GDT Issues

[jzgriffin]

I've recently implemented the GDT from http://www.osdever.net/bkerndev. When I compile GDT.c, I get a warning that I believe is causing the renewed Virtual PC errors. Here it is:

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Source/GDT.c:75: warning: assignment makes integer from pointer without a cast

Line 75 of Source/GDT.c:

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GDTPointer.Base = &GDTEntry;

GDTPointer:

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struct TGDTPointer GDTPointer;

TGDTPointer:

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struct TGDTPointer
{
    unsigned short Limit;
    unsigned int Base;
} __attribute__((packed));

GDTEntry:

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struct TGDTEntry GDTEntry[3];

TGDTEntry:

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struct TGDTEntry
{
    unsigned short LimitLow;
    unsigned short BaseLow;
    unsigned char BaseMiddle;
    unsigned char Access;
    unsigned char Granularity;
    unsigned char BaseHigh;
} __attribute__((packed));

Any ideas what might be going wrong here?

P.S.: If you need more code, just ask.

[turtling]

If you do this: GDTPointer.Base = (unsigned int)&GDTEntry; ?
It's just a warning, it's not a problem.

[jzgriffin]

That got rid of the warning, but I still can't boot the kernel...the full source code is on SVN at svn://svn.griffinsoft.net/grakos for anyone who wants to have a look.

[oscoder]

Do you know exactly what is causing it not to boot? Do you think its in your gdt code? There are a number of things that can go wrong. In order you might like to try instruction tracing, or simply halting the cpu (in c: asm("hlt");) before the points you think might be causing the error - the first thing I'd try is putting one of these just before you load the new gdt.

[jzgriffin]

I've commented out GDTInitialize() (where GDTPointer.Base = ... is located) in Kernel.c to stop the GDT from initializing, yet the problem is still there. The results of ConsoleInitialize(), which was before GDTInitialize(), are never printed. Based on facts, I think that the problem is in Start.asm.

[Combuster]

GDTEntry[x] is one gdt entry (equivalent to TGDTEntry)
GDTEntry is an array of gdt entries
which is has the same type as TGDTEntry *
which is a pointer

by setting GDT.base to &GDTEntry you are setting the base as a pointer to a pointer to the GDT. (!)

The correct form is GDTPointer.base = (unsigned int) GDTEntry;

Besides, warnings are symptoms of bad code.

[jzgriffin]

I've done that, and it just prevents warning. Also, I've added the IDT/ISR/IRQ, and it still fails to boot. All of that's on the SVN repository. Please help?

[Combuster]

hmm, you have serious trouble:

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OUTPUT_FORMAT("binary")
ENTRY(Start)

SECTIONS
{
    .text 0xFF800000 :
    {
        *(.text)
    }
    .data :
    {
        *(.data)
    }
    .bss :
    { 					
        *(.bss)
    }
}

Linking to higherhalf, WITHOUT Stub, WITHOUT Paging, WITHOUT bootloader and you still expect it to work?

[jzgriffin]

I tried changing the linker script (I used the one from that tutorial, the second time around) and it still didn't boot. *tries again*

Edit: No luck with that. *commits to SVN anyways*

[Combuster]

Just changing things randomly wont get you anywhere... Methinks you have no clue about the changes you just did to your linker script

What happens, your binary is loaded at some address (i dont know which - you dont use grub and your own bootloader has no sourcecode) but the code is designed to run at a different address. ATM its set to the 1MB mark, which simple bootloaders cant access as it lies outside bill gates' famous 640k. Check with whatever proprietary code you use where your kernel is actually loaded, and put the correct value into your linker script.

If that does not help, please post the bootloader's specs when you come back.

[Brynet-Inc]

He's using bootf02 last time I checked Combuster.. He also has a custom one in his repository.