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Posted: **Mon Sep 18, 2006 5:09 pm**

Hi,
I'm trying to follow the HigherHalf with GDT tutorial but I've run into a small problem. My Loader (boot.asm -> loader.o) reports it can't find the GP Struct (which I define in gdt.c and also extern in gdt.h) and gdt.c reports it can't find the function (which is created in the boot.asm) gdt_flush() which it prototypes.
What stupidly obvious thing am I missing here?

Posted: **Mon Sep 18, 2006 6:44 pm**

Sorry having an off day

Forgot to include any code

gdt.c -

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#include "../includes/gdt.h"

// We'll need at least 3 entries in our GDT...

struct gdt_entry gdt[3];
struct gdt_ptr gp;

// Extern assembler function
void gdt_flush();

// Very simple: fills a GDT entry using the parameters
void gdt_set_gate(int num, unsigned long base, unsigned long limit, unsigned char access, unsigned char gran)
{
        gdt[num].base_low = (base & 0xFFFF);
        gdt[num].base_middle = (base >> 16) & 0xFF;
        gdt[num].base_high = (base >> 24) & 0xFF;

        gdt[num].limit_low = (limit & 0xFFFF);
        gdt[num].granularity = ((limit >> 16) & 0x0F);

        gdt[num].granularity |= (gran & 0xF0);
        gdt[num].access = access;
}

// Sets our 3 gates and installs the real GDT through the assembler function
void gdt_install()
{
        gp.limit = (sizeof(struct gdt_entry) * 6) - 1;
        gp.base = (unsigned int)&gdt;

        gdt_set_gate(0, 0, 0, 0, 0);
        gdt_set_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF);
        gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF);

        gdt_flush();
};

gdt.h -

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#ifndef KERNEL_GDT_H
#define KERNEL_GDT_H

/*=============
   PROTOTYPES
==============*/

void gdt_install();


/*=============
   STRUCTURES
==============*/

struct gdt_entry
{
        unsigned short limit_low;
        unsigned short base_low;
        unsigned char base_middle;
        unsigned char access;
        unsigned char granularity;
        unsigned char base_high;
} __attribute__((packed));

struct gdt_ptr
{
        unsigned short limit;
        unsigned int base;
} __attribute__((packed));

/*=============
   EXTERNALS
==============*/

extern struct gdt_ptr gp;

#endif

Linking batch file -

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@echo off
set DJGPP=C:\DJGPP\DJGPP.ENV
set PATH=C:\DJGPP\BIN;%PATH%

echo Building Bootstrap...
nasmw -f elf boot.asm -o Release\loader.o

move Kernel\*.o .\Release
move Kernel\mem\*.o .\Release

echo Linking...

ld-elf -T kernel.ld -o Release\kernel.bin Release\loader.o Release\gdt.o Release\paging.o Release\main.o

Linker Script and Boot.asm are exactly as appear in the tutorial.

Posted: **Wed Sep 20, 2006 8:41 am**

I resolved it incidentally and thought I'd better include it here for anyone searching at a later date.

I simply renamed all instances of gdt_flush and gp in the boot.asm to _gdt_flush and _gp respectivly

Posted: **Wed Sep 20, 2006 8:51 am**

elaverick wrote: I resolved it incidentally and thought I'd better include it here for anyone searching at a later date.

I simply renamed all instances of gdt_flush and gp in the boot.asm to _gdt_flush and _gp respectivly

aah. the good old issue of auto-prepend'd underscores ... DOS & WIndows export C main() as "_main" while the rest of the known universe exports it as "main" ... if you're using GCC, you can use -fno-leading-underscore so that your cygwin/djgpp/mingw behave likes the rest of known universe (alternatively, you can use -fleading-underscore to ease following a cygwin/djgpp tutorial under linux).

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