source: bootcd/isolinux/syslinux-6.03/utils/isohybrid.in

#!/usr/bin/perl
## -----------------------------------------------------------------------
##
##   Copyright 2002-2008 H. Peter Anvin - All Rights Reserved
##   Copyright 2009 Intel Corporation; author: H. Peter Anvin
##
##   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, Inc., 53 Temple Place Ste 330,
##   Boston MA 02111-1307, USA; either version 2 of the License, or
##   (at your option) any later version; incorporated herein by reference.
##
## -----------------------------------------------------------------------

#
# Post-process an ISO 9660 image generated with mkisofs/genisoimage
# to allow "hybrid booting" as a CD-ROM or as a hard disk.
#

use bytes;
use Fcntl;

# User-specifyable options
%opt = (
    # Fake geometry (zipdrive-style...)
    'h'      => 64,
    's'      => 32,
    # Partition number
    'entry'  => 1,
    # Partition offset
    'offset' => 0,
    # Partition type
    'type'   => 0x17,           # "Windows hidden IFS"
    # MBR ID
    'id'     => undef,
);

%valid_range = (
    'h'      => [1, 256],
    's'      => [1, 63],
    'entry'  => [1, 4],
    'offset' => [0, 64],
    'type'   => [0, 255],
    'id'     => [0, 0xffffffff],
    'hd0'    => [0, 2],
    'partok' => [0, 1],
);

# Boolean options just set other options
%bool_opt = (
    'nohd0'    => ['hd0', 0],
    'forcehd0' => ['hd0', 1],
    'ctrlhd0'  => ['hd0', 2],
    'nopartok' => ['partok', 0],
    'partok'   => ['partok', 1],
);

sub usage() {
    print STDERR "Usage: $0 [options] filename.iso\n",
    "Options:\n",
    "  -h          Number of default geometry heads\n",
    "  -s          Number of default geometry sectors\n",
    "  -entry      Specify partition entry number (1-4)\n",
    "  -offset     Specify partition offset (default 0)\n",
    "  -type       Specify partition type (default 0x17)\n",
    "  -id         Specify MBR ID (default random)\n",
    "  -forcehd0   Always assume we are loaded as disk ID 0\n",
    "  -ctrlhd0    Assume disk ID 0 if the Ctrl key is pressed\n",
    "  -partok     Allow booting from within a partition\n";
    exit 1;
}

# Parse a C-style integer (decimal/octal/hex)
sub doh($) {
    my($n) = @_;
    return ($n =~ /^0/) ? oct $n : $n+0;
}

sub get_random() {
    # Get a 32-bit random number
    my $rfd, $rnd;
    my $rid;

    if (open($rfd, "< /dev/urandom\0") && read($rfd, $rnd, 4) == 4) {
        $rid = unpack("V", $rnd);
    }

    close($rfd) if (defined($rfd));
    return $rid if (defined($rid));

    # This sucks but is better than nothing...
    return ($$+time()) & 0xffffffff;
}

sub get_hex_data() {
    my $mbr = '';
    my $line, $byte;
    while ( $line = <DATA> ) {
        chomp $line;
        last if ($line eq '*');
        foreach $byte ( split(/\s+/, $line) ) {
            $mbr .= chr(hex($byte));
        }
    }
    return $mbr;
}

while ($ARGV[0] =~ /^\-(.*)$/) {
    $o = $1;
    shift @ARGV;
    if (defined($bool_opt{$o})) {
        ($o, $v) = @{$bool_opt{$o}};
        $opt{$o} = $v;
    } elsif (exists($opt{$o})) {
        $opt{$o} = doh(shift @ARGV);
        if (defined($valid_range{$o})) {
            ($l, $h) = @{$valid_range{$o}};
            if ($opt{$o} < $l || $opt{$o} > $h) {
                die "$0: valid values for the -$o parameter are $l to $h\n";
            }
        }
    } else {
        usage();
    }
}

($file) = @ARGV;

if (!defined($file)) {
    usage();
}

open(FILE, "+< $file\0") or die "$0: cannot open $file: $!\n";
binmode FILE;

#
# First, actually figure out where mkisofs hid isolinux.bin
#
seek(FILE, 17*2048, SEEK_SET) or die "$0: $file: $!\n";
read(FILE, $boot_record, 2048) == 2048 or die "$0: $file: read error\n";
($br_sign, $br_cat_offset) = unpack("a71V", $boot_record);
if ($br_sign ne ("\0CD001\1EL TORITO SPECIFICATION" . ("\0" x 41))) {
    die "$0: $file: no boot record found\n";
}
seek(FILE, $br_cat_offset*2048, SEEK_SET) or die "$0: $file: $!\n";
read(FILE, $boot_cat, 2048) == 2048 or die "$0: $file: read error\n";

# We must have a Validation Entry followed by a Default Entry...
# no fanciness allowed for the Hybrid mode [XXX: might relax this later]
@ve = unpack("v16", $boot_cat);
$cs = 0;
for ($i = 0; $i < 16; $i++) {
    $cs += $ve[$i];
}
if ($ve[0] != 0x0001 || $ve[15] != 0xaa55 || $cs & 0xffff) {
    die "$0: $file: invalid boot catalog\n";
}
($de_boot, $de_media, $de_seg, $de_sys, $de_mbz1, $de_count, 
 $de_lba, $de_mbz2) = unpack("CCvCCvVv", substr($boot_cat, 32, 32));
if ($de_boot != 0x88 || $de_media != 0 ||
    ($de_segment != 0 && $de_segment != 0x7c0) || $de_count != 4) {
    die "$0: $file: unexpected boot catalog parameters\n";
}

# Now $de_lba should contain the CD sector number for isolinux.bin
seek(FILE, $de_lba*2048+0x40, SEEK_SET) or die "$0: $file: $!\n";
read(FILE, $ibsig, 4);
if ($ibsig ne "\xfb\xc0\x78\x70") {
    die "$0: $file: bootloader does not have a isolinux.bin hybrid signature.".
        "Note that isolinux-debug.bin does not support hybrid booting.\n";
}

# Get the total size of the image
(@imgstat = stat(FILE)) or die "$0: $file: $!\n";
$imgsize = $imgstat[7];
if (!$imgsize) {
    die "$0: $file: cannot determine length of file\n";
}
# Target image size: round up to a multiple of $h*$s*512
$h = $opt{'h'};
$s = $opt{'s'};
$cylsize = $h*$s*512;
$frac = $imgsize % $cylsize;
$padding = ($frac > 0) ? $cylsize - $frac : 0;
$imgsize += $padding;
$c = int($imgsize/$cylsize);
if ($c > 1024) {
    print STDERR "Warning: more than 1024 cylinders ($c).\n";
    print STDERR "Not all BIOSes will be able to boot this device.\n";
    $cc = 1024;
} else {
    $cc = $c;
}

# Preserve id when run again
if (defined($opt{'id'})) {
    $id = pack("V", doh($opt{'id'}));
} else {
    seek(FILE, 440, SEEK_SET) or die "$0: $file: $!\n";
    read(FILE, $id, 4);
    if ($id eq "\x00\x00\x00\x00") {
        $id = pack("V", get_random());
    }
}

# Print the MBR and partition table
seek(FILE, 0, SEEK_SET) or die "$0: $file: $!\n";

for ($i = 0; $i <= $opt{'hd0'}+3*$opt{'partok'}; $i++) {
    $mbr = get_hex_data();
}
if ( length($mbr) > 432 ) {
    die "$0: Bad MBR code\n";
}

$mbr .= "\0" x (432 - length($mbr));

$mbr .= pack("VV", $de_lba*4, 0);       # Offset 432: LBA of isolinux.bin
$mbr .= $id;                            # Offset 440: MBR ID
$mbr .= "\0\0";                         # Offset 446: actual partition table

# Print partition table
$offset  = $opt{'offset'};
$psize   = $c*$h*$s - $offset;
$bhead   = int($offset/$s) % $h;
$bsect   = ($offset % $s) + 1;
$bcyl    = int($offset/($h*$s));
$bsect  += ($bcyl & 0x300) >> 2;
$bcyl   &= 0xff;
$ehead   = $h-1;
$esect   = $s + ((($cc-1) & 0x300) >> 2);
$ecyl    = ($cc-1) & 0xff;
$fstype  = $opt{'type'};        # Partition type
$pentry  = $opt{'entry'};       # Partition slot

for ( $i = 1 ; $i <= 4 ; $i++ ) {
    if ( $i == $pentry ) {
        $mbr .= pack("CCCCCCCCVV", 0x80, $bhead, $bsect, $bcyl, $fstype,
                     $ehead, $esect, $ecyl, $offset, $psize);
    } else {
        $mbr .= "\0" x 16;
    }
}
$mbr .= "\x55\xaa";

print FILE $mbr;

# Pad the image to a fake cylinder boundary
seek(FILE, $imgstat[7], SEEK_SET) or die "$0: $file: $!\n";
if ($padding) {
    print FILE "\0" x $padding;
}

# Done...
close(FILE);

exit 0;
__END__