GDT Help

[beyondsociety]

Im at the point where I want to implement the gdt trick for addressing 4GB of memory without enabling paging. How do I set this up?

0xA0000000 + base = 0x100000
base = 0x100000 - 0xA0000000
base = -9FF00000 = 60100000

Do I have to add this base to the gdt segments: code, data? Or do I need to add the base to the gdt pointer that gets loaded by lgdt? For instance:

Code: Select all

gdt_ptr: gdt_end - gdt_start - 1 ; Limit
gdt ;base

[Adek336]

Only GDT.base.

Look:
JMP CS:0xA000_0000

CS.base = 0x6010_0000

CS.base + offset = 0x6010_0000 + 0xA000_0000 = 0x1_0010_0000.

When you put 1_0010_0000 into a longword you get 10_0000. So what you have to do, is to get this overflow. By setting the GDT.base to 6010_0000. Not GDTR. Do you understand how this trick works?

Btw, you don?t access 4Gb with this technique, all it is useful for is to allow a kernel linked at 0xa000_0000 init paging.

Cheers,
Adrian.

[beyondsociety]

Do you understand how this trick works?

Could you explain it more throughly.

[Adek336]

JMP CS:0xA000_0000
the CPU puts the logical adress into a longword:

A000_0000

adds the cs.base:

A000_0000
6010_0000 +

and the value is

0010_0000 because the 33 bit is not saved in a longword.

So it is an overflow, where the 33 bit is ignored.

Cheers,
Adrian.

[Pype.Clicker]

there's a nice previous thread which discuss the whole trick. Hope it'll help