Improve explanation for BootInfoFrameAllocator::usable_frames (#897)

blog/content/edition-2/posts/09-paging-implementation/index.md

@@ -920,7 +920,7 @@ This function uses iterator combinator methods to transform the initial `MemoryM
 - First, we call the `iter` method to convert the memory map to an iterator of [`MemoryRegion`]s.
 - Then we use the [`filter`] method to skip any reserved or otherwise unavailable regions. The bootloader updates the memory map for all the mappings it creates, so frames that are used by our kernel (code, data or stack) or to store the boot information are already marked as `InUse` or similar. Thus we can be sure that `Usable` frames are not used somewhere else.
 - Afterwards, we use the [`map`] combinator and Rust's [range syntax] to transform our iterator of memory regions to an iterator of address ranges.
-- The next step is the most complicated: We convert each range to an iterator through the `into_iter` method and then choose every 4096th address using [`step_by`]. Since 4096 bytes (= 4 KiB) is the page size, we get the start address of each frame. The bootloader page aligns all usable memory areas so that we don't need any alignment or rounding code here. By using [`flat_map`] instead of `map`, we get an `Iterator<Item = u64>` instead of an `Iterator<Item = Iterator<Item = u64>>`.
+- Next, we use [`flat_map`] to transform the address ranges into an iterator of frame start addresses, choosing every 4096th address using [`step_by`]. Since 4096 bytes (= 4 KiB) is the page size, we get the start address of each frame. The bootloader page aligns all usable memory areas so that we don't need any alignment or rounding code here. By using [`flat_map`] instead of `map`, we get an `Iterator<Item = u64>` instead of an `Iterator<Item = Iterator<Item = u64>>`.
 - Finally, we convert the start addresses to `PhysFrame` types to construct the an `Iterator<Item = PhysFrame>`.
 
 [`MemoryRegion`]: https://docs.rs/bootloader/0.6.4/bootloader/bootinfo/struct.MemoryRegion.html
@@ -951,7 +951,7 @@ unsafe impl FrameAllocator<Size4KiB> for BootInfoFrameAllocator {
 }
 ```
 
-We first use an `usable_frames` method to get an iterator of usable frames from the memory map. Then, we use the [`Iterator::nth`] function to get the frame with index `self.next` (thereby skipping `(self.next - 1)` frames). Before returning that frame, we increase `self.next` by one so that we return the following frame on the next call.
+We first use the `usable_frames` method to get an iterator of usable frames from the memory map. Then, we use the [`Iterator::nth`] function to get the frame with index `self.next` (thereby skipping `(self.next - 1)` frames). Before returning that frame, we increase `self.next` by one so that we return the following frame on the next call.
 
 [`Iterator::nth`]: https://doc.rust-lang.org/core/iter/trait.Iterator.html#method.nth