12 KiB

Reference documentation on available primitives

Data Structures

For memory safety, the following data structures are opaque and only modified using functions described further down. I still find it useful to understand how they work under the hood.

  • Handles: addresses to objects allocated on the heap. They're augmented with book-keeping to guarantee memory-safety, and so cannot be stored in registers. See for details, but in brief:

    • You need addr values to access data they point to.
    • You can't store addr values in other types. They're temporary.
    • You can store handle values in other types.
    • To convert handle to addr, use lookup.
    • Reclaiming memory (currently unimplemented) invalidates all addr values.
  • Arrays: size-prefixed regions of memory containing multiple elements of a single type. Contents are preceded by 4 bytes (32 bits) containing the size of the array in bytes.

  • Slices: a pair of 32-bit addresses denoting a half-open

    `start`, `end`) interval to live memory with a consistent lifetime.
    Invariant: `start` <= `end`
  • Streams: strings prefixed by 32-bit write and read indexes that the next write or read goes to, respectively.

    • offset 0: write index
    • offset 4: read index
    • offset 8: size of array (in bytes)
    • offset 12: start of array data

    Invariant: 0 <= read <= write <= size

    By default, writes to a stream abort if it's full. Reads to a stream abort if it's empty.

  • Graphemes: a sequence of up to 4 utf-8 bytes that encode a single Unicode code-point.

  • Code-points: integer representing a Unicode character. Must be representable in 32 bits as utf-8; largest supported value is 0x10000.

Mu will let you convert between byte, code-point-utf8 and code-point using copy, and trust that you know what you're doing. Be aware that doing so is only correct for English/Latin characters, digits and symbols (ASCII).


The most useful functions from and later .mu files. Look in for their full type signatures.

  • abort: print a message in red on the bottom left of the screen and halt

assertions for tests

  • check: fails current test if given boolean is false (= 0).
  • check-not: fails current test if given boolean isn't false (!= 0).
  • check-ints-equal: fails current test if given ints aren't equal.
  • check-strings-equal: fails current test if given strings have different bytes.
  • check-stream-equal: fails current test if stream's data doesn't match string in its entirety. Ignores the stream's read index.
  • check-array-equal: fails if an array's elements don't match what's written in a whitespace-separated string.
  • check-next-stream-line-equal: fails current test if next line of stream until newline doesn't match string.


  • handle-equal?: checks if two handles point at the identical address. Does not compare payloads at their respective addresses.

  • array-equal?: checks if two arrays (of ints only for now) have identical elements.

  • string-equal?: compares two strings.

  • stream-data-equal?: compares a stream with a string.

  • next-stream-line-equal?: compares with string the next line in a stream, from read index to newline.

  • slice-empty?: checks if the start and end of a slice are equal.

  • slice-equal?: compares a slice with a string.

  • slice-starts-with?: compares the start of a slice with a string.

  • stream-full?: checks if a write to a stream would abort.

  • stream-empty?: checks if a read from a stream would abort.


  • populate: allocates space for n objects of the appropriate type.
  • copy-array: allocates enough space and writes out a copy of an array of some type.
  • slice-to-string: allocates space for an array of bytes and copies the slice into it.


  • populate-stream: allocates space in a stream for n objects of the appropriate type.

  • write-to-stream: writes arbitrary objects to a stream of the appropriate type.

  • read-from-stream: reads arbitrary objects from a stream of the appropriate type.

  • stream-to-array: allocates just enough space and writes out a stream's data between its read index (inclusive) and write index (exclusive).

  • clear-stream: resets everything in the stream to 0 (except its size).

  • rewind-stream: resets the read index of the stream to 0 without modifying its contents.

  • write: writes a string into a stream of bytes. Doesn't support streams of other types.

  • try-write: writes a string into a stream of bytes if possible. Doesn't support streams of other types.

  • write-stream: concatenates one stream into another.

  • write-slice: writes a slice into a stream of bytes.

  • append-byte: writes a single byte into a stream of bytes.

  • append-byte-hex: writes textual representation of lowest byte in hex to a stream of bytes. Does not write a '0x' prefix.

  • read-byte: reads a single byte from a stream of bytes.

  • read-code-point-utf8: reads a single unicode code-point-utf8 (up to 4 bytes) from a stream of bytes.

reading/writing hex representations of integers

  • write-int32-hex

  • hex-int?: checks if a slice contains an int in hex. Supports '0x' prefix.

  • parse-hex-int: reads int in hex from string

  • parse-hex-int-from-slice: reads int in hex from slice

  • parse-array-of-ints: reads in multiple ints in hex, separated by whitespace.

  • hex-digit?: checks if byte is in [0, 9] or [a, f] (lowercase only)

  • write-int32-decimal

  • parse-decimal-int

  • parse-decimal-int-from-slice

  • parse-decimal-int-from-stream

  • parse-array-of-decimal-ints

  • decimal-digit?: checks if a code-point-utf8 is in [0, 9]

printing to screen

pixel-on-real-screen draws a single pixel in one of 256 colors.

All text-mode screen primitives require a screen object, which can be either the real screen on the computer or a fake screen for tests.

The real screen on the Mu computer can currently display a subset of Unicode. There is only one font, and it's mostly fixed-width, with individual glyphs for code-points being either 8 or 16 pixels wide.

  • draw-code-point: draws a single code-point at a given coordinate, with given foreground and background colors. Returns the number of 8-pixel wide grid locations used (either 1 or 2).

  • render-code-point: like draw-code-point, but handles newlines and updates cursor position. Assumes text is printed left-to-right, top-to-bottom.

  • clear-screen

  • draw-text-rightward: draws a single line of text, stopping when it reaches either the provided bound or the right screen margin.

  • draw-stream-rightward

  • draw-text-rightward-over-full-screen: does not provide a bound.

  • draw-text-wrapping-right-then-down: draws multiple lines of text on screen with simplistic word-wrap (no hyphenation) within (x, y) bounds.

  • draw-stream-wrapping-right-then-down

  • draw-text-wrapping-right-then-down-over-full-screen

  • draw-int32-hex-wrapping-right-then-down

  • draw-int32-hex-wrapping-right-then-down-over-full-screen

  • draw-int32-decimal-wrapping-right-then-down

  • draw-int32-decimal-wrapping-right-then-down-over-full-screen

Similar primitives for writing text top-to-bottom, left-to-right.

  • draw-text-downward
  • draw-stream-downward
  • draw-text-wrapping-down-then-right
  • draw-stream-wrapping-down-then-right
  • draw-text-wrapping-down-then-right-over-full-screen
  • draw-int32-hex-wrapping-down-then-right
  • draw-int32-hex-wrapping-down-then-right-over-full-screen
  • draw-int32-decimal-wrapping-down-then-right
  • draw-int32-decimal-wrapping-down-then-right-over-full-screen

Screens remember the current cursor position. The following primitives automatically read and update the cursor position in various ways.

  • cursor-position

  • set-cursor-position

  • draw-code-point-at-cursor-over-full-screen: render-code-point at cursor position.

  • draw-cursor: highlights the current position of the cursor. Programs must pass in the code-point to draw at the cursor position, and are responsible for clearing the highlight when the cursor moves.

  • move-cursor-left, move-cursor-right, move-cursor-up, move-cursor-down. These primitives always silently fail if the desired movement would go out of screen bounds.

  • move-cursor-to-left-margin-of-next-line

  • move-cursor-rightward-and-downward: move cursor one code-point-utf8 to the right.

  • draw-text-rightward-from-cursor: truncate at some right margin.

  • draw-text-rightward-from-cursor-over-full-screen: truncate at right edge of screen.

  • draw-text-wrapping-right-then-down-from-cursor: wrap at some right margin.

  • draw-text-wrapping-right-then-down-from-cursor-over-full-screen: wrap at right edge of screen.

  • draw-int32-hex-wrapping-right-then-down-from-cursor

  • draw-int32-hex-wrapping-right-then-down-from-cursor-over-full-screen

  • draw-int32-decimal-wrapping-right-then-down-from-cursor

  • draw-int32-decimal-wrapping-right-then-down-from-cursor-over-full-screen

  • draw-text-wrapping-down-then-right-from-cursor: wrap at some bottom margin.

  • draw-text-wrapping-down-then-right-from-cursor-over-full-screen: wrap at bottom edge of screen.

Assertions for tests:

  • check-screen-row: compare a screen from the left margin of a given row index with a string. The row index counts downward from 0 at the top of the screen. String can be smaller or larger than a single row, and defines the region of interest. Strings longer than a row wrap around to the left margin of the next screen row. Currently assumes text is printed left-to-right on the screen.
  • check-screen-row-from: compare a fragment of a screen (left to write, top to bottom) starting from a given (x, y) coordinate with an expected string. Currently assumes text is printed left-to-right and top-to-bottom on the screen.
  • check-screen-row-in-color: like check-screen-row but:
    • also compares foreground color
    • ignores screen locations where the expected string contains spaces
  • check-screen-row-in-color-from
  • check-screen-row-in-background-color
  • check-screen-row-in-background-color-from
  • check-background-color-in-screen-row: unlike previous functions, this doesn't check screen contents, only background color. Ignores background color where expected string contains spaces, and compares background color where expected string does not contain spaces. Never compares the character at any screen location.
  • check-background-color-in-screen-row-from

pixel graphics

  • pixel: draw a single point at (x, y) with a given color between 0 and 255.
  • draw-line: between two points (x1, y1) and (x2, y2)
  • draw-horizontal-line
  • draw-vertical-line
  • draw-circle
  • draw-disc: takes an inner and outer radius
  • draw-monotonic-bezier: draw curved lines with a single control point. Doesn't support curves with "U-turns".


read-key reads a single key from the keyboard and returns it if it exists. Returns 0 if no key has been pressed. Currently only supports single-byte (ASCII) keys, which are identical to their code-point and code-point-utf8 representations.

read-line-from-keyboard reads keys from keyboard, echoes them to screen (with given fg/bg colors) and accumulates them in a stream until it encounters a newline.

read-mouse-event returns a recent change in x and y coordinate.

timer-counter returns a monotonically increasing counter with some fixed frequency. You can periodically poll it to check for intervals passing, but can't make assumptions about how much time has passed.

Mu doesn't currently support interrupt-based events.

We also don't yet have a fake keyboard.

persistent storage

read-ata-disk synchronously reads a whole number of sectors from a disk of persistent storage. The disk must follow the ATA specification with a 28-bit sector address. Each sector is 512 bytes. Therefore, Mu currently supports ATA hard disks of up to 128GB capacity.

Similarly, write-ata-disk synchronously writes a whole number of sectors to disk.

Mu doesn't currently support asynchronous transfers to or from a disk.