Add skeleton for next post

posts/DRAFT-multiboot-module.md

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+---
+layout: post
+title: 'A Multiboot Module'
+---
+TODO
+
+## The Multiboot Information Structure
+When a multiboot compliant bootloader loads a kernel, it passes a pointer to a boot information struct in the `ebx` register. We can use it to get information about available memory and loaded kernel sections. So let's write a module for it!
+
+TODO
+
+## Start and End of Kernel
+We can now use the ELF section tag to calculate the start and end address of our loaded kernel:
+
+TODO
+
+## A frame allocator
+When we create a paging module in the next post, we will need to map virtual pages to free physical frames. So we will need some kind of allocator that keeps track of physical frames and gives us a free one when needed. We can use the memory tag to write such a frame allocator.
+
+The allocator struct looks like this:
+
+```rust
+struct AreaFrameAllocator {
+    first_used_frame: Frame,
+    last_used_frame: Frame,
+    current_area: Option<MemoryArea>,
+    areas: MemoryAreaIter,
+}
+```
+TODO
+
+To allocate a frame we try to find one in the current area and update the first/last used bounds. If we can't find one, we look for the new area with the minimal start address, that still contains free frames. If the current area is `None`, there are no free frames left.
+
+TODO
+
+### Unit Tests
+TODO
+
+## Remapping the Kernel Sections
+We can use the ELF section tag to write a skeleton that remaps the kernel correctly:
+
+```rust
+for section in multiboot.elf_tag().sections() {
+    for page in start_page..end_page {
+        // TODO identity_map(page, section.writable(), section.executable())
+    }
+}
+```
+TODO