+++
title = "Printing to Screen"
date = "2015-10-23"
updated = "2016-10-06"
aliases = [
    "/2015/10/23/printing-to-screen/",
    "/rust-os/printing-to-screen.html",
]
+++

In the [previous post] we switched from assembly to [Rust], a systems programming language that provides great safety. But so far we are using unsafe features like [raw pointers] whenever we want to print to screen. In this post we will create a Rust module that provides a safe and easy-to-use interface for the VGA text buffer. It will support Rust's [formatting macros], too.

[previous post]: {{% relref "03-set-up-rust.md" %}}
[Rust]: https://www.rust-lang.org/
[raw pointers]: https://doc.rust-lang.org/book/raw-pointers.html
[formatting macros]: https://doc.rust-lang.org/std/fmt/#related-macros

<!--more--><aside id="toc"></aside>

This post uses recent unstable features, so you need an up-to-date nighly compiler. If you have any questions, problems, or suggestions please [file an issue] or create a comment at the bottom. The code from this post is also available on [Github][code repository].

[file an issue]: https://github.com/phil-opp/blog_os/issues
[code repository]: https://github.com/phil-opp/blog_os/tree/printing_to_screen

## The VGA Text Buffer
The text buffer starts at physical address `0xb8000` and contains the characters displayed on screen. It has 25 rows and 80 columns. Each screen character has the following format:

Bit(s) | Value
------ | ----------------
0-7    | ASCII code point
8-11   | Foreground color
12-14  | Background color
15     | Blink

The following colors are available:

Number | Color      | Number + Bright Bit | Bright Color
------ | ---------- | ------------------- | -------------
0x0    | Black      | 0x8                 | Dark Gray
0x1    | Blue       | 0x9                 | Light Blue
0x2    | Green      | 0xa                 | Light Green
0x3    | Cyan       | 0xb                 | Light Cyan
0x4    | Red        | 0xc                 | Light Red
0x5    | Magenta    | 0xd                 | Pink
0x6    | Brown      | 0xe                 | Yellow
0x7    | Light Gray | 0xf                 | White

Bit 4 is the _bright bit_, which turns for example blue into light blue. It is unavailable in background color as the bit is used to control if the text should blink. If you want to use a light background color (e.g. white) you have to disable blinking through a [BIOS function][disable blinking].

[disable blinking]: http://www.ctyme.com/intr/rb-0117.htm

## A basic Rust Module
Now that we know how the VGA buffer works, we can create a Rust module to handle printing:

```rust
// in src/lib.rs
mod vga_buffer;
```

The content of this module can live either in `src/vga_buffer.rs` or `src/vga_buffer/mod.rs`. The latter supports submodules while the former does not. But our module does not need any submodules so we create it as `src/vga_buffer.rs`.

All of the code below goes into our new module (unless specified otherwise).

### Colors
First, we represent the different colors using an enum:

```rust
#[allow(dead_code)]
#[repr(u8)]
pub enum Color {
    Black      = 0,
    Blue       = 1,
    Green      = 2,
    Cyan       = 3,
    Red        = 4,
    Magenta    = 5,
    Brown      = 6,
    LightGray  = 7,
    DarkGray   = 8,
    LightBlue  = 9,
    LightGreen = 10,
    LightCyan  = 11,
    LightRed   = 12,
    Pink       = 13,
    Yellow     = 14,
    White      = 15,
}
```
We use a [C-like enum] here to explicitly specify the number for each color. Because of the `repr(u8)` attribute each enum variant is stored as an `u8`. Actually 4 bits would be sufficient, but Rust doesn't have an `u4` type.

[C-like enum]: http://rustbyexample.com/custom_types/enum/c_like.html

Normally the compiler would issue a warning for each unused variant. By using the `#[allow(dead_code)]` attribute we disable these warnings for the `Color` enum.

To represent a full color code that specifies foreground and background color, we create a [newtype] on top of `u8`:

[newtype]: https://aturon.github.io/features/types/newtype.html

```rust
struct ColorCode(u8);

impl ColorCode {
    const fn new(foreground: Color, background: Color) -> ColorCode {
        ColorCode((background as u8) << 4 | (foreground as u8))
    }
}
```
The `ColorCode` contains the full color byte, containing foreground and background color. Blinking is enabled implicitly by using a bright background color (soon we will disable blinking anyway). The `new` function is a [const function] to allow it in static initializers. As `const` functions are unstable we need to add the `const_fn` feature in `src/lib.rs`.

[const function]: https://github.com/rust-lang/rfcs/blob/master/text/0911-const-fn.md

### The Text Buffer
Now we can add structures to represent a screen character and the text buffer:

```rust
#[repr(C)]
struct ScreenChar {
    ascii_character: u8,
    color_code: ColorCode,
}

const BUFFER_HEIGHT: usize = 25;
const BUFFER_WIDTH: usize = 80;

struct Buffer {
    chars: [[ScreenChar; BUFFER_WIDTH]; BUFFER_HEIGHT],
}
```
Since the field ordering in default structs is undefined in Rust, we need the [repr(C\)] attribute. It guarantees that the struct's fields are laid out exactly like in a C struct and thus guarantees the correct field ordering.

[repr(C\)]: https://doc.rust-lang.org/nightly/nomicon/other-reprs.html#reprc

To actually write to screen, we now create a writer type:

```rust
use core::ptr::Unique;

pub struct Writer {
    column_position: usize,
    color_code: ColorCode,
    buffer: Unique<Buffer>,
}
```
The writer will always write to the last line and shift lines up when a line is full (or on `\n`). The `column_position` field keeps track of the current position in the last row. The current foreground and background colors are specified by `color_code` and a pointer to the VGA buffer is stored in `buffer`. To make it possible to create a `static` Writer later, the `buffer` field stores an `Unique<Buffer>` instead of a plain `*mut Buffer`. [Unique] is a wrapper that implements Send/Sync and is thus usable as a `static`. Since it's unstable, you may need to add the `unique` feature to `lib.rs`:

[Unique]: https://doc.rust-lang.org/nightly/core/ptr/struct.Unique.html

```rust
// in src/lib.rs
#![feature(unique)]
```

## Printing Characters
Now we can use the `Writer` to modify the buffer's characters. First we create a method to write a single ASCII byte (it doesn't compile yet):

```rust
impl Writer {
    pub fn write_byte(&mut self, byte: u8) {
        match byte {
            b'\n' => self.new_line(),
            byte => {
                if self.column_position >= BUFFER_WIDTH {
                    self.new_line();
                }

                let row = BUFFER_HEIGHT - 1;
                let col = self.column_position;

                let color_code = self.color_code;
                self.buffer().chars[row][col] = ScreenChar {
                    ascii_character: byte,
                    color_code: color_code,
                };
                self.column_position += 1;
            }
        }
    }

    fn buffer(&mut self) -> &mut Buffer {
        unsafe{ self.buffer.get_mut() }
    }

    fn new_line(&mut self) {/* TODO */}
}
```
If the byte is the [newline] byte `\n`, the writer does not print anything. Instead it calls a `new_line` method, which we'll implement later. Other bytes get printed to the screen in the second match case.

[newline]: https://en.wikipedia.org/wiki/Newline

When printing a byte, the writer checks if the current line is full. In that case, a `new_line` call is required before to wrap the line. Then it writes a new `ScreenChar` to the buffer at the current position. Finally, the current column position is advanced.

The `buffer()` auxiliary method converts the raw pointer in the `buffer` field into a safe mutable buffer reference. The unsafe block is needed because the [get_mut()] method of `Unique` is unsafe. But our `buffer()` method itself isn't marked as unsafe, so it must not introduce any unsafety (e.g. cause segfaults). To guarantee that, it's very important that the `buffer` field always points to a valid `Buffer`. It's like a contract that we must stand to every time we create a `Writer`. To ensure that it's not possible to create an invalid `Writer` from outside of the module, the struct must have at least one private field and public creation functions are not allowed either.
[get_mut()]: https://doc.rust-lang.org/nightly/core/ptr/struct.Unique.html#method.get_mut

### Cannot Move out of Borrowed Content
When we try to compile it, we get the following error:

```
error[E0507]: cannot move out of borrowed content
  --> src/vga_buffer.rs:79:34
   |
79 | let color_code = self.color_code;
   |                  ^^^^ cannot move out of borrowed content
```
The reason it that Rust _moves_ values by default instead of copying them like other languages. And we cannot move `color_code` out of `self` because we only borrowed `self`. For more information check out the [ownership section] in the Rust book.

[ownership section]: https://doc.rust-lang.org/book/ownership.html
[by reference]: http://rust-lang.github.io/book/ch04-02-references-and-borrowing.html

To fix it, we can implement the [Copy] trait for the `ColorCode` type. The easiest way to do this is to use the built-in [derive macro]:

[Copy]: https://doc.rust-lang.org/nightly/core/marker/trait.Copy.html
[derive macro]: http://rustbyexample.com/trait/derive.html

{{< highlight rust "hl_lines=1" >}}
#[derive(Debug, Clone, Copy)]
struct ColorCode(u8);
{{< / highlight >}}

We also derive the [Clone] trait, since it's a requirement for `Copy`, and the [Debug] trait, which allows us to print this field for debugging purposes.

[Clone]: https://doc.rust-lang.org/nightly/core/clone/trait.Clone.html
[Debug]: https://doc.rust-lang.org/nightly/core/fmt/trait.Debug.html

Now our project should compile again.

However, the [documentation for Copy] says: _“if your type can implement Copy, it should”_. Therefore we also derive Copy for `Color` and `ScreenChar`:

[documentation for Copy]: https://doc.rust-lang.org/core/marker/trait.Copy.html#when-should-my-type-be-copy

{{< highlight rust "hl_lines=2 6" >}}
#[allow(dead_code)]
#[derive(Debug, Clone, Copy)]
#[repr(u8)]
pub enum Color {...}

#[derive(Debug, Clone, Copy)]
#[repr(C)]
struct ScreenChar {...}
{{< / highlight >}}

### Try it out!
To write some characters to the screen, you can create a temporary function:

```rust
pub fn print_something() {
    let mut writer = Writer {
        column_position: 0,
        color_code: ColorCode::new(Color::LightGreen, Color::Black),
        buffer: unsafe { Unique::new(0xb8000 as *mut _) },
    };

    writer.write_byte(b'H');
}
```
It just creates a new Writer that points to the VGA buffer at `0xb8000`. Then it writes the byte `b'H'` to it. The `b` prefix creates a [byte character], which represents an ASCII code point. When we call `vga_buffer::print_something` in main, a `H` should be printed in the _lower_ left corner of the screen in light green.

[byte character]: https://doc.rust-lang.org/reference.html#characters-and-strings

### Volatile
We just saw that our `H` was printed correctly. However, it might not work with future Rust compilers that optimize more aggressively.

The problem is that we only write to the `Buffer` and never read from it again. The compiler doesn't know about the side effect that some characters appear on the screen. So it might decide that these writes are unnecessary and can be omitted.

To avoid this erroneous optimization, we need to specify these writes as _[volatile]_. This tells the compiler that the write has side effects and should not be optimized away.

[volatile]: https://en.wikipedia.org/wiki/Volatile_(computer_programming)

In order to use volatile writes for the VGA buffer, we use the [volatile][volatile crate] library. This _crate_ (this is how packages are called in the Rust world) provides a `Volatile` wrapper type with `read` and `write` methods. These methods internally use the [read_volatile] and [write_volatile] functions of the standard library and thus guarantee that the reads/writes are not optimized away.

[volatile crate]: https://docs.rs/volatile
[read_volatile]: https://doc.rust-lang.org/nightly/core/ptr/fn.read_volatile.html
[write_volatile]: https://doc.rust-lang.org/nightly/core/ptr/fn.write_volatile.html

We can add a dependency on the `volatile` crate by adding it to the `dependencies` section of our `Cargo.toml`:

```toml
# in Cargo.toml

[dependencies]
volatile = "0.1.0"
```

The `0.1.0` is the [semantic] version number. For more information, see the [Specifying Dependencies] guide of the cargo documentation.

[semantic]: http://semver.org/
[Specifying Dependencies]: http://doc.crates.io/specifying-dependencies.html

Now we've declared that our project depends on the `volatile` crate and are able to import it in `src/lib.rs`:

```rust
// in src/lib.rs

extern crate volatile;
```

Let's use it to make writes to the VGA buffer volatile. We update our `Buffer` type as follows:

```rust
// in src/vga_buffer.rs

use volatile::Volatile;

struct Buffer {
    chars: [[Volatile<ScreenChar>; BUFFER_WIDTH]; BUFFER_HEIGHT],
}
```
Instead of a `ScreenChar`, we're now using a `Volatile<ScreenChar>`. (The `Volatile` type is [generic] and can wrap (almost) any type). This ensures that we can't accidentally write to it through a “normal” write. Instead, we have to use the `write` method now.

[generic]: https://doc.rust-lang.org/book/generics.html

This means that we have to update our `Writer::write_byte` method:

{{< highlight rust "hl_lines=8 11" >}}
impl Writer {
    pub fn write_byte(&mut self, byte: u8) {
        match byte {
            b'\n' => self.new_line(),
            byte => {
                ...

                self.buffer().chars[row][col].write(ScreenChar {
                    ascii_character: byte,
                    color_code: color_code,
                });
                ...
            }
        }
    }
    ...
}
{{< / highlight >}}

Instead of a normal assignment using `=`, we're now using the `write` method. This guarantees that the compiler will never optimize away this write.

## Printing Strings

To print whole strings, we can convert them to bytes and print them one-by-one:

```rust
// in `impl Writer`
pub fn write_str(&mut self, s: &str) {
    for byte in s.bytes() {
      self.write_byte(byte)
    }
}
```
You can try it yourself in the `print_something` function.

When you print strings with some special characters like `ä` or `λ`, you'll notice that they cause weird symbols on screen. That's because they are represented by multiple bytes in [UTF-8]. By converting them to bytes, we of course get strange results. But since the VGA buffer doesn't support UTF-8, it's not possible to display these characters anyway.

[core tracking issue]: https://github.com/rust-lang/rust/issues/27701
[UTF-8]: http://www.fileformat.info/info/unicode/utf8.htm

### Support Formatting Macros
It would be nice to support Rust's formatting macros, too. That way, we can easily print different types like integers or floats. To support them, we need to implement the [core::fmt::Write] trait. The only required method of this trait is `write_str` that looks quite similar to our `write_str` method. To implement the trait, we just need to move it into an `impl ::core::fmt::Write for Writer` block and add a return type:

```rust
impl ::core::fmt::Write for Writer {
    fn write_str(&mut self, s: &str) -> ::core::fmt::Result {
        for byte in s.bytes() {
          self.write_byte(byte)
        }
        Ok(())
    }
}
```
The `Ok(())` is just a `Ok` Result containing the `()` type. We can drop the `pub` because trait methods are always public.

Now we can use Rust's built-in `write!`/`writeln!` formatting macros:

```rust
// in the `print_something` function
use core::fmt::Write;
let mut writer = Writer {...};
writer.write_byte(b'H');
writer.write_str("ello! ");
write!(writer, "The numbers are {} and {}", 42, 1.0/3.0);
```
Now you should see a `Hello! The numbers are 42 and 0.3333333333333333` in strange colors at the bottom of the screen.

[core::fmt::Write]: https://doc.rust-lang.org/nightly/core/fmt/trait.Write.html

### Newlines
Right now, we just ignore newlines and characters that don't fit into the line anymore. Instead we want to move every character one line up (the top line gets deleted) and start at the beginning of the last line again. To do this, we add an implementation for the `new_line` method of `Writer`:

```rust
// in `impl Writer`

fn new_line(&mut self) {
    for row in 1..BUFFER_HEIGHT {
        for col in 0..BUFFER_WIDTH {
            let buffer = self.buffer();
            let character = buffer.chars[row][col].read();
            buffer.chars[row - 1][col].write(character);
        }
    }
    self.clear_row(BUFFER_HEIGHT-1);
    self.column_position = 0;
}

fn clear_row(&mut self, row: usize) {/* TODO */}
```
We iterate over all screen characters and move each characters one row up. Note that the range notation (`..`) is exclusive the upper bound. We also omit the 0th row (the first range starts at `1`) because it's the row that is shifted off screen.

Now we only need to implement the `clear_row` method to finish the newline code:

```rust
// in `impl Writer`
fn clear_row(&mut self, row: usize) {
    let blank = ScreenChar {
        ascii_character: b' ',
        color_code: self.color_code,
    };
    for col in 0..BUFFER_WIDTH {
        self.buffer().chars[row][col].write(blank);
    }
}
```
This method clears a row by overwriting all of its characters with a space character.

## Providing an Interface
To provide a global writer that can used as an interface from other modules, we can add a `static` writer:

```rust
pub static WRITER: Writer = Writer {
    column_position: 0,
    color_code: ColorCode::new(Color::LightGreen, Color::Black),
    buffer: unsafe { Unique::new(0xb8000 as *mut _) },
};
```

But we can't use it to print anything! You can try it yourself in the `print_something` function. The reason is that we try to take a mutable reference (`&mut`) to a immutable `static` when calling `WRITER.print_byte`.

To resolve it, we could use a [mutable static]. But then every read and write to it would be unsafe since it could easily introduce data races and other bad things. Using `static mut` is highly discouraged, there are even proposals to [remove it][remove static mut].

[mutable static]: https://doc.rust-lang.org/book/const-and-static.html#mutability
[remove static mut]: https://internals.rust-lang.org/t/pre-rfc-remove-static-mut/1437

But what are the alternatives? We could try to use a cell type like [RefCell] or even [UnsafeCell] to provide [interior mutability]. But these types aren't [Sync] \(with good reason), so we can't use them in statics.

[RefCell]: https://doc.rust-lang.org/nightly/core/cell/struct.RefCell.html
[UnsafeCell]: https://doc.rust-lang.org/nightly/core/cell/struct.UnsafeCell.html
[interior mutability]: https://doc.rust-lang.org/book/mutability.html#interior-vs.-exterior-mutability
[Sync]: https://doc.rust-lang.org/nightly/core/marker/trait.Sync.html

To get synchronized interior mutability, users of the standard library can use [Mutex]. It provides mutual exclusion by blocking threads when the resource is already locked. But our basic kernel does not have any blocking support or even a concept of threads, so we can't use it either. However there is a really basic kind of mutex in computer science that requires no operating system features: the [spinlock]. Instead of blocking, the threads simply try to lock it again and again in a tight loop and thus burn CPU time until the mutex is free again.

[Mutex]: https://doc.rust-lang.org/nightly/std/sync/struct.Mutex.html
[spinlock]: https://en.wikipedia.org/wiki/Spinlock

To use a spinning mutex, we can add the [spin crate] as a dependency:

[spin crate]: https://crates.io/crates/spin

```toml
# in Cargo.toml
[dependencies]
rlibc = "0.1.4"
spin = "0.3.4"
```

```rust
// in src/lib.rs
extern crate spin;
```

Then we can use the spinning Mutex to add interior mutability to our static writer:

```rust
// in src/vga_buffer.rs again
use spin::Mutex;
...
pub static WRITER: Mutex<Writer> = Mutex::new(Writer {
    column_position: 0,
    color_code: ColorCode::new(Color::LightGreen, Color::Black),
    buffer: unsafe { Unique::new(0xb8000 as *mut _) },
});
```
[Mutex::new] is a const function, too, so it can be used in statics.

Now we can easily print from our main function:

[Mutex::new]: https://mvdnes.github.io/rust-docs/spinlock-rs/spin/struct.Mutex.html#method.new

```rust
// in src/lib.rs
pub extern fn rust_main() {
    use core::fmt::Write;
    vga_buffer::WRITER.lock().write_str("Hello again");
    write!(vga_buffer::WRITER.lock(), ", some numbers: {} {}", 42, 1.337);
    loop{}
}
```
Note that we need to import the `Write` trait if we want to use its functions.

## A println macro
Rust's [macro syntax] is a bit strange, so we won't try to write a macro from scratch. Instead we look at the source of the [`println!` macro] in the standard library:

[macro syntax]: https://doc.rust-lang.org/nightly/book/macros.html
[`println!` macro]: https://doc.rust-lang.org/nightly/std/macro.println!.html

```rust
macro_rules! println {
    ($fmt:expr) => (print!(concat!($fmt, "\n")));
    ($fmt:expr, $($arg:tt)*) => (print!(concat!($fmt, "\n"), $($arg)*));
}
```
It just adds a `\n` and then invokes the [`print!` macro], which is defined as:

[`print!` macro]: https://doc.rust-lang.org/nightly/std/macro.print!.html

```rust
macro_rules! print {
    ($($arg:tt)*) => ($crate::io::_print(format_args!($($arg)*)));
}
```
It calls the `_print` method in the `io` module of the current crate (`$crate`), which is `std`. The [`_print` function] in libstd is rather complicated, as it supports different `Stdout` devices.

[`_print` function]: https://doc.rust-lang.org/nightly/src/std/io/stdio.rs.html#578

To print to the VGA buffer, we just copy the `println!` macro and modify the `print!` macro to use our static `WRITER` instead of `_print`:

```rust
// in src/vga_buffer.rs
macro_rules! print {
    ($($arg:tt)*) => ({
            use core::fmt::Write;
            let mut writer = $crate::vga_buffer::WRITER.lock();
            writer.write_fmt(format_args!($($arg)*)).unwrap();
    });
}
```
Instead of a `_print` function, we call the `write_fmt` method of our static `Writer`. Since we're using a method from the `Write` trait, we need to import it before. The additional `unwrap()` at the end panics if printing isn't successful. But since we always return `Ok` in `write_str`, that should not happen.

Note the additional `{}` scope around the macro: I wrote `=> ({…})` instead of `=> (…)`. The additional `{}` avoids that the `Write` trait is silently imported when `print` is used.

### Clearing the screen
We can now use `println!` to add a rather trivial function to clear the screen:

```rust
// in src/vga_buffer.rs
pub fn clear_screen() {
    for _ in 0..BUFFER_HEIGHT {
        println!("");
    }
}
```
### Hello World using `println`
To use `println` in `lib.rs`, we need to import the macros of the VGA buffer module first. Therefore we add a `#[macro_use]` attribute to the module declaration:

```rust
// in src/lib.rs

#[macro_use]
mod vga_buffer;

#[no_mangle]
pub extern fn rust_main() {
    // ATTENTION: we have a very small stack and no guard page
    vga_buffer::clear_screen();
    println!("Hello World{}", "!");

    loop{}
}
```
Since we imported the macros at crate level, they are available in all modules and thus provide an easy and safe interface to the VGA buffer.

## What's next?
In the next posts we will map the kernel pages correctly so that accessing `0x0` or writing to `.rodata` is not possible anymore. To obtain the loaded kernel sections we will read the Multiboot information structure. Then we will create a paging module and use it to switch to a new page table where the kernel sections are mapped correctly.

The [next post] describes the Multiboot information structure and creates a frame allocator using the information about memory areas.

[next post]: {{% relref "05-allocating-frames.md" %}}

## Other Rust OS Projects
Now that you know the very basics of OS development in Rust, you should also check out the following projects:

- [Rust Bare-Bones Kernel]: A basic kernel with roughly the same functionality as ours. Writes output to the serial port instead of the VGA buffer and maps the kernel to the [higher half] \(instead of our identity mapping).
_Note_: You need to [cross compile binutils] to build it (or you create some symbolic links[^fn-symlink] if you're on x86_64).
[Rust Bare-Bones Kernel]: https://github.com/thepowersgang/rust-barebones-kernel
[higher half]: http://wiki.osdev.org/Higher_Half_Kernel
[cross compile binutils]: {{% relref "cross-compile-binutils.md" %}}
[^fn-symlink]: You will need to symlink `x86_64-none_elf-XXX` to `/usr/bin/XXX` where `XXX` is in {`as`, `ld`, `objcopy`, `objdump`, `strip`}. The `x86_64-none_elf-XXX` files must be in some folder that is in your `$PATH`. But then you can only build for your x86_64 host architecture, so use this hack only for testing.

- [RustOS]: More advanced kernel that supports allocation, keyboard inputs, and threads. It also has a scheduler and a basic network driver.
[RustOS]: https://github.com/RustOS-Fork-Holding-Ground/RustOS

- ["Tifflin" Experimental Kernel]: Big kernel project by thepowersgang, that is actively developed and has over 650 commits. It has a separate userspace and supports multiple file systems, even a GUI is included. Needs a cross compiler.
["Tifflin" Experimental Kernel]:https://github.com/thepowersgang/rust_os

- [Redox]: Probably the most complete Rust OS today. It has an active community and over 1000 Github stars. File systems, network, an audio player, a picture viewer, and much more. Just take a look at the [screenshots][redox screenshots].
[Redox]: https://github.com/redox-os/redox
[redox screenshots]: https://github.com/redox-os/redox#what-it-looks-like