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Improve presentation of code snippets

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Philipp Oppermann
2016-04-25 22:17:14 +02:00
parent aeb3100ee4
commit fe9b742978
3 changed files with 81 additions and 49 deletions
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106
blog/post/2015-11-15-allocating-frames.md
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@@ -49,15 +49,20 @@ Instead of writing an own Multiboot module, we use the [multiboot2-elf64] crate.
[multiboot2-elf64]: https://github.com/phil-opp/multiboot2-elf64
[^fn-multiboot-crate]: All contributions are welcome! If you want to maintain it, please contact me!
So let's add a dependency on the git repository in the `Cargo.toml`:
So let's add a dependency on the git repository:
```toml
...
# in Cargo.toml
[dependencies.multiboot2]
git = "https://github.com/phil-opp/multiboot2-elf64"
```
Now we can add `extern crate multiboot2` and use it to print available memory areas.
```rust
// in src/lib.rs
extern crate multiboot2;
```
Now we can use it to print available memory areas.
### Available Memory
The boot information structure consists of various _tags_. See section 3.4 of the Multiboot specification ([PDF][multiboot specification]) for a complete list. The _memory map_ tag contains a list of all available RAM areas. Special areas such as the VGA text buffer at `0xb8000` are not available. Note that some of the available memory is already used by our kernel and by the multiboot information structure itself.
@@ -264,49 +269,64 @@ pub struct AreaFrameAllocator {
```
The `next_free_frame` field is a simple counter that is increased every time we return a frame. It's initialized to `0` and every frame below it counts as used. The `current_area` field holds the memory area that contains `next_free_frame`. If `next_free_frame` leaves this area, we will look for the next one in `areas`. When there are no areas left, all frames are used and `current_area` becomes `None`. The `{kernel, multiboot}_{start, end}` fields are used to avoid returning already used fields.
To implement the `FrameAllocator` trait, we need to implement the `allocate_frame` and the `deallocate_frame` methods. The former looks like this:
To implement the `FrameAllocator` trait, we need to implement the allocation and deallocation methods:
```rust
fn allocate_frame(&mut self) -> Option<Frame> {
if let Some(area) = self.current_area {
// "clone" the frame to return it if it's free. Frame doesn't
// implement Clone, but we can construct an identical frame.
let frame = Frame{ number: self.next_free_frame.number };
impl FrameAllocator for AreaFrameAllocator {
fn allocate_frame(&mut self) -> Option<Frame> {
// TODO (see below)
}
// the last frame of the current area
let current_area_last_frame = {
let address = area.base_addr + area.length - 1;
Frame::containing_address(address as usize)
};
if frame > current_area_last_frame {
// all frames of current area are used, switch to next area
self.choose_next_area();
} else if frame >= self.kernel_start && frame <= self.kernel_end {
// `frame` is used by the kernel
self.next_free_frame = Frame {
number: self.kernel_end.number + 1
};
} else if frame >= self.multiboot_start && frame <= self.multiboot_end {
// `frame` is used by the multiboot information structure
self.next_free_frame = Frame {
number: self.multiboot_end.number + 1
};
} else {
// frame is unused, increment `next_free_frame` and return it
self.next_free_frame.number += 1;
return Some(frame);
}
// `frame` was not valid, try it again with the updated `next_free_frame`
self.allocate_frame()
} else {
None // no free frames left
fn deallocate_frame(&mut self, frame: Frame) {
// TODO (see below)
}
}
```
The `choose_next_area` method isn't part of the trait and thus goes into an `impl AreaFrameAllocator` block:
The `allocate_frame` method looks like this:
```rust
// in `allocate_frame` in `impl FrameAllocator for AreaFrameAllocator`
if let Some(area) = self.current_area {
// "Clone" the frame to return it if it's free. Frame doesn't
// implement Clone, but we can construct an identical frame.
let frame = Frame{ number: self.next_free_frame.number };
// the last frame of the current area
let current_area_last_frame = {
let address = area.base_addr + area.length - 1;
Frame::containing_address(address as usize)
};
if frame > current_area_last_frame {
// all frames of current area are used, switch to next area
self.choose_next_area();
} else if frame >= self.kernel_start && frame <= self.kernel_end {
// `frame` is used by the kernel
self.next_free_frame = Frame {
number: self.kernel_end.number + 1
};
} else if frame >= self.multiboot_start && frame <= self.multiboot_end {
// `frame` is used by the multiboot information structure
self.next_free_frame = Frame {
number: self.multiboot_end.number + 1
};
} else {
// frame is unused, increment `next_free_frame` and return it
self.next_free_frame.number += 1;
return Some(frame);
}
// `frame` was not valid, try it again with the updated `next_free_frame`
self.allocate_frame()
} else {
None // no free frames left
}
```
The `choose_next_area` method isn't part of the trait and thus goes into a new `impl AreaFrameAllocator` block:
```rust
// in `impl AreaFrameAllocator`
fn choose_next_area(&mut self) {
self.current_area = self.areas.clone().filter(|area| {
let address = area.base_addr + area.length - 1;
@@ -330,8 +350,14 @@ If the `next_free_frame` is below the new `current_area`, it needs to be updated
We don't have a data structure to store free frames, so we can't implement `deallocate_frame` reasonably. Thus we use the `unimplemented` macro, which just panics when the method is called:
```rust
fn deallocate_frame(&mut self, _frame: Frame) {
unimplemented!()
impl FrameAllocator for AreaFrameAllocator {
fn allocate_frame(&mut self) -> Option<Frame> {
// described above
}
fn deallocate_frame(&mut self, _frame: Frame) {
unimplemented!()
}
}
```

8
blog/post/2015-12-09-modifying-page-tables.md
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@@ -101,7 +101,13 @@ features = ["no_std"]
The `no_std` feature is needed because `bitflags` depends on the standard library by default. But it has a [cargo feature] to use the core library instead. It will become the default as soon as `no_std` is stable in a stable Rust release.
[cargo feature]: http://doc.crates.io/manifest.html#the-[features]-section
Note that you need a `#[macro_use]` above the `extern crate` definition.
To import the macro, we need to use `#[macro_use]` above the `extern crate` definition:
```rust
// in src/lib.rs
#[macro_use]
extern crate bitflags;
```
Now we can model the various flags:

16
blog/post/2016-04-11-kernel-heap.md
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@@ -320,7 +320,7 @@ let heap_test = Box::new(42);
When we try to compile it using `make run`, we get several linker errors about a function named `_Unwind_Resume`:
```
target/x86_64-unknown-linux-gnu/debug/libblog_os.a(bump_allocator-947b648f2a584929.0.o):
target/x86_64-unknown-linux-gnu/debug/libblog_os.a(bump_allocator-[…].0.o):
In function `bump_allocator::__rust_allocate':
/home/…/blog_os/libs/bump_allocator/src/lib.rs:19:
undefined reference to `_Unwind_Resume'
@@ -461,8 +461,8 @@ The crate provides an [assert_has_not_been_called!] macro (sorry for the long na
pub fn init(boot_info: &BootInformation) {
assert_has_not_been_called!("memory::init must be called only once");
let memory_map_tag = …
…
let memory_map_tag = ...
...
}
```
That's it. Now our `memory::init` function can only be called once. The macro works by creating a static [AtomicBool] named `CALLED`, which is initialized to `false`. When the macro is invoked, it checks the value of `CALLED` and sets it to `true`. If the value was already `true` before, the macro panics.
@@ -481,7 +481,7 @@ pub fn remap_the_kernel<A>(allocator: &mut A, boot_info: &BootInformation)
-> ActivePageTable // new
where A: FrameAllocator
{
…
...
println!("guard page at {:#x}", old_p4_page.start_address());
active_table // new
@@ -494,9 +494,9 @@ Now we have full page table access in the `memory::init` function. This allows u
// in src/memory/mod.rs
pub fn init(boot_info: &BootInformation) {
…
...
let mut frame_allocator = …;
let mut frame_allocator = ...;
// below is the new part
@@ -520,10 +520,10 @@ The `Page::range_inclusive` function is just a copy of the `Frame::range_inclusi
// in src/memory/paging/mod.rs
#[derive(…, PartialEq, Eq, PartialOrd, Ord)]
pub struct Page {…}
pub struct Page {...}
impl Page {
…
...
pub fn range_inclusive(start: Page, end: Page) -> PageIter {
PageIter {
start: start,