - 1,1, with 0.5,0.5 being the pixel center.

fn cls(color: i32)

Clears the screen to the given color index. Also sets the text cursor to 0, 0 and disables graphical text mode.

fn setPixel(x: i32, y: i32, color: i32)

Sets the pixel at x, y to the given color index.

fn getPixel(x: i32, y: i32) -> i32

Returns the color index at x, y. Returns 0 if the given coordinates are outside the screen.

fn hline(left: i32, right: i32, y: i32, color: i32)

Fills the horizontal line [left, right), y with the given color index.

fn rectangle(x: f32, y: f32, w: f32, h: f32, color: i32)

Fills the rectangle x,y - x+w,y+h with the given color index.

(Sets all pixels where the pixel center lies inside the rectangle.)

fn circle(cx: f32, cy: f32, radius: f32, color: i32)

Fills the circle at cx, cy and with radius with the given color index.

(Sets all pixels where the pixel center lies inside the circle.)

fn rectangleOutline(x: f32, y: f32, w: f32, h: f32, color: i32)

Draws a one pixel outline on the inside of the given rectangle.

(Draws the outermost pixels that are still inside the rectangle area.)

fn circleOutline(cx: f32, cy: f32, radius: f32, color: i32)

Draws a one pixel outline on the inside of the given circle.

(Draws the outermost pixels that are still inside the circle area.)

fn line(x1: f32, y1: f32, x2: f32, y2: f32, color: i32)

Draws a line from x1,y1 to x2,y2 in the given color index.

fn blitSprite(spriteData: i32, size: i32, x: i32, y: i32, control: i32)

Copies the pixel data at spriteData onto the screen at x, y. The size of the sprite is passed as width | (height << 16). If the height is given as 0, the sprite is is treated as square (width x width).

The control parameter controls masking and flipping of the sprite:

fn grabSprite(spriteData: i32, size: i32, x: i32, y: i32, control: i32)

Copies the pixel data on the screen at x, y to spriteData. Parameters are exactly the same as blitSprite.


MicroW8 provides input from a gamepad with one D-Pad and 4 buttons, or a keyboard emulation thereof.

The buttons are numbered


In addition to using the API functions below, the gamepad state can also be read as a bitfield of pressed buttons at address 0x44. 0x48 holds the buttons that were pressed last frame.

fn isButtonPressed(btn: i32) -> i32

Returns whether the buttons with the given index is pressed this frame.

fn isButtonTriggered(btn: i32) -> i32

Returns whether the given button is newly pressed this frame.

fn time() -> f32

Returns the time in seconds since the start of the cart.

The integer time in milliseconds can also be read at address 0x40.

Text output

The default font can be seen here.

The font can be changed by writing 1bpp 8x8 characters to addresses 0x13400-0x13c00.

All text printing is done at the cursor position, which is advanced after printing each character. The cursor is not visible.

Text printing can operate in two modes - normal and graphics. After startup and after cls() normal mode is active.

Normal mode

In normal mode, text printing is constrained to an 8x8 character grid. Setting the cursor position to 2,3 will start printing at pixel coordinates 16,24.

When printing characters, the full 8x8 pixels are painted with the text and background colors according to the character graphics in the font.

When moving/printing past the left or right border the cursor will automatically wrap to the previous/next line. When moving/printing past the upper/lower border, the screen will be scrolled down/up 8 pixels, filling the fresh line with the background color.

Graphics mode

In graphics mode, text can be printed to any pixel position, the cursor position is set in pixel coordinates.

When printing characters only the foreground pixels are set, the background is "transparent".

Moving/printing past any border does not cause any special operation, the cursor just goes off-screen.

Text scale

An integer text scale factor in the range 1x-16x can be set with control char 30. An attempt to set a scale outside that range will reset the scale to 1x.

After startup and cls the scale is initialized to 1x.

Control chars

Characters 0-31 are control characters and don't print by default. They take the next 0-2 following characters as parameters. Avoid the reserved control chars, they are currently NOPs but their behavior can change in later MicroW8 versions.

1charPrint char (including control chars)
4-Switch to normal mode, reset cursor to 0,0
5-Switch to graphics mode
6-Switch output to (debug) console
7-Bell / trigger sound channel 0
8-Move cursor left
9-Move cursor right
10-Move cursor down
11-Move cursor up
12-do cls(background_color)
13-Move cursor to the left border
14colorSet the background color
15colorSet the text color
24-Swap text/background colors
30scaleSet text scale (1-16)
31x, ySet cursor position (*)

(*) In graphics mode, the x coordinate is doubled when using control char 31 to be able to cover the whole screen with one byte.

Debug output

Control code 6 switches all text output (except codes 4 and 5 to switch output back to the screen) to the console. Where exactly this ends up (if at all) is an implementation detail of the runtimes. The native dev-runtime writes the debug output to stdout, the web runtime to the debug console using console.log. Both implementations buffer the output until they encounter a newline character (10) in the output stream.

There may be future runtimes that ignore the debug output completely.

In CurlyWas, a simple way to log some value might look like this:

printChar('\06V: '); // switch to console out, print some prefix
printChar('\n\4'); // newline and switch back to screen

fn printChar(char: i32)

Prints the character in the lower 8 bits of char. If the upper 24 bits are non-zero, right-shifts char by 8 bits and loops back to the beginning.

fn printString(ptr: i32)

Prints the zero-terminated string at the given memory address.

fn printInt(num: i32)

Prints num as a signed decimal number.

fn setTextColor(color: i32)

Sets the text color.

fn setBackgroundColor(color: i32)

Sets the background color.

fn setCursorPosition(x: i32, y: i32)

Sets the cursor position. In normal mode x and y are multiplied by 8 to get the pixel position, in graphics mode they are used as is.


Low level operation

MicroW8 actually runs two instances of your module. On the first instance, it calls upd and displays the framebuffer found in its memory. On the second instance, it calls snd instead with an incrementing sample index and expects that function to return sound samples for the left and right channel at 44100 Hz. If your module does not export a snd function, it calls the api function sndGes instead.

As the only means of communication, 32 bytes starting at address 0x00050 are copied from main to sound memory after upd returns.

By default, the sndGes function generates sound based on the 32 bytes at 0x00050. This means that in the default configuration those 32 bytes act as sound registers. See the sndGes function for the meaning of those registers.

export fn snd(sampleIndex: i32) -> f32

If the module exports a snd function, it is called 88200 times per second to provide PCM sample data for playback (44.1kHz stereo). The sampleIndex will start at 0 and increments by 1 for each call. On even indices the function is expected to return a sample value for the left channel, on odd indices for the right channel.

fn playNote(channel: i32, note: i32)

Triggers a note (1-127) on the given channel (0-3). Notes are semitones with 69 being A4 (same as MIDI). A note value of 0 stops the sound playing on that channel. A note value 128-255 will trigger note-128 and immediately stop it (playing attack+release parts of envelope).

This function assumes the default setup, with the sndGes registers located at 0x00050.

fn sndGes(sampleIndex: i32) -> f32

This implements a sound chip, generating sound based on 32 bytes of sound registers.

The spec of this sound chip are:

This function requires 1024 bytes of working memory, the first 32 bytes of which are interpreted as the sound registers. The base address of its working memory can be configured by writing the address to 0x12c78. It defaults to 0x00050.

Here is a short description of the 32 sound registers.

00 - CTRL0
06 - CTRL1
0c - CTRL2
12 - CTRL3
  | 7  6 |   5  |   4  |  3  2  |    1    |    0    |
  | wave | ring | wide | filter | trigger | note on |

  note on: stay in decay/sustain part of envelope
  trigger: the attack part of the envlope is triggered when either this changes
           or note on is changed from 0 to 1.
  filter : 0 - no filter
           1 - fixed 6db 1-pole filter with cutoff two octaves above note
           2 - programmable filter 0
           3 - programmable filter 1
  wide   : use wide stereo panning
  ring   : ring modulate with triangle wave at frequency of previous channel
  wave   : 0 - rectangle
           1 - saw
           2 - triangle
           3 - noise

01 - PULS0
07 - PULS1
0d - PULS2
13 - PULS3
  Pulse width 0-255, with 0 being the plain version of each wave form.
  rectangle - 50%-100% pulse width
  saw       - inverts 0%-100% of the saw wave form around the center
  triangle  - morphs into an octave up triangle wave
  noise     - blends into a decimated saw wave (just try it out)

02 - FINE0
08 - FINE1
0e - FINE2
14 - FINE3
  Fractional note

03 - NOTE0
09 - NOTE1
0f - NOTE2
15 - NOTE3
  Note, 69 = A4

04 - ENVA0
0a - ENVA1
10 - ENVA2
16 - ENVA3
  | 7 6 5 4 | 3 2 1 0 |
  | decay   | attack  |

05 - ENVB0
0b - ENVB1
11 - ENVB2
17 - ENVB3
  | 7 6 5 4 | 3 2 1 0 |
  | release | sustain |

18 - VO01
  | 7 6 5 4  | 3 2 1 0  |
  | volume 1 | volume 0 |

19 - VO23
  | 7 6 5 4  | 3 2 1 0  |
  | volume 3 | volume 2 |

1a - FCTR0
1b - FCTR1
  | 7 6 5 4   | 3 | 2    | 1    | 0   |
  | resonance | 0 | band | high | low |

1c - FFIN0
1e - FFIN1
  cutoff frequency - fractional note

1d - FNOT0
1f - FNOT1
  cutoff frequency - note

The uw8 tool

The uw8 tool included in the MicroW8 download includes a number of useful tools for developing MicroW8 carts. For small productions written in wat or CurlyWas you don't need anything apart from uw8 and a text editor of your choice.

uw8 run


uw8 run [<options>] <file>

Runs <file> which can be a binary WebAssembly module, an .uw8 cart, a wat (WebAssembly text format) source file or a CurlyWas source file.


when using the native runtime:

Note that the cpu-only window does not support fullscreen nor upscale filters.

Unless --no-gpu is given, uw8 will first try to open a gpu accelerated window, falling back to the old cpu-only window if that fails. Therefore you should rarely need to manually pass --no-gpu. If you prefer the old pixel doubling look to the now default crt filter, you can just pass --filter nearest or --filter 1.

The upscale filter options are:

1, nearest              : Anti-aliased nearest filter
2, fast_crt             : Very simple, cheap crt filter, not very good below a window size of 960x720
3, ss_crt               : Super sampled crt filter, a little more demanding on the GPU but scales well to smaller window sizes
4, chromatic_crt        : Variant of fast_crt with a slight offset of the three color dots of a pixel, still pretty cheap
5, auto_crt (default)   : ss_crt below 960x720, chromatic_crt otherwise

You can switch the upscale filter at any time using the keys 1-5. You can toggle fullscreen with F.

uw8 pack


uw8 pack [<options>] <infile> <outfile>

Packs the WebAssembly module or text file, or CurlyWas source file into a .uw8 cart.


uw8 unpack


uw8 unpack <infile> <outfile>

Unpacks a MicroW8 module into a standard WebAssembly module.

uw8 compile


uw8 compile [<options>] <infile> <outfile>

Compiles a CurlyWas source file to a standard WebAssembly module. Most useful together with the --debug option to get a module that works well in the Chrome debugger.


uw8 filter-exports


uw8 filter-exports <infile> <outfile>

Reads a binary WebAssembly module, removes all exports not used by the MicroW8 platform + everything that is unreachable without those exports and writes the resulting module to outfile.

When compiling C code (or Rust, zig or others) to WebAssembly, you end up with a few exported global variables that are used for managing the heap and C stack, even if the code doesn't actually use those features. You can use this command to automatically remove them and gain a few bytes. See the C, Rust and zig examples in the MicroW8 repository.

Other useful tools

The Web Assembly Binary Toolkit includes a few useful tools, eg. wat2wasm to compile the WebAssemby text format to binary wasm and wasm2wat to disassemble wasm binaries.

Binaryen includes wasm-opt which enable additional optimizations over what LLVM (the backend that is used by most compilers that target WebAssembly) can do.


The classical distribution option is just to put the .uw8 cart into a zip file, let people run it themselves, either in the uw8 tool or in the web runtime.

If you want to go this way, you might consider including microw8.html in your download. It's specifically designed to be a small (~10KB at the moment), self-contained HTML file for just this reason. That way, anyone who has downloaded you production can run it, even when offline, provided they have a modern web browser at hand. Also, should future versions of MicroW8 ever introduce any kind of incompatibilities, they'd still have a compatible version right there without hunting arround for an old version.

For small productions (<= 1024 bytes), when you load them in the web runtime, the URL is automatically updated to include the cart as base64 encoded data. You can just give that URL to others for them to run your prod.

url parameter

Another option is to put the cart on a webserver and add #url=url/to/the/cart.uw8 to the end of the web runtime URL. (Like this)

If the cart and the web runtime are on different domains, you'll have to make sure that CORS header are enabled for the cart, otherwise the web runtime won't be able to load it.

Feel free to put the web runtime on your own server if it makes sense to you, its license allows you to do anything you want with it.

.html + .uw8

At startup the web runtime will try to load a cart in the same directory as the .html file. If the URL of the web runtime ends in .html it will try to load a cart with the same name and the extension .uw8. If the URL of the web runtime ends in a / it will try to load a cart.uw8 at that location.

So, you could for example serve the web runtime as and the cart as and send people to the HTML page to run the cart. Or you could put them up as and and send people to

If a cart is found and loaded in this way, the load button is hidden.

The above .html + .uw8 option works great on as well. Put these two files into a zip archive:

Upload the zip file to and make sure to set the embedded viewport size to exactly (!) 640x480 pixel. At that exact size the web runtime hides everything except for the MicroW8 screen.

If instead you actually want to display the border around the screen and the byte size you can try a size of about 720x620.

See here for an example upload.

.uw8 format

The first byte of the file specifies the format version:

Format version 00:

This file is simply a standard WebAssembly module

Format version 01:

The rest of this file is the same as a WebAssembly module with the 8 byte header removed. This module can leave out sections which are then taken from a base module provided by MicroW8.

You can generate this base module yourself using uw8-tool. As a quick summary, it provides all function types with up to 5 parameters (i32 or f32) where the f32 parameters always preceed the i32 parameters. Then it includes all imports that MicroW8 provides, a function section with a single function of type () -> void and an export section that exports the first function in the file under the name upd.

Format version 02:

Same as version 01 except everything after the first byte is compressed using a custom LZ compression scheme.

The web runtime

Load carts into the web runtime either by using the "Load cart..." button, or by dragging the file onto the screen area.


For input, you can either use a standard gamepad or keyboard. On a keyboard use the arrow keys and the keys Z, X, A and S to emulate the A, B, X and Y buttons.

Video recording

Press F10 to start recording, press again to stop. Then a download dialog will open for the video file. The file might miss some metadata needed to load in some video editing tools, in that case you can run it through ffmpeg like this `ffmpeg -i IN_NAME.webm -c copy -o OUT_NAME.webm to fix it up.

To convert it to 1280x720, for example for a lovebyte upload, you can use:

ffmpeg -i IN.webm -vf "scale=960:720:flags=neighbor,pad=1280:720:160:0" -r 60 OUT.mp4


Pressing F9 opens a download dialog with a screenshot.

Devkit mode

Append #devkit to the web runtime url in order to switch to devkit mode. In devkit mode, standard web assembly modules are loaded bypassing the loader, removing all size restrictions. At the same time, the memory limit is increased to 1GB.