gdt.c

/*
 * =====================================================================================
 *
 *       Filename:  gdt.c
 *
 *    Description:  
 *
 *        Version:  1.0
 *        Created:  Wednesday 15 October 2014 02:52:30  IST
 *       Revision:  none
 *       Compiler:  gcc
 *
 *         Author:  YOUR NAME (), 
 *   Organization:  
 *
 * =====================================================================================
 */

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));



/* Our GDT, with 3 entries, and finally our special GDT pointer */

struct gdt_entry gdt[3];

struct gdt_ptr gp;



/* This is in start.asm. We use this to properly reload

*  the new segment registers */

extern void gdt_flush();



/* Setup a descriptor in the Global Descriptor Table */

void gdt_set_gate(int num, unsigned long base, unsigned long limit, unsigned char access, unsigned char gran)

{

    /* Setup the descriptor base address */

    gdt[num].base_low = (base & 0xFFFF);

    gdt[num].base_middle = (base >> 16) & 0xFF;

    gdt[num].base_high = (base >> 24) & 0xFF;



    /* Setup the descriptor limits */

    gdt[num].limit_low = (limit & 0xFFFF);

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



    /* Finally, set up the granularity and access flags */

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

    gdt[num].access = access;

}



/* Should be called by main. This will setup the special GDT

*  pointer, set up the first 3 entries in our GDT, and then

*  finally call gdt_flush() in our assembler file in order

*  to tell the processor where the new GDT is and update the

*  new segment registers */

void gdt_install()

{

    /* Setup the GDT pointer and limit */

    gp.limit = (sizeof(struct gdt_entry) * 3) - 1;

    gp.base = &gdt;



    /* Our NULL descriptor */

    gdt_set_gate(0, 0, 0, 0, 0);



    /* The second 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(1, 0, 0xFFFFFFFF, 0x9A, 0xCF);



    /* The third entry is our Data Segment. It's EXACTLY the

    *  this entry's access byte says it's a Data Segment */

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



    /* Flush out the old GDT and install the new changes! */

    gdt_flush();

}