gdt.c

一个微型操作系统源码

/*
 * OSV
 * Copyright (C) 2002 Ciprian DOSOFTEI <[e-mail]>
 * All rights reserved.
 * 
 * http://backster.free.fr/osv
 *
 * This file is part of the OSV project. OSV is free software, also known as
 * "open source"; you can redistribute it and/or modify it under the terms 
 * of the GNU General Public License (GPL), version 2, as published by the Free
 * Software Foundation (FSF). To explore alternate licensing terms, contact 
 * the author at [e-mail] or [phone].
 * 
 * OSV is distributed in the hope that it will be useful, but WITHOUT ANY
 * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
 * FOR A PARTICULAR PURPOSE.  See the GPL for more details.  You should have
 * received a copy of the GPL along with OSV; see the file COPYING.  If
 * not, write to the FSF, 59 Temple Place #330, Boston, MA 02111-1307, USA.
 */

#include <gdt.h>

descriptor *GDT = (descriptor *)0;

void setupGDT(void) {
    dword GDTR[2];

    memset((void *)GDT, 0, GDT_SIZE * sizeof(descriptor));
    setupDescriptor(&GDT[1], 0, LIMIT_4GB, 8, 0, 1, 1); /* 0x08 -- KERNEL CODE */
    setupDescriptor(&GDT[2], 0, LIMIT_4GB, 2, 0, 1, 1); /* 0x10 -- KERNEL DATA */
    setupDescriptor(&GDT[3], 0, LIMIT_4GB, 8, 3, 1, 1); /* 0x1B -- USER CODE */
    setupDescriptor(&GDT[4], 0, LIMIT_4GB, 2, 3, 1, 1); /* 0x23 -- USER DATA */

    GDTR[0] = ((GDT_SIZE * sizeof(descriptor)) - 1) << 16;
    GDTR[1] = (dword)GDT;

    __asm__ __volatile__("lgdt (%0)" : : "r" (((byte *)GDTR) + 2));

    __asm__ __volatile__("ljmp $8, $__flush\n"
          "__flush:");

    __asm__ __volatile__("movl $0x10, %eax\n"
          "movw %ax, %ss\n"
          "movl $0x23, %eax\n"
          "movw %ax, %ds\n"
          "movw %ax, %es\n"
          "movw %ax, %fs\n"
          "movw %ax, %gs");
}

void setupDescriptor(descriptor *d, dword base, dword limit, byte type, byte dpl, byte s, byte db) {
    if (limit > LIMIT_1MB) {
	limit = limit >> 12;
        d->g = 1;
    } else d->g = 0;

    d->base0_15 = base;
    d->base16_23 = base >> 16;
    d->base24_31 = base >> 24;

    d->limit0_15 = limit;
    d->limit16_19 = limit >> 16;

    d->type = type;
    d->s = s;
    d->dpl = dpl;
    d->p = 1;
    d->avl = 0;
    d->zero = 0;
    d->db = db;
}