source: bootcd/isolinux/syslinux-6.03/gpxe/src/arch/i386/image/bootsector.c

/*
 * Copyright (C) 2007 Michael Brown <mbrown@fensystems.co.uk>.
 *
 * 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 any later version.
 *
 * This program 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 this program; if not, write to the Free Software
 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 */

FILE_LICENCE ( GPL2_OR_LATER );

/**
 * @file
 *
 * x86 bootsector image format
 *
 */

#include <errno.h>
#include <realmode.h>
#include <biosint.h>
#include <bootsector.h>

/** Vector for storing original INT 18 handler
 *
 * We do not chain to this vector, so there is no need to place it in
 * .text16.
 */
static struct segoff int18_vector;

/** Vector for storing original INT 19 handler
 *
 * We do not chain to this vector, so there is no need to place it in
 * .text16.
 */
static struct segoff int19_vector;

/** Restart point for INT 18 or 19 */
extern void bootsector_exec_fail ( void );

/**
 * Jump to preloaded bootsector
 *
 * @v segment           Real-mode segment
 * @v offset            Real-mode offset
 * @v drive             Drive number to pass to boot sector
 * @ret rc              Return status code
 */
int call_bootsector ( unsigned int segment, unsigned int offset,
                      unsigned int drive ) {
        int discard_b, discard_D, discard_d;

        DBG ( "Booting from boot sector at %04x:%04x\n", segment, offset );

        /* Hook INTs 18 and 19 to capture failure paths */
        hook_bios_interrupt ( 0x18, ( unsigned int ) bootsector_exec_fail,
                              &int18_vector );
        hook_bios_interrupt ( 0x19, ( unsigned int ) bootsector_exec_fail,
                              &int19_vector );

        /* Boot the loaded sector
         *
         * We assume that the boot sector may completely destroy our
         * real-mode stack, so we preserve everything we need in
         * static storage.
         */
        __asm__ __volatile__ ( REAL_CODE ( /* Save return address off-stack */
                                           "popw %%cs:saved_retaddr\n\t"
                                           /* Save stack pointer */
                                           "movw %%ss, %%ax\n\t"
                                           "movw %%ax, %%cs:saved_ss\n\t"
                                           "movw %%sp, %%cs:saved_sp\n\t"
                                           /* Jump to boot sector */
                                           "pushw %%bx\n\t"
                                           "pushw %%di\n\t"
                                           "sti\n\t"
                                           "lret\n\t"
                                           /* Preserved variables */
                                           "\nsaved_ss: .word 0\n\t"
                                           "\nsaved_sp: .word 0\n\t"
                                           "\nsaved_retaddr: .word 0\n\t"
                                           /* Boot failure return point */
                                           "\nbootsector_exec_fail:\n\t"
                                           /* Restore stack pointer */
                                           "movw %%cs:saved_ss, %%ax\n\t"
                                           "movw %%ax, %%ss\n\t"
                                           "movw %%cs:saved_sp, %%sp\n\t"
                                           /* Return via saved address */
                                           "jmp *%%cs:saved_retaddr\n\t" )
                               : "=b" ( discard_b ), "=D" ( discard_D ),
                                 "=d" ( discard_d )
                               : "b" ( segment ), "D" ( offset ),
                                 "d" ( drive )
                               : "eax", "ecx", "esi", "ebp" );

        DBG ( "Booted disk returned via INT 18 or 19\n" );

        /* Unhook INTs 18 and 19 */
        unhook_bios_interrupt ( 0x18, ( unsigned int ) bootsector_exec_fail,
                                &int18_vector );
        unhook_bios_interrupt ( 0x19, ( unsigned int ) bootsector_exec_fail,
                                &int19_vector );
        
        return -ECANCELED;
}