geos/log/2024-03-27.md

VGA text

Hello. Today I succesfully made a println! macro.

std

To following conventions I decided to make a std library. To do this I made a workspace in kernel/.

kernel/Cargo.toml

[package]
name = "kernel"
version = "0.1.0"
edition = "2021"

[workspace]
members = ["std"]

[dependencies]
std = { path = "std" }

lib.rs

To not have one big spaghetti file I separated out all the individal modules into different files. kernel/std/src/lib.rs

#![no_std]

pub mod vga_buffer;

vga_buffer.rs

First step for a VGA text driver is to be able to represent data.

kernel/std/src/lib.rs

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[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,
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(transparent)] // Store this as a u8
pub struct ColorCode(u8);

impl ColorCode {
    pub fn new(foreground: Color, background: Color) -> ColorCode {
        ColorCode((background as u8) << 4 | (foreground as u8))
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(C)] // Tells rust to store this like a struct in C, i.e. store this as 2 bytes
struct ScreenChar {
    ascii_character: u8,
    color_code: ColorCode,
}

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

#[repr(transparent)]
struct Buffer {
    chars: [[ScreenChar; BUFFER_WIDTH]; BUFFER_HEIGHT],
}

Now I can represent the VGA text buffer. To write to this I want to make a writer struct that implements fmt::Write.

pub struct Writer {
    col: usize,
    row: usize,
    buffer: &'static mut Buffer,
}

impl Writer {
    pub fn write_byte(&mut self, byte: u8) {
        self.write_byte_color(byte, ColorCode::new(Color::White, Color::Black));
    }

    pub fn write_byte_color(&mut self, byte: u8, color: ColorCode) {
        match byte {
            b'\n' => self.new_line(),
            byte => {
                if self.col >= BUFFER_WIDTH {
                    self.new_line();
                }

                let row = self.row;
                let col = self.col;

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

    pub fn write_string(&mut self, s: &str) {
        self.write_string_color(s, ColorCode::new(Color::White, Color::Black));
    }

    pub fn write_string_color(&mut self, s: &str, color: ColorCode) {
        for char in s.bytes() {
            match char {
                0x20..=0x7e | b'\n' => self.write_byte_color(char, color), // Ascii Character
                _ => self.write_byte(0xfe), // Non ascii
            }
        }
    }

    fn new_line(&mut self) {
        self.row += 1;
        self.col = 0;
    }
}

impl fmt::Write for Writer {
    fn write_str(&mut self, s: &str) -> fmt::Result {
        self.write_string(s);
        Ok(())
    }
}

The problem now becomes that I can create a Writer but I don't have a global position, so it will just overwrite what I wrote last time. To solve this I can use a static variable. But a static variable won't be mutable. This can be solved with Mutex in the spin crate and lazy_static.

kernel/std/Cargo.toml

[package]
name = "std"
version = "0.1.0"
edition = "2021"

[dependencies]
lazy_static = { version = "1.4.0", features = ["spin_no_std"] }
spin = "0.9.8"

and to make a static variable I wrote (copied from here) this

lazy_static! {
    pub static ref WRITER: Mutex<Writer> = Mutex::new(Writer {
        col: 0,
        row: 0,
        buffer: unsafe { &mut *(0xB8000 as *mut Buffer) },
    });
}

in kernel/std/src/vga_buffer.rs.

Last step is just to add a function and 2 macros

#[doc(hidden)]
pub fn _print(args: fmt::Arguments) {
    WRITER.lock().write_fmt(args).unwrap()
}

#[macro_export]
macro_rules! print {
    ($($arg:tt)*) => ($crate::vga_buffer::_print(format_args!($($arg)*)));
}

#[macro_export]
macro_rules! println {
    () => ($crate::print!("\n"));
    ($($arg:tt)*) => ($crate::print!("{}\n", format_args!($($arg)*)));
}

Huge thanks to @phil-opp for os.phil-opp.com