src/arch/i386/image/bootsector.c

   1 /*
   2  * Copyright (C) 2007 Michael Brown <mbrown@fensystems.co.uk>.
   3  *
   4  * This program is free software; you can redistribute it and/or
   5  * modify it under the terms of the GNU General Public License as
   6  * published by the Free Software Foundation; either version 2 of the
   7  * License, or any later version.
   8  *
   9  * This program is distributed in the hope that it will be useful, but
  10  * WITHOUT ANY WARRANTY; without even the implied warranty of
  11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12  * General Public License for more details.
  13  *
  14  * You should have received a copy of the GNU General Public License
  15  * along with this program; if not, write to the Free Software
  16  * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  17  */
  18
  19 FILE_LICENCE ( GPL2_OR_LATER );
  20
  21 /**
  22  * @file
  23  *
  24  * x86 bootsector image format
  25  *
  26  */
  27
  28 #include <errno.h>
  29 #include <realmode.h>
  30 #include <biosint.h>
  31 #include <bootsector.h>
  32
  33 /** Vector for storing original INT 18 handler
  34  *
  35  * We do not chain to this vector, so there is no need to place it in
  36  * .text16.
  37  */
  38 static struct segoff int18_vector;
  39
  40 /** Vector for storing original INT 19 handler
  41  *
  42  * We do not chain to this vector, so there is no need to place it in
  43  * .text16.
  44  */
  45 static struct segoff int19_vector;
  46
  47 /** Restart point for INT 18 or 19 */
  48 extern void bootsector_exec_fail ( void );
  49
  50 /**
  51  * Jump to preloaded bootsector
  52  *
  53  * @v segment           Real-mode segment
  54  * @v offset            Real-mode offset
  55  * @v drive             Drive number to pass to boot sector
  56  * @ret rc              Return status code
  57  */
  58 int call_bootsector ( unsigned int segment, unsigned int offset,
  59                       unsigned int drive ) {
  60         int discard_b, discard_D, discard_d;
  61
  62         DBG ( "Booting from boot sector at %04x:%04x\n", segment, offset );
  63
  64         /* Hook INTs 18 and 19 to capture failure paths */
  65         hook_bios_interrupt ( 0x18, ( unsigned int ) bootsector_exec_fail,
  66                               &int18_vector );
  67         hook_bios_interrupt ( 0x19, ( unsigned int ) bootsector_exec_fail,
  68                               &int19_vector );
  69
  70         /* Boot the loaded sector
  71          *
  72          * We assume that the boot sector may completely destroy our
  73          * real-mode stack, so we preserve everything we need in
  74          * static storage.
  75          */
  76         __asm__ __volatile__ ( REAL_CODE ( /* Save return address off-stack */
  77                                            "popw %%cs:saved_retaddr\n\t"
  78                                            /* Save stack pointer */
  79                                            "movw %%ss, %%ax\n\t"
  80                                            "movw %%ax, %%cs:saved_ss\n\t"
  81                                            "movw %%sp, %%cs:saved_sp\n\t"
  82                                            /* Jump to boot sector */
  83                                            "pushw %%bx\n\t"
  84                                            "pushw %%di\n\t"
  85                                            "sti\n\t"
  86                                            "lret\n\t"
  87                                            /* Preserved variables */
  88                                            "\nsaved_ss: .word 0\n\t"
  89                                            "\nsaved_sp: .word 0\n\t"
  90                                            "\nsaved_retaddr: .word 0\n\t"
  91                                            /* Boot failure return point */
  92                                            "\nbootsector_exec_fail:\n\t"
  93                                            /* Restore stack pointer */
  94                                            "movw %%cs:saved_ss, %%ax\n\t"
  95                                            "movw %%ax, %%ss\n\t"
  96                                            "movw %%cs:saved_sp, %%sp\n\t"
  97                                            /* Return via saved address */
  98                                            "jmp *%%cs:saved_retaddr\n\t" )
  99                                : "=b" ( discard_b ), "=D" ( discard_D ),
 100                                  "=d" ( discard_d )
 101                                : "b" ( segment ), "D" ( offset ),
 102                                  "d" ( drive )
 103                                : "eax", "ecx", "esi", "ebp" );
 104
 105         DBG ( "Booted disk returned via INT 18 or 19\n" );
 106
 107         /* Unhook INTs 18 and 19 */
 108         unhook_bios_interrupt ( 0x18, ( unsigned int ) bootsector_exec_fail,
 109                                 &int18_vector );
 110         unhook_bios_interrupt ( 0x19, ( unsigned int ) bootsector_exec_fail,
 111                                 &int19_vector );
 112
 113         return -ECANCELED;
 114 }