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Merge pull request phil-opp/phil-opp.github.io#3 from phil-opp/printing_wip
Finish and publish post about printing to screen
This commit is contained in:
Philipp Oppermann
@@ -1,263 +0,0 @@
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---
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layout: post
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title: 'Printing to Screen'
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category: 'rust-os'
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---
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TODO Introduction
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## The VGA Text Buffer
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The text buffer starts at physical address `0xb8000` and contains the characters displayed on screen. It has 80 rows and 25 columns. Each screen character has the following format:
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Bit(s) | Value
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------ | ----------------
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0-7 | ASCII code point
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8-11 | Foreground color
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12-14 | Background color
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15 | Blink
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The following colors are available:
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Number | Color | Number + Bright Bit | Bright Color
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------ | ---------- | ------------------- | -------------
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0x0 | Black | 0x8 | Dark Gray
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0x1 | Blue | 0x9 | Light Blue
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0x2 | Green | 0xa | Light Green
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0x3 | Cyan | 0xb | Light Cyan
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0x4 | Red | 0xc | Light Red
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0x5 | Magenta | 0xd | Light Magenta
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0x6 | Brown | 0xe | Yellow
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0x7 | Light Gray | 0xf | White
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Bit 4 is the _bright bit_, which is unavailable in background color as it's the blink bit. But it's possible to disable blinking through a [BIOS function][disable blinking] and use the full 16 colors as background.
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[disable blinking]: http://www.ctyme.com/intr/rb-0117.htm
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## Creating a Rust Module
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Let's create the Rust module `vga_buffer`. Therefor we create a file named `src/vga_buffer.rs` and add a `mod vga_buffer` line to `src/lib.rs`. Now we can create an enum for the colors ([full file](#TODO)):
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```rust
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#[repr(u8)]
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pub enum Color {
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Black = 0,
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Blue = 1,
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...
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Yellow = 14,
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White = 15,
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}
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struct ColorCode(u8);
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impl ColorCode {
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const fn new(foreground: Color, background: Color) -> ColorCode {
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ColorCode((background as u8) << 4 | (foreground as u8))
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}
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}
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```
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We use the `repr(u8)` attribute to represent each variant of `Color` as an `u8`. The `ColorCode` contains the full color byte, containing foreground and background color. The `new` function is a [const function] to allow it in static initializers. It's unstable so we need to add the `const_fn` feature in `src/lib.rs`. We ignore the blink bit here to keep the code short. Now we can represent a screen character and the text buffer:
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```rust
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struct ScreenChar {
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ascii_character: u8,
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color_code: ColorCode,
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}
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const BUFFER_HEIGHT: usize = 25;
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const BUFFER_WIDTH: usize = 80;
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struct Buffer {
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chars: [[ScreenChar; BUFFER_WIDTH]; BUFFER_HEIGHT],
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}
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```
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Now we can represent the actual screen writer:
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```rust
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pub struct Writer {
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column_position: usize,
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color_code: ColorCode,
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}
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impl Writer {
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pub const fn new(foreground: Color, background: Color) -> Writer {
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Writer {
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column_position: 0,
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color_code: ColorCode::new(foreground, background),
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}
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}
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}
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```
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It's `pub` because it's part of the public interface and `const` to allow it in static initializers. The plan is to write always to the last line and shift lines up when a line is full (or on `\n`). So we just need to store the current column position and the current `ColorCode`.
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### Writing to screen
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Now we can create a `write_byte` function:
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```rust
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impl Writer {
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pub fn write_byte(&mut self, byte: u8) {
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const NEWLINE: u8 = '\n' as u8;
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const BUFFER: *mut Buffer = 0xb8000 as *mut _;
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match byte {
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NEWLINE => {}, // TODO
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byte => {
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if self.column_position >= BUFFER_WIDTH {
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//TODO
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}
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let row = BUFFER_HEIGHT - 1;
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let col = self.column_position;
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Self::buffer().chars[row][col] = ScreenChar {
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ascii_character: byte,
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color_code: self.color_code,
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};
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self.column_position += 1;
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}
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}
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}
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}
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```
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Some Notes:
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- We write a new `ScreenChar` to the current field in the buffer and increase the column position
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- We take `&mut self` as we modify the column position
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- `Self` refers to the `Writer` type
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The `buffer()` function just converts the raw pointer to a mutable reference. We need the `unsafe` block because Rust doesn't know if the pointer is valid. It looks like this:
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```rust
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impl Writer {
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fn buffer() -> &'static mut Buffer {
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const BUFFER: *mut Buffer = 0xb8000 as *mut _;
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unsafe{&mut *BUFFER}
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}
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}
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```
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Now we can test it in `main`:
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```rust
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pub extern fn main() {
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use vga_buffer::{Writer, Color};
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...
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let mut writer = Writer::new(Color::Blue, Color::LightGreen);
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writer.write_byte(b'H');
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}
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```
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The `b'H'` is a [byte character] and represents the ASCII code point for `H`. It should be printed in the _lower_ left corner of the screen on `make run`.
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To print whole strings, we can convert them to bytes[^utf8-problems] and print them one-by-one:
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```rust
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pub fn write_str(&mut self, s: &str) {
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for byte in s.bytes() {
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self.write_byte(byte)
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}
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}
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```
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[byte character]: https://doc.rust-lang.org/reference.html#characters-and-strings
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[^utf8-problems]: This approach works well for strings that contain only ASCII characters. For other Unicode characters, however, we get weird symbols on the screen. But they can't be printed in the VGA text buffer anyway.
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## Support Formatting Macros
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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` and looks quite similar to our `write_str` method. We just need to move it into an `impl ::core::fmt::Write for Writer` block and add a return type:
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```rust
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impl ::core::fmt::Write for Writer {
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fn write_str(&mut self, s: &str) -> ::core::fmt::Result {
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for byte in s.bytes() {
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self.write_byte(byte)
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}
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Ok(())
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}
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}
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```
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The `Ok(())` is just the `Ok` Result containing the `()` type. We can drop the `pub` because trait methods are always public.
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Now we can use Rust's built-in `write!`/`writeln!` formatting macros:
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```rust
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...
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let mut writer = Writer::new(Color::Blue, Color::LightGreen);
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writer.write_byte(b'H');
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writer.write_str("ello! ");
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write!(writer, "The numbers are {} and {}", 42, 1.0/3.0);
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```
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Now you should see a `Hello! The numbers are 42 and 0.3333333333333333` in strange colors at the bottom of the screen.
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[core::fmt::Write]: https://doc.rust-lang.org/nightly/core/fmt/trait.Write.html
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## Newlines
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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 a `new_line` method to `Writer`:
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```rust
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fn new_line(&mut self) {
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for row in 0..(BUFFER_HEIGHT-1) {
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Self::buffer().chars[row] = Self::buffer().chars[row + 1]
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}
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self.clear_row(BUFFER_HEIGHT-1);
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self.column_position = 0;
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}
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```
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We just move each line to the line above. Notice that the range notation (`..`) is exclusive the upper bound.
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The `clear_row` method looks like this:
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```rust
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fn clear_row(&mut self, row: usize) {
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let blank = ScreenChar {
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ascii_character: b' ',
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color_code: self.color_code,
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};
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Self::buffer().chars[row] = [blank; BUFFER_WIDTH];
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}
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```
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Now we just need to call the `new_line()` method in the 2 cases marked with `//TODO` and our writer supports newlines.
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## A `println!` macro
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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:
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```
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macro_rules! println {
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($fmt:expr) => (print!(concat!($fmt, "\n")));
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($fmt:expr, $($arg:tt)*) => (print!(concat!($fmt, "\n"), $($arg)*));
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}
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```
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It just refers to the [`print!` macro] that is defined as:
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```
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macro_rules! print {
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($($arg:tt)*) => ($crate::io::_print(format_args!($($arg)*)));
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}
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```
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It just calls the _print_ method in the `io` module of the current crate (`$crate`), which is `std`. The [`_print` function] is rather complicated, as it supports different `Stdout`s.
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To print to the VGA buffer, we just copy both macros and replace the `io` module with the `vga_buffer` buffer in the `print!` macro:
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```
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macro_rules! print {
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($($arg:tt)*) => ($crate::vga_buffer::_print(format_args!($($arg)*)));
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}
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```
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Now we can write our own `_print` function:
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```rust
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pub fn _print(fmt: ::core::fmt::Arguments) {
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use core::fmt::Write;
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static mut WRITER: Writer = Writer::new(Color::LightGreen, Color::Black);
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unsafe{WRITER.write_fmt(fmt)};
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}
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```
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The function needs to be public because every `print!(…)` is expanded to `::vga_buffer::_print(…)`. It uses a `static mut` to store a writer and calls the `write_fmt` method of the `core::fmt::Write` trait (hence the import). It's highly discouraged to use `static mut`s because they introduce all kinds of data races (that's why every access is unsafe). We use it here anyway, as we have only a single thread at the moment. But we already have another data race: We can create multiple `Writer`s, that write to the same memory at `0xb8000`. So as soon as we add multithreading, we need to revisit this module again and find better solutions.
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[macro syntax]: https://doc.rust-lang.org/nightly/book/macros.html
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[`println!` macro]: https://doc.rust-lang.org/nightly/std/macro.println!.html
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[`print!` macro]: https://doc.rust-lang.org/nightly/std/macro.print!.html
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## Clearing the screen
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We can now add a rather trivial last function:
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```
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pub fn clear_screen() {
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for _ in 0..BUFFER_HEIGHT {
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println!("");
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}
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}
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```
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## What's next?
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Soon we will tackle virtual memory management and map the kernel sections correctly. This will cause many strange bugs and boot loops. To understand what's going on a real debugger is indispensable. In the [next post] we will setup [GDB] to work with QEMU.
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405
_posts/2015-10-23-printing-to-screen.md
Normal file
405
_posts/2015-10-23-printing-to-screen.md
Normal file
@@ -0,0 +1,405 @@
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---
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layout: post
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title: 'Printing to Screen'
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category: 'rust-os'
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---
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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.
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[previous post]: {{ site.url }}{{ page.previous.url }}
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[Rust]: https://www.rust-lang.org/
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[raw pointers]: https://doc.rust-lang.org/book/raw-pointers.html
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[formatting macros]: https://doc.rust-lang.org/std/fmt/#related-macros
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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].
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[file an issue]: https://github.com/phil-opp/phil-opp.github.io/issues
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[code repository]: https://github.com/phil-opp/blog_os/tree/printing_to_screen/src
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## The VGA Text Buffer
|
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The text buffer starts at physical address `0xb8000` and contains the characters displayed on screen. It has 80 rows and 25 columns. Each screen character has the following format:
|
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|
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Bit(s) | Value
|
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------ | ----------------
|
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0-7 | ASCII code point
|
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8-11 | Foreground color
|
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12-14 | Background color
|
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15 | Blink
|
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|
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The following colors are available:
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|
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Number | Color | Number + Bright Bit | Bright Color
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------ | ---------- | ------------------- | -------------
|
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0x0 | Black | 0x8 | Dark Gray
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0x1 | Blue | 0x9 | Light Blue
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0x2 | Green | 0xa | Light Green
|
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0x3 | Cyan | 0xb | Light Cyan
|
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0x4 | Red | 0xc | Light Red
|
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0x5 | Magenta | 0xd | Light Magenta
|
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0x6 | Brown | 0xe | Yellow
|
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0x7 | Light Gray | 0xf | White
|
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|
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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][disable blinking]. Then the full 16 colors can be used as background.
|
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|
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[disable blinking]: http://www.ctyme.com/intr/rb-0117.htm
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## A basic Rust Module
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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`.
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### Colors
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First, we represent the different colors using an enum ([full file](#TODO)):
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```rust
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#[repr(u8)]
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pub enum Color {
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Black = 0,
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Blue = 1,
|
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...
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Yellow = 14,
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White = 15,
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}
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```
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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.
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[C-like enum]: http://rustbyexample.com/custom_types/enum/c_like.html
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To represent a full color code that specifies foreground and background color, we create a [newtype] on top of `u8`:
|
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[newtype]: https://aturon.github.io/features/types/newtype.html
|
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|
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```rust
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struct ColorCode(u8);
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|
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impl ColorCode {
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const fn new(foreground: Color, background: Color) -> ColorCode {
|
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ColorCode((background as u8) << 4 | (foreground as u8))
|
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}
|
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}
|
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```
|
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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`.
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[const function]: https://github.com/rust-lang/rfcs/blob/master/text/0911-const-fn.md
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|
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### The Text Buffer
|
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Now we can add structures to represent a screen character and the text buffer:
|
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|
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```rust
|
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struct ScreenChar {
|
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ascii_character: u8,
|
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color_code: ColorCode,
|
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}
|
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|
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const BUFFER_HEIGHT: usize = 25;
|
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const BUFFER_WIDTH: usize = 80;
|
||||
|
||||
struct Buffer {
|
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chars: [[ScreenChar; BUFFER_WIDTH]; BUFFER_HEIGHT],
|
||||
}
|
||||
```
|
||||
To ensure that `ScreenChar` is exactly 16-bits, one might be tempted to use the [repr(packed)] attribute. But Rust does not insert any padding around two `u8` values, so it's not needed here. And using `repr(packed)` is generally discouraged because it can [cause undefined behavior][repr(packed) issue].
|
||||
|
||||
[repr(packed)]: https://doc.rust-lang.org/nightly/nomicon/other-reprs.html#reprpacked
|
||||
[repr(packed) issue]: https://github.com/rust-lang/rust/issues/27060
|
||||
|
||||
To actually write to screen, we now create a writer type:
|
||||
|
||||
```rust
|
||||
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`.
|
||||
|
||||
[Unique]: https://doc.rust-lang.org/nightly/core/ptr/struct.Unique.html
|
||||
|
||||
## 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:
|
||||
|
||||
```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;
|
||||
|
||||
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.
|
||||
|
||||
[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
|
||||
|
||||
### 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: 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
|
||||
|
||||
### Printing Strings
|
||||
|
||||
To print whole strings, we can convert them to bytes and print them one-by-one:
|
||||
|
||||
```rust
|
||||
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].
|
||||
|
||||
[UTF-8]: http://www.fileformat.info/info/unicode/utf8.htm
|
||||
[Byte String]: https://doc.rust-lang.org/reference.html#characters-and-strings
|
||||
|
||||
### 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
|
||||
...
|
||||
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.
|
||||
|
||||
[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
|
||||
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) {/* see below */}
|
||||
```
|
||||
We just move each line to the line above. Notice that the range notation (`..`) is exclusive the upper bound.
|
||||
|
||||
The `clear_row` method looks like this:
|
||||
|
||||
```rust
|
||||
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/Printing from outside/Synchronization/...
|
||||
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: Unique::new(0xb8000 as *mut _),
|
||||
};
|
||||
```
|
||||
|
||||
This won't work! 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 in Cargo.toml:
|
||||
|
||||
[spin crate]: https://crates.io/crates/spin
|
||||
|
||||
```toml
|
||||
...
|
||||
[dependencies]
|
||||
rlibc = "*"
|
||||
spin = "*"
|
||||
```
|
||||
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:
|
||||
|
||||
```rust
|
||||
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:
|
||||
|
||||
[Mutex::new]: https://mvdnes.github.io/rust-docs/spinlock-rs/spin/struct.Mutex.html#method.new
|
||||
|
||||
```rust
|
||||
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
|
||||
|
||||
```
|
||||
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
|
||||
|
||||
```
|
||||
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`:
|
||||
|
||||
```
|
||||
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:
|
||||
|
||||
```rust
|
||||
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
|
||||
#[macro_use]
|
||||
mod vga_buffer;
|
||||
|
||||
#[no_mangle]
|
||||
pub extern fn rust_main() {
|
||||
// ATTENTION: we have a very small stack and no guard page
|
||||
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.
|
||||
|
||||
## 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 a [cross compiler] to build it (or you create some symbolic links[^fn-symlink]).
|
||||
[Rust Bare-Bones Kernel]: https://github.com/thepowersgang/rust-barebones-kernel
|
||||
[higher half]: http://wiki.osdev.org/Higher_Half_Kernel
|
||||
[cross compiler]: http://wiki.osdev.org/GCC_Cross-Compiler
|
||||
[^fn-symlink]: You will need symlink `x86_64-none_elf-XXX` to `/usr/bin/XXX` where `XXX` is in {`as`, `gcc`, `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, too.
|
||||
["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
|
||||
Reference in New Issue
Block a user