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Some wording improvements

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Philipp Oppermann
2015-09-26 13:25:33 +02:00
parent 9d34da7c13
commit 517c5eab0d
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_drafts/printing-to-screen.md
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@@ -33,7 +33,9 @@ Bit 4 is the _bright bit_, which turns for example blue into light blue. It is u
[disable blinking]: http://www.ctyme.com/intr/rb-0117.htm
## Creating a Rust Module
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)):
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`.
First, we represent the different colors using an enum ([full file](#TODO)):
```rust
#[repr(u8)]
@@ -81,23 +83,23 @@ struct Buffer {
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 `repr(packed)` can cause [undefined behavior][repr(packed) issue] and that's always bad.
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#repr(packed)
[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<'a> {
pub struct Writer {
column_position: usize,
color_code: ColorCode,
buffer: &'a mut Buffer,
buffer: Unique<Buffer>,
}
```
The explicit lifetime tells Rust that the writer lives as long as the mutual buffer reference. Thus Rust ensures statically that the writer does not write to invalid memory.
The writer will always write to the last line and shift lines up when a line is full (or on `\n`). So just the current column position needs to be stored. The current foreground and background colors are specified by `color_code`. 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` (we want to add static Writer later).
The writer will always write to the last line and shift lines up when a line is full (or on `\n`). So the current row is always the last row and just the current column position needs to be stored. The current foreground and background colors are specified by `color_code`.
[Unique]: https://doc.rust-lang.org/nightly/core/ptr/struct.Unique.html
### Printing Characters
Now we can use the `Writer` to modify the buffer's characters. First we create a method to write a single ASCII byte: