Add a FrameAllocator trait

posts/DRAFT-allocating-frames.md

@@ -200,6 +200,16 @@ impl Frame {
 }
 ```
 
+We also add a `FrameAllocator` trait:
+
+```rust
+pub trait FrameAllocator {
+    fn allocate_frame(&mut self) -> Option<Frame>;
+    fn deallocate_frame(&mut self, frame: Frame);
+}
+```
+This allows us to create another, more advanced frame allocator in the future.
+
 ### The Allocator
 Now we can put everything together and create the frame allocator. It looks like this:
 
@@ -248,7 +258,7 @@ fn choose_next_area(&mut self) {
 ```
 
 ```rust
-pub fn allocate_frame(&mut self) -> Option<Frame> {
+fn allocate_frame(&mut self) -> Option<Frame> {
     match self.current_area {
         None => None,
         Some(area) => {

src/lib.rs

@@ -58,6 +58,7 @@ pub extern fn rust_main(multiboot_information_address: usize) {
         kernel_end as usize, multiboot_start, multiboot_end, memory_map_tag.memory_areas());
 
     for i in 0.. {
+        use memory::FrameAllocator;
         if let None = frame_allocator.allocate_frame() {
             println!("allocated {} frames", i);
             break;

src/memory/area_frame_allocator.rs

@@ -1,4 +1,4 @@
-use memory::Frame;
+use memory::{Frame, FrameAllocator};
 use multiboot2::{MemoryAreaIter, MemoryArea};
 
 /// A frame allocator that uses the memory areas from the multiboot information structure as
@@ -33,7 +33,16 @@ impl AreaFrameAllocator {
         allocator
     }
 
-    pub fn allocate_frame(&mut self) -> Option<Frame> {
+    fn choose_next_area(&mut self) {
+        self.current_area = self.areas.clone().filter(|area| {
+            let address = area.base_addr + area.length - 1;
+            Frame::containing_address(address as usize) >= self.next_free_frame
+        }).min_by(|area| area.base_addr);
+    }
+}
+
+impl FrameAllocator for AreaFrameAllocator {
+    fn allocate_frame(&mut self) -> Option<Frame> {
         match self.current_area {
             None => None,
             Some(area) => {
@@ -58,10 +67,5 @@ impl AreaFrameAllocator {
         }
     }
 
-    fn choose_next_area(&mut self) {
-        self.current_area = self.areas.clone().filter(|area| {
-            let address = area.base_addr + area.length - 1;
-            Frame::containing_address(address as usize) >= self.next_free_frame
-        }).min_by(|area| area.base_addr);
-    }
+    fn deallocate_frame(&mut self, _frame: Frame) {unimplemented!()}
 }

src/memory/mod.rs

@@ -14,3 +14,8 @@ impl Frame {
         Frame{ number: address / PAGE_SIZE }
     }
 }
+
+pub trait FrameAllocator {
+    fn allocate_frame(&mut self) -> Option<Frame>;
+    fn deallocate_frame(&mut self, frame: Frame);
+}