problems entering protected mode

[Seven11]

My second bootloader is loaded from a FAT12 disk by the bootsector to the addr 10000h and it uses this code:

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cli
o32 lgdt[gdtr]
o32 lidt[idtr]
mov eax,cr0
or al,1
mov cr0,eax
jmp code32sel:pmode
bits32
pmode:
jmp $

To get into pmode, this code has always worked but now when I changed the bootloader addr from 7e00h (right after the bootsector) to 10000h it stopped working. Also I can say that all the segments are cleared before running the above code except CS which is 1000h.

Also, bochs tells me this:
"fetch_raw_descriptor: GDT: index (f)1 > limit (0)"

Looking at the NASM output for the lgdt and lidt instructions:

000023A2 660F0116[D621] o32 lgdt[gdtr]
000023A8 660F011E[F421] o32 lidt[idtr]

makes me think that the cpu never get's the GDT or IDT table since they are located att addr 121D6h (GDT) and 121F4h (IDT)...

any good solutions?

[Brendan]

Hi,

Which assembler are you using, what do you use for "ORG", what is in DS when this code is run and what data is at gdtr and idtr?


Cheers,

Brendan

[Dex]

First are you setting any org in your second stage and if so, what is it ?
Next bubach had a problem like this, with BOS have a look at that code to see how he fixed it. http://bos.asmhackers.net/downloads.php

I think he had to do something like this:

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	  lgdt	  [gdtr - 0x10000]		      ; Load the GDT descriptor
	  lidt	  [idtr - 0x10000]		      ; Load the IDT descriptor

PS: Whats the "o32" ?

[Seven11]

brendan: as I wrote, the base addr och the bootloader is 10000h and "all the segments are cleared before running the above code except CS which is 1000h." => DS = 0. The assembler is NASM.

Dex: "o32" tells NASM to use 32-bits operand (in other words adding the instruction prefix 66h), look in the Instruction Set Reference from Intel on the instruction "LGDT/LIDT".

The GDT table looks like this:

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%macro GDT_ENTRY 6
  %%LIMIT_15_0 dw %1
  %%BASE_15_0 dw %2
  %%BASE_23_16 db %3
  %%OPT_1 db %4
  %%OPT_2_LIMIT_19_16 db %5
  %%BASE_31_24 db %6
%endmacro

gdtr
  dw gdt_end-gdt-1
  dd gdt
gdt
   nullseg equ ($-gdt)
   GDT_ENTRY 0h, 0h, 0h, 0h, 0h, 0h ;nullseg
   code32sel equ ($-gdt)
   GDT_ENTRY 0FFFFh, 0h, 0h, 9ah, 0cfh, 0h ;Kernel Code
   data32sel equ ($-gdt)
   GDT_ENTRY 0FFFFh, 0h, 0h, 92h, 0cfh, 0h ;Kernel
gdt_end

PS! I have removed the "lidt" instruction for now to make debugging easier, it will be added again when the lgdt thing works.

PS #2! BOS seems to be entering Pmode in the bootsector, this is not an option for me because the bootloader probes the BIOS for VESA, APM and Memory information...

One last option is to move back the bootloader to under the 10000h mark, but shouldn't it be possible to get into Pmode when your above 10000h???

[Brendan]

Hi,

brendan: as I wrote, the base addr och the bootloader is 10000h and "all the segments are cleared before running the above code except CS which is 1000h." => DS = 0. The assembler is NASM.

The "o32" will change the operand size, but won't change the address size. To do that you'd need:

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     o32 lgdt   [dword gdtr]          ; Load the GDT descriptor

In this case, NASM should generate an "o32 lgdt [0x0001????] instruction, which is what you want (because DS base is zero), but will also cause a general protection fault (because the address is above the 64 KB limit for DS).

To fix this, either:

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     mov ax,0x1000
     mov ds,ax
     o32 lgdt   [gdtr]          ; Load the GDT descriptor

Or perhaps:

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     o32 lgdt   [cs:gdtr]          ; Load the GDT descriptor

You should also have a problem with your far jump, which probably needs to be:

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     jmp dword code32sel:pmode

Cheers,

Brendan

[Seven11]

for some reason:

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o32 lgdt   [dword gdtr]          ; Load the GDT descriptor

doesn't work... however...

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 jmp dword code32sel:pmode

did it

so If anybody else has the problem here is the code to solve it:

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;Enter Pmode from above 10000h
cli
o32 lgdt[gdtr] ;assumes BASE(DS)*16+gdtr points to GDT structure.
o32 lgdt[idtr]  ;BASE(DS)*16+idtr should point to the IDT structure.
mov eax,cr0
or al,1
mov cr0,eax
jmp dword code32sel:pmode ;where "code32sel" is your 32-bit kernel code seg.
bits 32
pmode:
mov ax,data32sel ;where "data32sel" is your 32-bit kernel data seg.
mov ds,ax
mov es,ax
mov fs,ax
mov gs,ax
mov ss,ax

Thanks for the help!