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Print “Hello” instead of “OK”

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Philipp Oppermann
2018-02-04 00:14:13 +01:00
parent 9fc0a07453
commit bff0c4401c
2 changed files with 20 additions and 8 deletions
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18
blog/content/second-edition/posts/02-minimal-rust-kernel/index.md
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@@ -54,7 +54,7 @@ If you are interested in building your own bootloader, check out our “[Booting
### UEFI
## A Minimal Kernel
Now that we know how a computer boots, it's time to create our own minimal kernel. Our goal is to create a bootable disk image that prints a green “OK” to the screen when booted. For that we build upon the [freestanding Rust binary] we created in the previous post.
Now that we know how a computer boots, it's time to create our own minimal kernel. Our goal is to create a bootable disk image that prints a green “Hello” to the screen when booted. For that we build upon the [freestanding Rust binary] we created in the previous post.
We already have our `_start` entry point, which will be called by the boot loader. So let's output something to screen from it.
@@ -65,26 +65,32 @@ The easiest way to print text to the screen at this stage is the [VGA text buffe
![screen output for common ASCII characters](https://upload.wikimedia.org/wikipedia/commons/6/6d/Codepage-737.png)
We will discuss the exact layout of the VGA buffer in the next post, where we write a first small driver for it. For printing “OK”, we just need to know that the buffer is located at address `0xb8000` and that each character cell consists of an ASCII byte and a color byte.
We will discuss the exact layout of the VGA buffer in the next post, where we write a first small driver for it. For printing “Hello”, we just need to know that the buffer is located at address `0xb8000` and that each character cell consists of an ASCII byte and a color byte.
So let's extend our `main.rs` to write `OK` to the screen:
The implementation looks like this:
```rust
#[no_mangle]
pub fn _start(boot_info: &'static mut BootInfo) -> ! {
let vga_buffer = 0xb8000 as *const u8 as *mut u8;
unsafe {
*vga_buffer.offset(0) = b'O';
*vga_buffer.offset(0) = b'H';
*vga_buffer.offset(1) = 0xa; // foreground color green
*vga_buffer.offset(2) = b'K';
*vga_buffer.offset(2) = b'e';
*vga_buffer.offset(3) = 0xa; // foreground color green
*vga_buffer.offset(4) = b'l';
*vga_buffer.offset(5) = 0xa;
*vga_buffer.offset(6) = b'l';
*vga_buffer.offset(7) = 0xa;
*vga_buffer.offset(8) = b'o';
*vga_buffer.offset(9) = 0xa;
}
loop {}
}
```
First, we cast the integer `0xb8000` into a [raw pointer]. Then we use the [`offset`] method to write the first four bytes individually. We write the ASCII character `b'O'` (the `b` prefix creates an single-byte ASCII character instead of a four-byte Unicode character), then we write the color `0xa` (which translates to “green foreground, black background”). We repeat the same for the second character.
First, we cast the integer `0xb8000` into a [raw pointer]. Then we use the [`offset`] method to write the first ten bytes individually. We write the ASCII character `b'H'` (the `b` prefix creates an single-byte ASCII character instead of a four-byte Unicode character), then we write the color `0xa` (which translates to “green foreground, black background”). We repeat the same for the other four characters.
[raw pointer]: https://doc.rust-lang.org/stable/book/second-edition/ch19-01-unsafe-rust.html#dereferencing-a-raw-pointer
[`offset`]: https://doc.rust-lang.org/std/primitive.pointer.html#method.offset