blog_os/posts/DRAFT-remapping-the-kernel.md

layout, title

layout	title
post	Remapping the Kernel

TODO

The Multiboot Information Structure

When a Multiboot compliant bootloader loads a kernel, it passes a pointer to a boot information structure in the ebx register. We can use it to get information about available memory and loaded kernel sections.

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:

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:

for section in multiboot.elf_tag().sections() {
    for page in start_page..end_page {
        // TODO identity_map(page, section.writable(), section.executable())
    }
}

TODO