gdt.c

#include <gdt.h>

gdt_entry_t gdt[10];
gdt_ptr_t   gp;

void gdt_install(void);
void gdt_set_gate(int num, u32_t, u32_t, u8_t, u8_t);

/* Setup a descriptor in the Global Descriptor Table */
void gdt_set_gate(int num, u32_t base, u32_t limit, u8_t access, u8_t 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;
}

void gdt_install(void)
{
    /* Setup the GDT pointer and limit */
    gp.limit = sizeof(gdt_entry_t)*3 - 1;
    gp.base = (u32_t)&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_set_gate(3, 4, 0xFFFFFFF0, 0x92, 0xCF);

    gdt_flush((u32_t)&gp);
}