[Write an OS in Rust] Day 2

[Write an OS in Rust] Day 2

Today's task is implement a write function for vga text mode followed by post

目录

• [Write an OS in Rust] Day 2
  • • Rust
      • • Attribute
        • use
        • mut
        • volatile
    • OS

Rust

Attribute

• #[allow(dead_code)]: The compiler provides a dead_code lint that will warn about unused functions. Use it allow a unused strcut、function...
• #[repr(u8)]: Give a alignment.

use

Like from xxx import xxxx in python. According to Use declarations, use will be the top in a module or a block usually.

mut

Sometimes I can't understand mut clearly. Today I did a new test about mut, I wrote this code :

#[warn(dead_code)]
struct Rectangle {
    height: i32,
    width: i32,
}

impl Rectangle {
    pub fn change_height(&mut self, val: i32) {
        self.height = val;
    }
}

fn main()
{
    let mut test = Rectangle{
        height: 10,
        width: 5,
    };

    test.change_height(9);
    
}

then, compiler tell me, I need set a mutable var:

➜  hello-world git:(master) ✗ cargo build
   Compiling hello-world v0.1.0 (./hello-world)
error[E0596]: cannot borrow `test` as mutable, as it is not declared as mutable
   --> src/main.rs:193:5
    |
188 |     let test = Rectangle{
    |         ---- help: consider changing this to be mutable: `mut test`
...
193 |     test.change_height(9);
    |     ^^^^^^^^^^^^^^^^^^^^^ cannot borrow as mutable

For more information about this error, try `rustc --explain E0596`.
error: could not compile `hello-world` due to previous error

so, the correct code is :

fn main()
{
    let mut test = Rectangle{
        height: 10,
        width: 5,
    };

    test.change_height(9);
    
}

volatile

ablou volatile, I also search its usages in C++.

A volatile specifier is a hint to a compiler that an object may change its value in ways not specified by the language so that aggressive optimizations must be avoided.

One variable use volatile ask the compiler not to optimize this variable.volatile indicates the variable maybe changed when you access, but this change maybe cant be percieved by compiler, volatile guarent that every time you access the variable, you should access it address.

In Rust, volatile is a crate

OS

This post just decribed VGA Text Mode.