Pmode help

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xleelz

Joined: 12 Mar 2011
Posts: 86
Location: In Google Code Server... waiting for someone to download me

xleelz 10 Jun 2012, 21:40

Been way too long since I've tried my hand at OS Dev..
I'm working on a 2 stage FAT12 bootloader, and upon reaching PMode in the second stage, it seems to just restart... any idea what went wrong?
here's the code:

Code:

use16
org 500

start:
       cli
 xor ax,ax
   mov ds,ax
   mov fs,ax
   mov gs,ax
   mov es,ax
   mov ax,0x9000
       mov ss,ax
   mov sp,0xffff


       pusha
       lgdt [gdtr]
 popa

    mov eax,cr0
 or eax,1
    mov cr0,eax

     jmp codeSeg:Pmode

gdt:
       dq 0
        
codeSeg:
    dw 0xffff
   dw 0
        db 0
        db 10011010b
        db 11001111b
        db 0
dataSeg:
        dw 0xffff
   dw 0
        db 0
        db 10010010b
        db 11001111b
        db 0
gdtEnd:

gdtr:
        dw gdtEnd - gdt - 1
 dd gdt

use32

Pmode:
       mov ax,dataSeg
      mov es,ax
   mov fs,ax
   mov gs,ax
   mov ds,ax
   mov ss,ax
   mov esp,90000h

  mov edi, 0xB8000
    mov ah,1fh
  mov al,'A'
        mov word [edi],ax

loopr:
 hlt
 jmp loopr

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10 Jun 2012, 21:40

cod3b453

Joined: 25 Aug 2004
Posts: 618

cod3b453 10 Jun 2012, 21:59

The pusha/popa around the lgdt isn't needed. You probably just need to rebase the selectors:

Code:

; ...
jmp (codeSeg - gdt):Pmode
; ...
mov ax,(dataSeg - gdt)
; ...

10 Jun 2012, 21:59

xleelz

Joined: 12 Mar 2011
Posts: 86
Location: In Google Code Server... waiting for someone to download me

xleelz 11 Jun 2012, 01:02

thanks for the quick response but it still reboots... I did some debugging and it seems to make it into protected mode, fs and gs aren't set to the data segment but ds, ss, and esp are..

11 Jun 2012, 01:02

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20632
Location: In your JS exploiting you and your system

revolution 11 Jun 2012, 02:03

xleelz wrote:

Code:

        mov sp,0xffff

It is probably a good idea to align your stack properly.

Perhaps this instead:

Code:

xor sp,sp ;sp=0 to give a 64k stack aligned to 0 mod 4

11 Jun 2012, 02:03

BAiC

Joined: 22 Mar 2011
Posts: 272
Location: California

BAiC 11 Jun 2012, 07:12

"codeSeg" is a label just as much as Pmode (Pmode is the label you want)...

codeSeg needs to be a selector.. from your code it looks like it needs to be 8..

Code:

jmp 8:Pmode

I agree with cod3b453; get rid of the pusha/popa. I also agree with revolution; align your stack...

furthermore; your "hlt" loop at the end is terminal unless you enable interrupts.. since you don't have an IDT that won't actually matter.

11 Jun 2012, 07:12

sinsi

Joined: 10 Aug 2007
Posts: 794
Location: Adelaide

sinsi 11 Jun 2012, 07:22

"org 500" - how do you load it there? That's part of the interrupt table.
Possibly at the top of 640K there is the EBDA, maybe your stack is causing some problem?

11 Jun 2012, 07:22

xleelz

Joined: 12 Mar 2011
Posts: 86
Location: In Google Code Server... waiting for someone to download me

xleelz 11 Jun 2012, 15:07

Quote:

Possibly at the top of 640K there is the EBDA, maybe your stack is causing some problem?

maybe, but it does seem to work past that point. Debugging says it reboots when it loads the gdt, apparently it's invalid.

Thanks for all the replies and I made the respective changes.

11 Jun 2012, 15:07

cod3b453

Joined: 25 Aug 2004
Posts: 618

cod3b453 11 Jun 2012, 17:37

The IVT is at 0x00000-0x00400 (256 * (2+2)). The BDA is 0x00400-0x00500 so org 0x500 should be fine. (The EBDA is usually at 0xE0000). Also might be worth trying:

Code:

jmp pword (codeSeg - gdt):Pmode
; ...
align 4
Pmode:

11 Jun 2012, 17:37

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