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A guided tour of Mu on x86 computers

by Kartik Agaram and sejo

Mu shrinks all the software in a computer until it can (in principle) fit in a single head. Sensible error messages with as little code as possible, starting all the way from your (x86) processor's instruction set. Everything easy to change to your needs (habitable), everything easy to check up on (auditable).

This page is a guided tour through Mu's Readme and reference documentation. We'll start out really slow and gradually accelerate as we build up skills. By the end of it all, I hope you'll be able to program your processor to run some small graphical programs. The programs will only use a small subset of your computer's capabilities; there's still a lot I don't know and therefore cannot teach. However, the programs will run on a real processor without needing any other intermediary software.

Prerequisites

You will need:

A computer with an x86 processor running Linux. Mu is designed to eventually escape Linux, but still needs some host environment for now. Other platforms will also do (BSD, Mac OS, Windows Subsystem for Linux), but be warned that things will be much (~20x) slower.
Some fluency in typing commands at the terminal and interpreting their output.
Fluency with some text editor. Things like undo, copying and pasting text, and saving work in files. A little experience programming in some language is also handy.
Git for version control.
QEMU for emulating a processor without Linux.
Basic knowledge of number bases, and the difference between decimal and hexadecimal numbers.
Basic knowledge of the inside of a processor, such as the difference between a small number of registers and a large number of locations in memory.

If you have trouble with any of this, I'm always nearby and available to answer questions. The prerequisites are just things I haven't figured out how to explain yet. In particular, I want this page to be accessible to people who are in the process of learning programming, but I'm sure it isn't good enough yet for that. Ask me questions and help me improve it.

Task 1: getting started

Read the first half of the Readme (stop before the section on “Syntax”). Can you figure out the commands to prepare Mu on your computer?

Open a terminal and run the following commands to prepare Mu on your computer:

git clone https://github.com/akkartik/mu
cd mu

Run a small program to start:

./translate tutorial/task1.mu
qemu-system-i386 code.img

If you aren't on Linux (or Windows Subsystem for Linux), the command for creating code.img will be slightly different:

./translate_emulated tutorial/task1.mu
qemu-system-i386 code.img

Either way, you should see this:

screenshot of hello world on the Mu computer

If you have any trouble at this point, don't waste any time thinking about it. Just get in touch.

(You can look at tutorial/task1.mu at this point if you like. It's just 3 lines long. But don't worry if it doesn't make much sense.)

Task 2: running automated tests

Here's a new program to run:

./translate tutorial/task2.mu
qemu-system-i386 code.img

(As before, I'll leave you to substitute translate with translate_emulated if you're not on Linux.)

This time the screen will look like this:

screenshot of failing test on the Mu computer

Each of the dots represents an automated test, a little self-contained and automated program run and its results verified. Mu comes with a lot of tests (every function starting with 'test-' is a test), and it always runs all tests on boot before it runs any program. You may have missed the dots when you ran Task 1 because there were no failures. They were printed on the screen and then immediately erased. In Task 2, however, we've deliberately included a failing test. When any tests fail, Mu will immediately stop, showing you messages from failing tests and implicitly asking you to first fix them. A lot of learning programming is about building a sense for when you need to write tests for the code you write.

(Don't worry just yet about what the message in the middle of all the dots means.)

Task 3: configure your text editor

So far we haven't used a text editor yet, but we will now be starting to do so. Before we do, it's worth spending a little bit of time setting your preferred editor up to be a little more ergonomic. Mu comes with syntax highlighting settings for a few common text editors in the editor/ sub-directory. If you don't see your text editor there, or if you don't know what to do with those files, get in touch! Here's what my editor (Vim) looks like with these settings on the program of Task 1:

Vim text editor rendering some colors in a Mu program

It's particularly useful to highlight comments which the computer ignores (everything on a line after a # character) and strings within "" double quotes.

Task 4: your first Mu statement

Mu is a statement-oriented language. Most statements translate into a single instruction to the x86 processor. Quickly read the first two sections of the Mu reference (about functions and variables) to learn a little bit about it. It's ok if it doesn't all make sense just yet. We'll reread it later.

Here's a skeleton of a Mu function that's missing a single statement.

fn the-answer -> _/eax: int {
  var result/eax: int <- copy 0
  # insert your statement below {

  # }
  return result
}

Try running it now:

./translate tutorial/task4.mu
qemu-system-i386 code.img

(As before, I'll leave you to substitute translate with translate_emulated if you're not on Linux.)

You should see a failing test that looks something like this:

screenshot of the initial (failing) state of task 4

Open tutorial/task4.mu in your text editor. Think about how to add a line between the {} lines to make the-answer return 42. Rerun the above commands. You'll know you got it right when all the tests pass, i.e. when the rows of dots and text above are replaced by an empty screen.

Don't be afraid to run the above commands over and over again as you try out different solutions. Here's a way to run them together so they're easy to repeat.

./translate tutorial/task4.mu  &&  qemu-system-i386 code.img

In programming there is no penalty for making mistakes, and once you arrive at the correct solution you have it forever. As always, feel free to ping me and ask questions or share your experience.

Mu statements can have outputs on the left (before the <-) and inouts (either inputs or outputs) on the right, after the instruction name. The order matters.

One gotcha to keep in mind is that numbers in Mu must always be in hexadecimal notation, starting with 0x. Use a calculator on your computer or phone to convert 42 to hexadecimal, or this page on your web browser.

Task 5: variables in registers, variables in memory

We'll now practice managing one variable in a register (like last time) and a second one in memory. To prepare for this, reread the first two sections of the Mu reference. The section on integer arithmetic also provides a useful cheatsheet of the different forms of instructions you will need.

Here's the exercise, with comments starting with # highlighting the gaps in the program:

fn foo -> _/eax: int {
  var x: int
  # statement 1: store 3 in x
  # statement 2: define a new variable 'y' in register eax and store 4 in it
  # statement 3: add y to x, storing the result in x
  return x
}

Again, you're encouraged to repeatedly try out your programs by running this command as often as you like:

./translate tutorial/task5.mu  &&  qemu-system-i386 code.img

The section on integer arithmetic shows that Mu consistently follows a few rules:

Instructions that write to a register always have an output before the <-.
Instructions that use an argument in memory always have it as the first inout.
Instructions that write to memory have a preposition in their name. Contrast add to a register vs add-to a memory location, subtract from a register vs subtract-from a memory location, and so on.

If you're stuck, as always, my door is open. You can also see a solution in the repository, though I won't link to it lest it encourage peeking.

Where possible, try to store variables in registers rather than the stack. The two main reasons to use the stack are:

when you need lots of variables and run out of registers, and
when you have types that don't fit in 32 bits.

Task 6: getting used to a few error messages

If you're like me, seeing an error message can feel a bit stressful. It usually happens when you're trying to get somewhere, it can feel like the computer is being deliberately obtrusive, there's uncertainty about what's wrong.

Well, I'd like to share one trick I recently learned to stop fearing error messages: deliberately trigger them at a time and place of your choosing, when you're mentally prepared to see them. That takes the stress right out.

Here's the skeleton for tutorial/task6.mu:

fn main {
  var m: int
  var r/edx: int <- copy 0
  # insert a single statement below

}

(Reminder: m here is stored somewhere in memory, while r is stored in register edx. Variables in registers must always be initialized when they're created. Variables in memory must never be initialized, because they're always implicitly initialized to 0.)

Now, starting from this skeleton, type the following statements in, one at a time. Your program should only ever have one more statement than the above skeleton. We'll try out the following statements, one by one:

m <- copy 3
r <- copy 3
copy-to r, 3
copy-to m, 3

Before typing in each one, write down whether you expect an error. After trying it out, compare your answer. It can also be useful to write down the exact error you see, and what it means, in your own words.

(Also, don't forget to delete the statement you typed in before you move on to trying out the next one.)

Making notes about error messages is an example of a more general trick called a runbook. Runbooks are aids to memory, scripts for what to do when you run into a problem. People think worse in the presence of stress, and runbooks can help reduce the need for thinking in the presence of stress. They're a way of programming people (your future self or others) rather than computers.

Task 7: variables in registers, variables in memory (again)

Go back to your program in Task 5. Replace the first statement declaring variable x:

var x: int

so it looks like this:

var x/edx: int <- copy 0

Run translate (or translate_emulated) as usual. Use your runbook from Task 6 to address the errors that arise.

Task 8: primitive statements vs function calls

Managing variables in memory vs register is one of two key skills to programming in Mu. The second key skill is calling primitives (which are provided by the x86 instruction set) vs functions (which are defined in terms of primitives).

To prepare for this task, reread the very first section of the Mu reference, on functions and function calls.

Now look at the following programs. In each case, write down whether you expect translation to return any errors and why.

fn f a: int {
}

fn main {
  f 0
  var r/eax: int <- copy 3
  f r
  var m: int
  f m
}

(When you're ready, try the above program out as ./translate tutorial/task8a.mu.)

fn f -> _/eax: int {
  var result/ecx: int <- copy 0
  return result
}

fn main {
  var x/eax: int <- f
}

(When you're ready, try the above program out as ./translate tutorial/task8b.mu.)

fn f -> _/eax: int {
  return 3
}

fn main {
  var x/ecx: int <- f
}

(When you're ready, try the above program out as ./translate tutorial/task8c.mu.)

Functions have fewer restrictions than primitives on inouts, but more restrictions on outputs. Inouts can be registers, or memory, or even literals. This is why the first example above is legal. Outputs, however, must hard-code specific registers, and function calls must write their outputs to matching registers. This is why the third example above is illegal.

One subtlety here is that we only require agreement on output registers between function call and function header. We don't actually have to return the precise register a function header specifies. The return value can even be a literal integer or in memory somewhere. The return is really just a copy to the appropriate register(s). This is why the second example above is legal.

Task 9: juggling registers between function calls

Here's a program:

fn f -> _/eax: int {
  return 2
}

fn g -> _/eax: int {
  return 3
}

fn add-f-and-g -> _/eax: int {
  var x/eax: int <- f
  var y/eax: int <- g
  x <- add y
  return x
}

What's wrong with this program? How can you fix it and pass all tests by modifying just function add-f-and-g?

By convention, most functions in Mu return their results in register eax. That creates a fair bit of contention for this register, and we often end up having to move the output of a function call around to some other location to free up space for the next function we need to call.

An alternative approach would be to distribute the load between registers so that different functions use different output registers. That would reduce the odds of conflict, but not eradicate them entirely. It would also add some difficulty in calling functions; now you have to remember what register they write their outputs to. It's unclear if the benefits of this alternative outweigh the costs, so Mu follows long-established conventions in other Assembly languages. I do, however, violate the eax convention in some cases where a helper function is only narrowly useful in a single sort of circumstance and registers are at a premium. See, for example, the definition of the helper _read-dithering-error when rendering images. The leading underscore indicates that it's an internal detail of render-image, and not really intended to be called by itself.

Task 10: operating with fractional numbers

All our variables so far have had type int (integer), but there are limits to what you can do with just whole integers. For example, here's the formula a visitor to the US will require to convert distances mentioned on road signs from miles to kilometers:

distance * 1.609

Write a function to perform this conversion. Some starting points:

Reread the section on variables and registers with special attention to the float type.
Read the section on fractional arithmetic.
One wrinkle is that the x86 instruction set doesn't permit literal fractional arguments. So you'll need to create 1.609 somehow. See the section on moving values around under operations on simple types.

This task has four source files in the repo that reveal more and more of the answer. Start from the first, and bump down if you need a hint.

tutorial/task10.mu
tutorial/task10-hint1.mu
tutorial/task10-hint2.mu
tutorial/task10-hint3.mu

Task 11: conditionally executing statements

Here's a fragment of Mu code:

{
  compare x, 0
  break-if->=
  x <- copy 0
}

The combination of compare and break results in the variable x being assigned 0 if and only if its value was less than 0 at the beginning. The break family of instructions is used to jump to the end of an enclosing {} block if some condition is satisfied, skipping all intervening instructions.

To prepare for this task, read the sections in the Mu reference on compare and branches.

Now make the tests pass in tutorial/task11.mu. The goal is to implement our colloquial understanding of the “difference” between two numbers. In lay English, we say the difference between the first-place and third-place runner in a race is two places. This answer doesn't depend on the order in which we mention the runners; the difference between third and first is also two.

The section on integer arithmetic is again worth referring to when working on this task.

Task 12: fun with graphics

Here's a program to draw a rectangle on screen:

fn main screen: (addr screen) {
  draw-line screen, 0x100/x1 0x100/y1, 0x300/x2 0x100/y2, 3/color=green
  draw-line screen, 0x100/x1 0x200/y1, 0x300/x2 0x200/y2, 3/color=green
  draw-line screen, 0x100/x1 0x100/y1, 0x100/x2 0x200/y2, 3/color=green
  draw-line screen, 0x300/x1 0x100/y1, 0x300/x2 0x200/y2, 3/color=green
}

Play around with this function a bit, commenting out some statements with a leading # and rerunning the program. Build up a sense for how the statements map to lines on screen.

Modify the rectangle to start at the top-left corner on screen. How about other corners?

Notice the screen variable. The main function always has access to a screen variable, and any function wanting to draw to the screen will need this variable. Later you'll learn to create and pass fake screens within automated tests, so that we can maintain confidence that our graphics functions work as expected.

The “real” screen on a Mu computer is sized to 1024 (0x400) pixels wide and 768 (0x300) pixels tall by default. Each pixel can take on 256 colors. Many other screen configurations are possible, but it'll be up to you to learn how to get to them.

Graphics in Mu often involve literal integer constants. To help remember what they mean, you can attach comment tokens -- any string without whitespace -- to a literal integer after a /. For example, an argument of 1 can sometimes mean the width of a line, and at other times mean a boolean with a true value. Getting into the habit of including comment tokens is an easy way to make your programs easier to understand.

Another thing to notice in this program is the commas. Commas are entirely optional in Mu, and it can be handy to drop them selectively to group arguments together.

This is a good time to skim Mu's vocabulary of functions for pixel graphics. They're fun to play with. Once you want to use a specific function, look for details on the arguments it expects in signatures.mu.

Task 13: reading input from keyboard

Read the section on events from Mu's vocabulary. Write a program to read a key from the keyboard. Mu receives a keyboard object as the second argument of main:

fn main screen: (addr screen), keyboard: (addr keyboard) {
  # edit in here {

  # }
}

The signature of read-key -- along with many other functions -- is in 400.mu.

One wrinkle in this problem is that read-key may not actually return a key. You have to keep retrying until it does. You may have already encountered the list of loop operations in the section on branches. It might be a good time to refresh your knowledge there.

Task 14: streams and scanning input from the keyboard

Here's a skeleton of a program for processing text typed in at a keyboard:

fn main screen: (addr screen), keyboard: (addr keyboard) {
  var in-storage: (stream byte 0x80)
  var in/esi: (addr stream byte) <- address in-storage
  read-line-from-keyboard keyboard, in, screen, 0xf/fg 0/bg
  {
    var done?/eax: boolean <- stream-empty? in
    compare done?, 0/false
    break-if-!=
    var g/eax: code-point-utf8 <- read-code-point-utf8 in
    # do stuff with g here
    loop
  }
}

read-line-from-keyboard reads keystrokes from the keyboard until you press the Enter (also called newline) key, and accumulates them into a stream of bytes. The loop then repeatedly reads code points from the stream, and these code points are encoded in a special encoding of Unicode called UTF-8. Mu doesn't yet support non-Qwerty keyboards, but support for keyboards in other parts of the world should be easy to add thanks to our support for UTF-8.

This is a good time to skim the section in the Mu reference on streams, just to give yourself a sense of what you can do with them. Does the above program make sense now? Feel free to experiment to make sense of it.

Can you modify it to print out the line a second time, after you've typed it out until the Enter key? Can you print a space after every code point when you print the line out a second time? You'll need to skim the section on printing to screen from Mu's vocabulary. One common pattern: Mu programs read characters in UTF-8, but print raw code-points. Use to-code-point to decode UTF-8 and get at the underlying raw code-point, and use to-utf8 to encode a code-point into a code-point-utf8. (This stuff is still under construction; I hope to simplify it over time.)

Task 15: generating cool patterns

Back to drawing to screen. Here's a program that draws every pixel on screen with a color equal to the value of its x coordinate.

fn main screen: (addr screen) {
  var y/eax: int <- copy 0
  {
    compare y, 0x300/screen-height=768
    break-if->=
    var x/edx: int <- copy 0
    {
      compare x, 0x400/screen-width=1024
      break-if->=
      var color/ecx: int <- copy x
      pixel screen x, y, color
      x <- increment
      loop
    }
    y <- increment
    loop
  }
}

Before you run it, form a hypothesis about what the picture will look like. The screen is 1024 pixels wide, but there are only 256 colors. What are the implications of these facts?

After you run this program, try to modify it so every pixel gets a color equal to the sum of its x and y coordinates. Can you guess what pattern will result? Play around with more complex formulae. I particularly like the sum of squares of x and y coordinates. Check out the Mandelbrot set for a really complex example of this sort of procedural graphics.

Task 16: a simple app

We now know how to read keys from keyboard and draw on the screen. Try out tutorial/counter.mu which implements a simple counter app.

Now look at its code. Do all the parts make sense? Reread the extensive vocabulary of functions for drawing text to screen.

Here's a more challenging problem. Build an app to convert celsius to fahrenheit and vice versa. Two text fields, the <Tab> key to move the cursor between them, type in either field and hit <Enter> to populate the other field.

After you build it, compare your solution with tutorial/converter.mu. A second version breaks the program down into multiple functions, in tutorial/converter2.mu. Can you see how the two do the same thing? Which one do you like better?

There's lots more programs in this repository. Look in the apps/ directory. Check out the Mu shell/, which persists data between runs to a separate data disk. Hopefully this gives you some sense for how little software it takes to build useful programs for yourself. Do you have any new ideas for programs to write in Mu? Tell me about them! I'd love to jam with you.

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