2 * GRUB -- GRand Unified Bootloader
3 * Copyright (C) 1999,2000,2001,2002,2003,2005,2006,2007 Free Software Foundation, Inc.
5 * GRUB is free software: you can redistribute it and/or modify
6 * it under the terms of the GNU General Public License as published by
7 * the Free Software Foundation, either version 3 of the License, or
8 * (at your option) any later version.
10 * GRUB is distributed in the hope that it will be useful,
11 * but WITHOUT ANY WARRANTY; without even the implied warranty of
12 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 * GNU General Public License for more details.
15 * You should have received a copy of the GNU General Public License
16 * along with GRUB. If not, see <http://www.gnu.org/licenses/>.
21 * Note: These functions defined in this file may be called from C.
22 * Be careful of that you must not modify some registers. Quote
23 * from gcc-2.95.2/gcc/config/i386/i386.h:
25 1 for registers not available across function calls.
26 These must include the FIXED_REGISTERS and also any
27 registers that can be used without being saved.
28 The latter must include the registers where values are returned
29 and the register where structure-value addresses are passed.
30 Aside from that, you can include as many other registers as you like.
32 ax,dx,cx,bx,si,di,bp,sp,st,st1,st2,st3,st4,st5,st6,st7,arg
33 { 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1 }
37 * Note: GRUB is compiled with the options -mrtd and -mregparm=3.
38 * So the first three arguments are passed in %eax, %edx, and %ecx,
39 * respectively, and if a function has a fixed number of arguments
40 * and the number if greater than three, the function must return
41 * with "ret $N" where N is ((the number of arguments) - 3) * 4.
45 * This is the area for all of the special variables.
48 .p2align 2 /* force 4-byte alignment */
51 .long GRUB_MEMORY_MACHINE_PROT_STACK
54 * This is the Global Descriptor Table
56 * An entry, a "Segment Descriptor", looks like this:
59 * ------------------------------------------------------------
60 * | | |B| |A| | | |1|0|E|W|A| |
61 * | BASE 31..24 |G|/|L|V| LIMIT |P|DPL| TYPE | BASE 23:16 | 4
62 * | | |D| |L| 19..16| | |1|1|C|R|A| |
63 * ------------------------------------------------------------
65 * | BASE 15..0 | LIMIT 15..0 | 0
67 * ------------------------------------------------------------
69 * Note the ordering of the data items is reversed from the above
73 .p2align 2 /* force 4-byte alignment */
79 * base = 0x00000000, limit = 0xFFFFF (4 KiB Granularity), present
80 * type = 32bit code execute/read, DPL = 0
83 .byte 0, 0x9A, 0xCF, 0
86 * base = 0x00000000, limit 0xFFFFF (4 KiB Granularity), present
87 * type = 32 bit data read/write, DPL = 0
90 .byte 0, 0x92, 0xCF, 0
92 /* -- 16 bit real mode CS --
93 * base = 0x00000000, limit 0x0FFFF (1 B Granularity), present
94 * type = 16 bit code execute/read only/conforming, DPL = 0
99 /* -- 16 bit real mode DS --
100 * base = 0x00000000, limit 0x0FFFF (1 B Granularity), present
101 * type = 16 bit data read/write, DPL = 0
107 /* this is the GDT descriptor */
109 .word 0x27 /* limit */
113 * These next routine, "prot_to_real" is structured in a very
114 * specific way. Be very careful when changing it.
116 * NOTE: Use of it messes up %eax and %ebp.
120 /* just in case, set GDT */
123 /* save the protected mode stack */
127 /* get the return address */
129 movl %eax, GRUB_MEMORY_MACHINE_REAL_STACK
131 /* set up new stack */
132 movl $GRUB_MEMORY_MACHINE_REAL_STACK, %eax
136 /* set up segment limits */
137 movw $GRUB_MEMORY_MACHINE_PSEUDO_REAL_DSEG, %ax
144 /* this might be an extra step */
145 /* jump to a 16 bit segment */
146 ljmp $GRUB_MEMORY_MACHINE_PSEUDO_REAL_CSEG, $tmpcseg
151 /* clear the PE bit of CR0 */
153 andl $(~GRUB_MEMORY_MACHINE_CR0_PE_ON), %eax
156 /* flush prefetch queue, reload %cs */
157 DATA32 ljmp $0, $realcseg
160 /* we are in real mode now
161 * set up the real mode segment registers : DS, SS, ES
172 /* restore interrupts */
175 /* return on new stack! */