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;asm
_gdt_flush:
    lgdt [_gp]        ; Load the GDT with our '_gp' which is a special pointer
    mov ax, 0x8      ; 0x10 is the offset in the GDT to our data segment
    mov ds, ax
    mov es, ax
    mov fs, ax
    mov gs, ax
    pop edx
    pop ecx
    mov ax,0x18 ;stack gets special selector
    mov ss,ax
    jmp 0x10:flush2   ; 0x08 is the offset to our code segment: Far jump!
flush2:
mov esp,0x1000 ;now if we go past the stack limit we get a nice page fault
mov ebp,0x1000
push ecx
push edx
    ret

//C code
void gdt_install()
{
    /* Setup the GDT pointer and limit */
    gp.limit = (sizeof(struct gdt_entry) * 4) - 1;
    gp.base = &gdt;

    /* Our NULL descriptor */
    gdt_set_gate(0, 0, 0, 0, 0);

    /* The thrid entry is our Code Segment. The base address
    *  is 0, the limit is 4GBytes, it uses 4KByte granularity,
    *  uses 32-bit opcodes, and is a Code Segment descriptor.
    *  Please check the table above in the tutorial in order
    *  to see exactly what each value means */
    gdt_set_gate(2, 0, 0xFFFFFFFF, 0x9A, 0xCF);

    /* The second entry is our Data Segment. It's EXACTLY the
    *  same as our code segment, but the descriptor type in
    *  this entry's access byte says it's a Data Segment */
    gdt_set_gate(1, 0, 0xFFFFFFFF, 0x92, 0xCF);
    /*The fourth entry is our stack segment. The base address
    * is 0x1000, the limit is 0x1000 or 1 4k page(1024 stack entries), it uses 4Kbyte 
    * granularity, is 32bit, and is a data/stack segment
    */
    gdt_set_gate(3,0x1000,1,0x92,0xCF);

    /* Flush out the old GDT and install the new changes! */
   
    gdt_flush();
    __asm("pop %eax");
    __asm("ret");
}

Code: Select all

    gdt_flush();
    __asm("pop %eax");
    __asm("ret");