Implement higher half kernel

[robert-w-gries]

Per OSDev:

• Map kernel to 0xC0100000
• Follow OSDev's guide
• Note: next_table_address doesn't always yield a canonical address - sign extension bug phil-opp/blog_os#362 reveals an issue with next_table_address sign extension

[IsaacWoods]

Since this is aimed at Long Mode only, I would argue we should make use of more of the virtual address space. Why not map the kernel at 0xffffffff80000000 (-2GiB), as with many 64-bit kernels; this allows the compiler to use relative relocations (the signed offsets fit into the ELF fields) when compiling the kernel as well.

[robert-w-gries]

Hey @IsaacWoods, my kernel supports both 32 bit and 64 bit targets. I think you may have wandered into my repo through your blog_os issue :-)

Your suggestion is good for 64 bit targets, and I'll look into using that mapping when I implement the higher half kernel

[IsaacWoods]

So I have! Very sorry; completely missed that this was a different repo somehow!

Edit: feel free to delete these comments - didn't mean to clutter your issue tracker!

[robert-w-gries]

No need to apologize! I appreciate the contribution