Higher Half Kernel
It is traditional and generally good to have your kernel mapped in every user process. Linux, for instance (and many other unices) reside at the virtual addresses 0xC0000000..0xffffffff of every address space, leaving the range 0x00000000..0xbfffffff for user code, data, stacks, libraries, etc. Kernels that have such design are said to be "in the higher half" by opposition to kernels that use lowest virtual addresses for themselves, and leave higher addresses for the applications.
Advantages of a higher half kernel are:
- Easier to set up VM86 processes since the region below 1MB is userspace.
- More generically, user applications are not dependent of how many memory is kernel space (Your app can be linked for 0x400000 regardless of whether kernel is at 0xC0000000, 0x80000000 or 0xE0000000 ...), which makes ABI's nicer.
- If your OS also supports 64-bits, 32-bit applications will be able to use the full 32-bit address space in the 64-bit version.
- 'mnemonic' invalid pointers such as 0xcafebabe, 0xdeadbeef, 0xdeadc0de, etc. can be used.
Initialization
To setup a higher half kernel, you have to map your kernel to the appropriate virtual address. How to do this basically depends on when you'd like your kernel to believe it's in the higher end, and when you set up paging.
Custom Bootloader
The easiest way is to load your kernel to any physical location you wish (for instance in the lowest 1MB) and prepare page tables that will perform the appropriate translation. Let's say you loaded your kernel starting at 0x00010000 up to 0x0009ffff and want it to appear at 0xC0010000.
- Pick 3 4096-aligned addresses where you'll put your page directory and system tables. Wipe them (with zeroes).
- Fill the lowest 256 entries of one table to set up Identity Paging for at least the BIOS and your bootloader (I'd even use 1:1 mapping for the whole lowest 1MB if I were you :)).
- In the other table, fill entry #0x10 with 0x00010003, entry #0x11 with 0x00011003 ... (For as much pages as your kernel has.).
- Fill the entry #0 of the directory with the address of the first table (and make it present).
- Fill the entry #768 of the directory with the address of the second table (and make it present).
When switching to Protected Mode, use this assembly example:
mov eax, physical_address_of_the_directory mov cr3,eax mov eax,cr0 or eax,0x80000001 ; enables both pmode and paging mov cr0,eax
Tim Robinson's GDT Trick
If we don't want to enable paging right from the start, it is still possible to have your kernel appearing in the higher half by using an appropriate base for the code and data segments. Say we have loaded the kernel at 0x10000 and we want it to appear at 0xC0000000. All we need is to find a base _X_ such as _X_+0xC0000000 = 0x10000. The bootloader will then initialize the GDT with cs.base = 0x40010000 = ds.base. This also means that special care must be taken for vram access, as (void*)0xb8000 is now somewhere above 1GB. Either use a special 0-based additional data-segment or use
#define logical_to_physical(x) (((void*)x)+0x40010000) #define physical_to_logical(x) (((void*)x)-0x40010000) short* vram=physical_to_logical(0xb8000)
When you eventually enable paging, create the page tables as mentioned above (keeping in mind that you need to undo the address conversion again, e.g.
pgentry *pagedirectory=physical_to_logical(0x9D000); pgentry *lowesttable=physical_to_logical(0x9C000); pgentry *kerneltable=physical_to_logical(0x9B000); /* prepare lowesttable and kerneltable */ pagedirectory[0]=mkpgentry(0x9C000,PG_PRESENT|whatever); pagedirectory[0xC0000000>>12]=mkpgentry(0x9B000,PG_PRESENT|PG_SYSTEM|whatever); set_cr3_and_update_gdt(0x9D000);
Along with setting CR3's value, you'll have to clear the base of all the previous segments and reload segment registers so that, at the exit of set_cr3_and_update_gdt, any memory references now use the 0-based code and data segment and that the page tables perform the translation required to still make 0x10000 appear at 0xC0000000.
With GRUB From the Start
GRUB will load your kernel at the desired physical target address, but it will leave segments in the GDT with a 0 base and it will not enable paging.
With GRUB Later On
Even with GRUB it's still possible to use TimRobinson's GDT trick: just set up a "fake" GDT then jump to your C code and enable paging. See Higher Half With GDT