gdt.cpp

/*
 * Copyright (c) 2012 Marco Hinz
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 * - Redistributions of source code must retain the above copyright notice, this
 *   list of conditions and the following disclaimer.
 * - Redistributions in binary form must reproduce the above copyright notice,
 *   this list of conditions and the following disclaimer in the documentation
 *   and/or other materials provided with the distribution.
 * - Neither the name of the author nor the names of its contributors may be
 *   used to endorse or promote products derived from this software without
 *   specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

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

/*
 * Failing setting up at least the following five entries will lead
 * to a state of considerable discontent:
 *
 *   - null entry
 *   - kernel code segment
 *   - kernel data segment
 *   - user code segment
 *   - user data segment
 */

static gdt_t       gdt;
static gdt_entry_t gdt_entries[3];

void set_entry(uint8_t num, uint64_t base, uint64_t limit, uint8_t access, uint8_t 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);
     * Shut up, gcc -Wconversion.
     */
    gdt_entries[num].granularity  |= gran & 0xf0;
    gdt_entries[num].access       = access;
}

void gdt_init(void)
{
    gdt.limit = sizeof(gdt_entries)*3 - 1;
    gdt.base  = (uintptr_t)&gdt_entries;

	set_entry(0, 0, 0, 0, 0);//Null segment
    set_entry(1, 0, 0xffffffff, 0x9a, 0xcf);  // kernel code segment
    set_entry(2, 0, 0xffffffff, 0x92, 0xcf);  // kernel data segment

    gdt_flush((uintptr_t)&gdt);
}