GDT C code

#include <stdint.h>
#include "gdt.h"

/* defines a GDT entry */

struct gdt_descriptor {
    uint16_t limit_low;         /* bits 0-15 of segment limit */
    uint16_t base_low;          /* bits 0-23 */
    uint8_t base_middle;        /* of base address */
    uint8_t access;             /* access byte */
    uint8_t granularity;            /* granularity */
    uint8_t base_high;          /* bits 24-32 of base address */
} __attribute__ ((packed));

/* defined GDT pointer including limit:
 * the max bytes taken up by the GDT - 1
*/

struct gdt_pointer {
    uint16_t limit;             /* size of GDT */
    uint32_t base;              /* base address of GDT */
} __attribute__ ((packed));

struct gdt_descriptor  gdt_desc[MAX_DESCRIPTORS];
struct gdt_pointer     gdt_p;

int gdt_set_descriptor(uint32_t i, uint64_t base, uint64_t limit, uint8_t access, uint8_t granularity)
{
    if (i > MAX_DESCRIPTORS)
        return

    /* null out the descriptor */

    memset((void *) gdt_desc, 0, sizeof(gdt_desc));

    /* Setup the descriptor base address */
    gdt_desc[i].base_low     = (base & 0xFFFF);
    gdt_desc[i].base_middle  = (base >> 16) & 0xFF;
    gdt_desc[i].base_high    = (base >> 24) & 0xFF;

    /* Setup the descriptor limits */
    gdt_desc[i].limit_low    = (limit & 0xFFFF);
    gdt_desc[i].granularity  = ((limit >> 16) & 0x0F);

    /* Finally, set up the granularity and access flags */
    gdt_desc[i].granularity |= (granularity & 0xF0);
    gdt_desc[i].access       = access;
}

int gdt_install()
{
    gdt_p.limit = (sizeof(gdt_desc) * MAX_DESCRIPTORS) - 1;
    gdt_p.base  = (uint32_t) &gdt_desc;

    /* set null descriptor */

    gdt_set_descriptor(0, 0, 0, 0, 0);

    /* set code descriptor */

    gdt_set_descriptor(1, 0, 0xFFFFFFFF, 0x9A, 0xCF);

    /* set data descriptor */

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

    /* install the GDT */

    _gdt_install();
}