isohybrid: fix partition table generation, documentsyslinux-3.72-pre3

Fix the generation of the partition table in isohybrid; also document
the operation of the partition table.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

2 files changed, 9 insertions, 10 deletions

diff --git a/doc/isolinux.txt b/doc/isolinux.txt
index dd8bca5a..eca2a974 100644
--- a/doc/isolinux.txt
+++ b/doc/isolinux.txt
@@ -96,6 +96,11 @@ This image can then be copied using any raw disk writing tool (on Unix
 systems, typically "dd" or "cat") to a USB disk, or written to a
 CD-ROM using standard CD burning tools.
 
+The ISO 9660 filesystem is encapsulated in a partition (which starts
+at offset zero, which may confuse some systems.)  This makes it
+possible for the operating system, once booted, to use the remainder
+of the device for persistent storage by creating a second partition.
+
 
       ++++ BOOTING DOS (OR OTHER SIMILAR OPERATING SYSTEMS) ++++
 
diff --git a/utils/isohybrid.in b/utils/isohybrid.in
index a0487ca0..5e3f785a 100644
--- a/utils/isohybrid.in
+++ b/utils/isohybrid.in
@@ -124,7 +124,7 @@ $mbr .= pack("V", $id);		# Offset 440: MBR ID
 $mbr .= "\0\0";			# Offset 446: actual partition table
 
 # Print partition table
-$psize = $c*$h*$s-$s;
+$psize   = $c*$h*$s;
 $bhead   = 0;
 $bsect   = 1;
 $bcyl    = 0;
@@ -132,18 +132,12 @@ $ehead   = $h-1;
 $esect   = $s + ((($cc-1) & 0x300) >> 2);
 $ecyl    = ($cc-1) & 0xff;
 $fstype  = 0x83;		# Linux (any better ideas?)
-$pentry  = 1;
-if ( $c > 1024 ) {
-    $fstype = 0x0e;
-} elsif ( $psize > 65536 ) {
-    $fstype = 0x06;
-} else {
-    $fstype = 0x04;
-}
+$pentry  = 1;			# First partition slot
+
 for ( $i = 1 ; $i <= 4 ; $i++ ) {
     if ( $i == $pentry ) {
 	$mbr .= pack("CCCCCCCCVV", 0x80, $bhead, $bsect, $bcyl, $fstype,
-		     $ehead, $esect, $ecyl, $s, $psize);
+		     $ehead, $esect, $ecyl, 0, $psize);
     } else {
 	$mbr .= "\0" x 16;
     }