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Diffstat (limited to 'grub-core/kern/i386/realmode.S')
| -rw-r--r-- | grub-core/kern/i386/realmode.S | 281 |
1 files changed, 281 insertions, 0 deletions
diff --git a/grub-core/kern/i386/realmode.S b/grub-core/kern/i386/realmode.S new file mode 100644 index 0000000..265cdcb --- /dev/null +++ b/grub-core/kern/i386/realmode.S @@ -0,0 +1,281 @@ +/* + * GRUB -- GRand Unified Bootloader + * Copyright (C) 1999,2000,2001,2002,2003,2005,2006,2007,2009,2010 Free Software Foundation, Inc. + * + * GRUB 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 3 of the License, or + * (at your option) any later version. + * + * GRUB 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 + * GNU General Public License for more details. + * + * You should have received a copy of the GNU General Public License + * along with GRUB. If not, see <http://www.gnu.org/licenses/>. + */ + +#include <grub/machine/memory.h> + +/* + * Note: These functions defined in this file may be called from C. + * Be careful of that you must not modify some registers. Quote + * from gcc-2.95.2/gcc/config/i386/i386.h: + + 1 for registers not available across function calls. + These must include the FIXED_REGISTERS and also any + registers that can be used without being saved. + The latter must include the registers where values are returned + and the register where structure-value addresses are passed. + Aside from that, you can include as many other registers as you like. + + ax,dx,cx,bx,si,di,bp,sp,st,st1,st2,st3,st4,st5,st6,st7,arg +{ 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1 } + */ + +/* + * Note: GRUB is compiled with the options -mrtd and -mregparm=3. + * So the first three arguments are passed in %eax, %edx, and %ecx, + * respectively, and if a function has a fixed number of arguments + * and the number if greater than three, the function must return + * with "ret $N" where N is ((the number of arguments) - 3) * 4. + */ + +/* + * This is the area for all of the special variables. + */ + +protstack: + .long GRUB_MEMORY_MACHINE_PROT_STACK + + .macro PROT_TO_REAL + call prot_to_real + .endm + + .macro REAL_TO_PROT + calll real_to_prot + .endm + +/* + * This is the Global Descriptor Table + * + * An entry, a "Segment Descriptor", looks like this: + * + * 31 24 19 16 7 0 + * ------------------------------------------------------------ + * | | |B| |A| | | |1|0|E|W|A| | + * | BASE 31..24 |G|/|L|V| LIMIT |P|DPL| TYPE | BASE 23:16 | 4 + * | | |D| |L| 19..16| | |1|1|C|R|A| | + * ------------------------------------------------------------ + * | | | + * | BASE 15..0 | LIMIT 15..0 | 0 + * | | | + * ------------------------------------------------------------ + * + * Note the ordering of the data items is reversed from the above + * description. + */ + + .p2align 5 /* force 32-byte alignment */ +gdt: + .word 0, 0 + .byte 0, 0, 0, 0 + + /* -- code segment -- + * base = 0x00000000, limit = 0xFFFFF (4 KiB Granularity), present + * type = 32bit code execute/read, DPL = 0 + */ + .word 0xFFFF, 0 + .byte 0, 0x9A, 0xCF, 0 + + /* -- data segment -- + * base = 0x00000000, limit 0xFFFFF (4 KiB Granularity), present + * type = 32 bit data read/write, DPL = 0 + */ + .word 0xFFFF, 0 + .byte 0, 0x92, 0xCF, 0 + + /* -- 16 bit real mode CS -- + * base = 0x00000000, limit 0x0FFFF (1 B Granularity), present + * type = 16 bit code execute/read only/conforming, DPL = 0 + */ + .word 0xFFFF, 0 + .byte 0, 0x9E, 0, 0 + + /* -- 16 bit real mode DS -- + * base = 0x00000000, limit 0x0FFFF (1 B Granularity), present + * type = 16 bit data read/write, DPL = 0 + */ + .word 0xFFFF, 0 + .byte 0, 0x92, 0, 0 + + + .p2align 5 +/* this is the GDT descriptor */ +gdtdesc: + .word 0x27 /* limit */ + .long gdt /* addr */ +LOCAL(realidt): + .word 0x400 + .long 0 +protidt: + .word 0 + .long 0 + +/* + * These next two routines, "real_to_prot" and "prot_to_real" are structured + * in a very specific way. Be very careful when changing them. + * + * NOTE: Use of either one messes up %eax and %ebp. + */ + +real_to_prot: + .code16 + cli + + /* load the GDT register */ + xorw %ax, %ax + movw %ax, %ds +#ifdef GRUB_MACHINE_QEMU + /* + qemu is special: gdtdesc is in ROM. + %cs = 0xf000 + _start + GRUB_BOOT_MACHINE_SIZE = 0x100000 + So + _start + GRUB_BOOT_MACHINE_SIZE - 0x10000 points to the same point + as %cs. + gdtdesc - (_start + GRUB_BOOT_MACHINE_SIZE - 0x10000) + = gdtdesc - _start - GRUB_BOOT_MACHINE_SIZE + 0x10000 + but the later can be computed by assembly. + */ + lgdtl %cs:(gdtdesc - _start - GRUB_BOOT_MACHINE_SIZE + 0x10000) +#else + lgdtl gdtdesc +#endif + + /* turn on protected mode */ + movl %cr0, %eax + orl $GRUB_MEMORY_CPU_CR0_PE_ON, %eax + movl %eax, %cr0 + + /* jump to relocation, flush prefetch queue, and reload %cs */ + ljmpl $GRUB_MEMORY_MACHINE_PROT_MODE_CSEG, $protcseg + + .code32 +protcseg: + /* reload other segment registers */ + movw $GRUB_MEMORY_MACHINE_PROT_MODE_DSEG, %ax + movw %ax, %ds + movw %ax, %es + movw %ax, %fs + movw %ax, %gs + movw %ax, %ss + + /* put the return address in a known safe location */ + movl (%esp), %eax + movl %eax, GRUB_MEMORY_MACHINE_REAL_STACK + + /* get protected mode stack */ + movl protstack, %eax + movl %eax, %esp + movl %eax, %ebp + + /* get return address onto the right stack */ + movl GRUB_MEMORY_MACHINE_REAL_STACK, %eax + movl %eax, (%esp) + + /* zero %eax */ + xorl %eax, %eax + + sidt LOCAL(realidt) + lidt protidt + + /* return on the old (or initialized) stack! */ + ret + /* prot_to_real assumes that this code is under 64K which is not + true for qemu. */ +#ifndef GRUB_MACHINE_QEMU +/* + * GRUB -- GRand Unified Bootloader + * Copyright (C) 1999,2000,2001,2002,2003,2005,2006,2007,2009,2010 Free Software Foundation, Inc. + * + * GRUB 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 3 of the License, or + * (at your option) any later version. + * + * GRUB 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 + * GNU General Public License for more details. + * + * You should have received a copy of the GNU General Public License + * along with GRUB. If not, see <http://www.gnu.org/licenses/>. + */ + +prot_to_real: + /* just in case, set GDT */ + lgdt gdtdesc + + sidt protidt + lidt LOCAL(realidt) + + /* save the protected mode stack */ + movl %esp, %eax + movl %eax, protstack + + /* get the return address */ + movl (%esp), %eax + movl %eax, GRUB_MEMORY_MACHINE_REAL_STACK + + /* set up new stack */ + movl $GRUB_MEMORY_MACHINE_REAL_STACK, %eax + movl %eax, %esp + movl %eax, %ebp + + /* set up segment limits */ + movw $GRUB_MEMORY_MACHINE_PSEUDO_REAL_DSEG, %ax + movw %ax, %ds + movw %ax, %es + movw %ax, %fs + movw %ax, %gs + movw %ax, %ss + + /* this might be an extra step */ + /* jump to a 16 bit segment */ + ljmp $GRUB_MEMORY_MACHINE_PSEUDO_REAL_CSEG, $tmpcseg + +tmpcseg: + .code16 + + /* clear the PE bit of CR0 */ + movl %cr0, %eax + andl $(~GRUB_MEMORY_CPU_CR0_PE_ON), %eax + movl %eax, %cr0 + + /* flush prefetch queue, reload %cs */ + ljmpl $0, $realcseg + +realcseg: + /* we are in real mode now + * set up the real mode segment registers : DS, SS, ES + */ + /* zero %eax */ + xorl %eax, %eax + + movw %ax, %ds + movw %ax, %es + movw %ax, %fs + movw %ax, %gs + movw %ax, %ss + +#ifdef GRUB_MACHINE_PCBIOS + /* restore interrupts */ + sti +#endif + + /* return on new stack! */ + retl +#endif + .code32 |