21 KiB
layout, title, redirect_from
| layout | title | redirect_from |
|---|---|---|
| post | Printing to Screen | /2015/10/23/printing-to-screen/ |
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]: {{ page.previous.url }} [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
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.
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 enable blinking. However, it's possible to disable blinking through a BIOS function. Then the full 16 colors can be used as background.
A basic Rust Module
Now that we know how the VGA buffer works, we can create a Rust module to handle printing. To create a new module named vga_buffer, we just need to create a file named src/vga_buffer.rs and add a mod vga_buffer line to src/lib.rs.
Colors
First, we represent the different colors using an enum:
#[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.
To represent a full color code that specifies foreground and background color, we create a newtype on top of u8:
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.
The Text Buffer
Now we can add structures to represent a screen character and the text buffer:
#[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.
To actually write to screen, we now create a writer type:
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.
Printing to Screen
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):
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;
self.buffer().chars[row][col] = ScreenChar {
ascii_character: byte,
color_code: self.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.
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: cannot move out of borrowed content [E0507]
color_code: self.color_code,
^~~~
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. To fix it, we can implement the [Copy trait] for the ColorCode type by adding #[derive(Clone, Copy)] to its struct.
[ownership section]: https://doc.rust-lang.org/book/ownership.html
[Copy trait]: https://doc.rust-lang.org/nightly/core/marker/trait.Copy.html
Try it out!
To write some characters to the screen, you can create a temporary function:
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.
Printing Strings
To print whole strings, we can convert them to bytes and print them one-by-one:
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. To ensure that a string contains only ASCII characters, you can prefix a b to create a Byte String.
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:
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:
...
let mut writer = Writer::new(Color::Blue, Color::LightGreen);
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.
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:
fn new_line(&mut self) {
for row in 0..(BUFFER_HEIGHT-1) {
let buffer = self.buffer();
buffer.chars[row] = buffer.chars[row + 1]
}
self.clear_row(BUFFER_HEIGHT-1);
self.column_position = 0;
}
fn clear_row(&mut self, row: usize) {/* TODO */}
We just move each line to the line above. Notice that the range notation (..) is exclusive the upper bound. But when we try to compile it, we get an borrow checker error again:
error: cannot move out of indexed content [E0507]
buffer.chars[row] = buffer.chars[row + 1]
^~~~~~~~~~~~~~~~~~~~~
It's because of Rust's move semantics again: We try to move out the ScreenChar at row + 1. If Rust would allow that, the array would become invalid as it would contain some valid and some moved out values. Fortunately, the ScreenChar type meets all criteria for the [Copy trait], so we can fix the problem by adding #derive(Clone, Copy) to ScreenChar.
Now we only need to implement the clear_row method to finish the newline code:
fn clear_row(&mut self, row: usize) {
let blank = ScreenChar {
ascii_character: b' ',
color_code: self.color_code,
};
self.buffer().chars[row] = [blank; BUFFER_WIDTH];
}
Providing an Interface
To provide a global writer that can used as an interface from other modules, we can add a static writer:
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.
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.
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.
To use a spinning mutex, we can add the spin crate as a dependency in Cargo.toml:
...
[dependencies]
rlibc = "0.1.4"
spin = "0.3.4"
and a extern crate spin; definition in src/lib.rs. Then we can use the spinning Mutex to provide interior mutability to our static writer:
use spin::Mutex;
...
pub static WRITER: Mutex<Writer> = Mutex::new(Writer {
column_position: 0,
color_code: ColorCode::new(Color::LightGreen, Color::Black),
buffer: 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:
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_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:
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.
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:
macro_rules! print {
($($arg:tt)*) => ({
use core::fmt::Write;
$crate::vga_buffer::WRITER.lock().write_fmt(format_args!($($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:
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:
#[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]: {{ page.next.url }}
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 links1 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]: /cross-compile-binutils.html
-
[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
-
You will need to symlink
x86_64-none_elf-XXXto/usr/bin/XXXwhereXXXis in {as,ld,objcopy,objdump,strip}. Thex86_64-none_elf-XXXfiles 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. ↩︎