descriptor_tables.c

/* descriptor_talbles.c - Initialises the GDT and IDT Defines the default ISR
 *                        and IRQ handler.
 *
 * This work is licensed under the Creative Commons Attribution-NonCommercial-
 * ShareAlike 3.0 Unported License. To view a copy of this license, visit
 * http://creativecommons.org/licenses/by-nc-sa/3.0/ or send a letter to:
 * Creative Commons
 * 444 Castro Street, Suite 900
 * Mountain View, California, 94041, USA.
 *
 * Author(s): Drew Cross <drew@ddcross.com>
 */

#include "common.h"
#include "descriptor_tables.h"

// Allow access the ASM functions from C code
//
extern void gdt_flush(u32int);

// Internal function prototypes
static void init_gdt();
static void gdt_set_gate(s32int, u32int, u8int, u8int);

gdt_entry_t gdt_enteries[5];
gdt_ptr_t gdt_ptr;
idt_entry_t idt_entries[256];
idt_ptr_t idt_ptr;


void init_descrpitor_tables()
{
    // Initializes the GDT
    init_gtd();
}

static void init_gdt()
{
    gdt_ptr.limit = (sizeof(gdt_entry_t) *5) -1;
    gdt_ptr.base = (u32int)&gdt_entries;

    gdt_set_gate(0, 0, 0, 0, 0);
    gdt_set_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF); // Code segment
    gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF); // Data segment
    gdt_set_gate(3, 0, 0xFFFFFFFF, 0xFA, 0xCF); // User mode code segment
    gdt_set_gate(4, 0, 0xFFFFFFFF, 0xF2, 0xCF); // User mode data segment

    gdt_flush((u32int)&gdt_ptr);
}


tatic void gdt_set_gate(s32int num, u32int base, u32int limit, u8int access, u8int gran)
{
    gdt_entries[num].base_low    = (base & 0xFFFF);
    gdt_entries[num].base_middle = (base >> 16) & 0xFF;
    gdt_entries[num].base_high   = (base >> 24) & 0xFF;

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

    gdt_entries[num].granularity |= gran & 0xF0;
    gdt_entries[num].access      = access;
}