Booting and entering long mode help please

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aaa

Joined: 01 Feb 2010
Posts: 1

aaa 21 Sep 2013, 21:19

Sorry for my english.

Long mode OS development project. Kernel will loaded at 0001:0000. Kernel consist of two parts: 64-bit kernel & 16-bit loader (init). So I want to get something like this:

Code:

org 10000h ; <<<< PROBLEM

use16
start:
jmp loader

use64
kernel:
; 64-bit code here
**********

use16
loader:
; 16-bit init code here
**********
jmp kernel ; go to kernel

I can't do this because of "org". If I write "org 0", 64-bit code will run with wrong base. If I write "org 10000h", 16-bit code can't be compiled.

Please, help me.

21 Sep 2013, 21:19

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20761
Location: In your JS exploiting you and your system

revolution 21 Sep 2013, 21:52

In 16-bit mode "0001:0000" is linear address 0x1000 (1 shl 12). But regardless, you need to use org 0 for the 16-bit code because your IP is zero. And use a long mode segment that maps to address 0x1000.

There are already code examples on this board about booting and entering long mode. A search might generate some fruitful source code for you.

21 Sep 2013, 21:52

Bargest

Joined: 09 Feb 2012
Posts: 79
Location: Russia

Bargest 22 Sep 2013, 21:56

I misunderstand why do you need to place loader after kernel. Also, you cannot switch direclty in long-mode without using protected (x32) mode.
About your question, you can use a trick:

Code:

org 0h ; as revolution said - "0001:0000" -> ip == 0
start16:
; code
jmp sel32:code32
end16:

org 1000h + end16 - start16
code32:
; 32-bit code

22 Sep 2013, 21:56

SeproMan

Joined: 11 Oct 2009
Posts: 74
Location: Belgium

SeproMan 25 Sep 2013, 10:24

revolution wrote :

Quote:

In 16-bit mode "0001:0000" is linear address 0x1000 (1 shl 12).

0001 represents the segment
0000 represents the offset

So this address actually means linear address 16 (NOT 0x1000 = 4096)

_________________
Real Address Mode.

25 Sep 2013, 10:24

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20761
Location: In your JS exploiting you and your system

revolution 26 Sep 2013, 07:44

SeproMan wrote:

revolution wrote :

Quote:

In 16-bit mode "0001:0000" is linear address 0x1000 (1 shl 12).

0001 represents the segment
0000 represents the offset

So this address actually means linear address 16 (NOT 0x1000 = 4096)

Thank you for the correction.

26 Sep 2013, 07:44

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