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.

First, we need to pass this pointer to our kernel as an argument to rust_main. To find out how arguments are passed to functions, we can look at the calling convention of Linux:

The first six integer or pointer arguments are passed in registers RDI, RSI, RDX, RCX, R8, and R9

So to pass the pointer to our kernel, we need to move it to rdi before calling the kernel. Since we're not using the rdi/edi register in our bootstrap code right now, we can simply set the edi register right after booting (in boot.asm):

start:
    mov esp, stack_top
    mov edi, ebx       ; Move Multiboot info pointer to edi

Now we can add the argument to our rust_main:

pub extern fn rust_main(multiboot_information_address: usize) { ... }

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