178 lines
4.9 KiB
ArmAsm
178 lines
4.9 KiB
ArmAsm
/*
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* GRUB -- GRand Unified Bootloader
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* Copyright (C) 1999,2000,2001,2002,2003,2005,2006,2007 Free Software Foundation, Inc.
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*
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* GRUB is free software: you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* GRUB is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with GRUB. If not, see <http://www.gnu.org/licenses/>.
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*/
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/*
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* Note: These functions defined in this file may be called from C.
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* Be careful of that you must not modify some registers. Quote
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* from gcc-2.95.2/gcc/config/i386/i386.h:
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1 for registers not available across function calls.
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These must include the FIXED_REGISTERS and also any
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registers that can be used without being saved.
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The latter must include the registers where values are returned
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and the register where structure-value addresses are passed.
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Aside from that, you can include as many other registers as you like.
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ax,dx,cx,bx,si,di,bp,sp,st,st1,st2,st3,st4,st5,st6,st7,arg
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{ 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1 }
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*/
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/*
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* Note: GRUB is compiled with the options -mrtd and -mregparm=3.
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* So the first three arguments are passed in %eax, %edx, and %ecx,
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* respectively, and if a function has a fixed number of arguments
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* and the number if greater than three, the function must return
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* with "ret $N" where N is ((the number of arguments) - 3) * 4.
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*/
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/*
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* This is the area for all of the special variables.
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*/
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.p2align 2 /* force 4-byte alignment */
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protstack:
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.long GRUB_MEMORY_MACHINE_PROT_STACK
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/*
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* This is the Global Descriptor Table
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*
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* An entry, a "Segment Descriptor", looks like this:
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*
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* 31 24 19 16 7 0
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* ------------------------------------------------------------
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* | | |B| |A| | | |1|0|E|W|A| |
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* | BASE 31..24 |G|/|L|V| LIMIT |P|DPL| TYPE | BASE 23:16 | 4
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* | | |D| |L| 19..16| | |1|1|C|R|A| |
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* ------------------------------------------------------------
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* | | |
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* | BASE 15..0 | LIMIT 15..0 | 0
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* | | |
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* ------------------------------------------------------------
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*
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* Note the ordering of the data items is reversed from the above
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* description.
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*/
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.p2align 2 /* force 4-byte alignment */
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gdt:
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.word 0, 0
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.byte 0, 0, 0, 0
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/* -- code segment --
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* base = 0x00000000, limit = 0xFFFFF (4 KiB Granularity), present
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* type = 32bit code execute/read, DPL = 0
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*/
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.word 0xFFFF, 0
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.byte 0, 0x9A, 0xCF, 0
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/* -- data segment --
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* base = 0x00000000, limit 0xFFFFF (4 KiB Granularity), present
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* type = 32 bit data read/write, DPL = 0
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*/
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.word 0xFFFF, 0
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.byte 0, 0x92, 0xCF, 0
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/* -- 16 bit real mode CS --
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* base = 0x00000000, limit 0x0FFFF (1 B Granularity), present
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* type = 16 bit code execute/read only/conforming, DPL = 0
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*/
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.word 0xFFFF, 0
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.byte 0, 0x9E, 0, 0
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/* -- 16 bit real mode DS --
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* base = 0x00000000, limit 0x0FFFF (1 B Granularity), present
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* type = 16 bit data read/write, DPL = 0
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*/
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.word 0xFFFF, 0
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.byte 0, 0x92, 0, 0
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/* this is the GDT descriptor */
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gdtdesc:
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.word 0x27 /* limit */
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.long gdt /* addr */
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/*
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* These next routine, "prot_to_real" is structured in a very
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* specific way. Be very careful when changing it.
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*
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* NOTE: Use of it messes up %eax and %ebp.
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*/
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prot_to_real:
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/* just in case, set GDT */
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lgdt gdtdesc
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/* save the protected mode stack */
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movl %esp, %eax
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movl %eax, protstack
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/* get the return address */
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movl (%esp), %eax
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movl %eax, GRUB_MEMORY_MACHINE_REAL_STACK
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/* set up new stack */
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movl $GRUB_MEMORY_MACHINE_REAL_STACK, %eax
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movl %eax, %esp
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movl %eax, %ebp
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/* set up segment limits */
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movw $GRUB_MEMORY_MACHINE_PSEUDO_REAL_DSEG, %ax
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movw %ax, %ds
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movw %ax, %es
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movw %ax, %fs
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movw %ax, %gs
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movw %ax, %ss
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/* this might be an extra step */
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/* jump to a 16 bit segment */
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ljmp $GRUB_MEMORY_MACHINE_PSEUDO_REAL_CSEG, $tmpcseg
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tmpcseg:
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.code16
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/* clear the PE bit of CR0 */
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movl %cr0, %eax
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andl $(~GRUB_MEMORY_MACHINE_CR0_PE_ON), %eax
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movl %eax, %cr0
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/* flush prefetch queue, reload %cs */
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DATA32 ljmp $0, $realcseg
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realcseg:
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/* we are in real mode now
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* set up the real mode segment registers : DS, SS, ES
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*/
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/* zero %eax */
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xorl %eax, %eax
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movw %ax, %ds
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movw %ax, %es
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movw %ax, %fs
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movw %ax, %gs
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movw %ax, %ss
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/* restore interrupts */
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sti
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/* return on new stack! */
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DATA32 ret
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.code32
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