gdt.c

一个操作系统的源代码

/** gdt.c
 **
 ** Original Author: Kasper Verdich Lund
 ** Date: 10/11/99
 **
 ** Description:
 ** Global Descriptor Table functions
 **
 ** This program is free software, you can redistribute it and/or
 ** modify it under the terms of the GNU General Public License
 ** as published by the Free Software Foundation; either version
 ** 2 of the License, or (at your option) any later version.
 **
 ** This program is distributed in the hope that it will be
 ** useful, but WITHOUT ANY WARRANTY; without even the implied
 ** warranty or MERCHANTABILITY or FITNESS FOR A PARTICULAR
 ** PURPOSE.  See the GNU General Public License for more
 ** details.
 **
 ** You should have received a copy of the GNU General Public
 ** License along with this program; if not, write to the
 ** Free Software Foundation, Inc., 59 Temple Place, Suite 330,
 ** Boston, MA 02111-1307 USA
 **
 *********************************************************Apostle OS**/

#include <gdt.h>
#include <tss.h>
#include <mem.h>
#include <types.h>
#include <string.h>

descriptor *GDT = (descriptor *)0;

void setupGDT(void)
{
  /* it is assumed that GDT[1] is 
   * the flat code segment descriptor 
   * and that GDT[2] is the flat data
   * segment descriptor */
  dword GDTR[2];

  memset((void *)GDT, 0, GDT_SIZE * sizeof(descriptor));
  setupDescriptor(&GDT[1], 0, LIMIT_4GB, 8, 0, 1, 1); /* 0x08 */
  setupDescriptor(&GDT[2], 0, LIMIT_4GB, 2, 0, 1, 1); /* 0x10 */
  setupDescriptor(&GDT[3], 0, LIMIT_4GB, 8, 3, 1, 1); /* 0x1B */
  setupDescriptor(&GDT[4], 0, LIMIT_4GB, 2, 3, 1, 1); /* 0x23 */
  
  /* the following computation of the GDTR
   * could/should be done much more readable by
   * describing the memory layout of the GDTR
   * in a packed struct in gdt.h */
  GDTR[0] = ((GDT_SIZE * sizeof(descriptor)) - 1) << 16;
  GDTR[1] = (dword)GDT;

  /* load the GDTR - it's only 48-bits and therefore the 
   * two first bytes are not used */
  __asm__("lgdt (%0)" : : "r" (((byte *)GDTR) + 2));
  
  /* flush the instruction queue and set CS */
  __asm__("ljmp $8, $__flush\n"
          "__flush:");

  /* set DS, SS, ES, FS, GS */
  __asm__("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)
{
  /* calculate limit and granularity */
  if (limit > LIMIT_1MB) 
    {
      limit = limit >> 12;
      d->g = 1;
    }
  else
    d->g = 0;

  /* set base address */
  d->base0_15 = base;
  d->base16_23 = base >> 16;
  d->base24_31 = base >> 24;
  
  /* set segment limit */
  d->limit0_15 = limit;
  d->limit16_19 = limit >> 16;
  
  /* set rest of descriptor */
  d->type = type;
  d->s = s;
  d->dpl = dpl;
  d->p = 1;
  d->avl = 0;
  d->zero = 0;
  d->db = db; 
}