source: bootcd/isolinux/syslinux-6.03/diag/geodsp/README

GeoDsp: Images to display the geometry as the BIOS has choosen to
interpret it.  Both images are intended to be written to the first ~8MiB
of a raw device (ie /dev/hda, /dev/sda) and be over one large cylinder
of 255*63 512-byte sectors in size.

To save the existing data for restore later:

        dd bs=1M iflag=fullblock count=8 if=/dev/sda of=sda.img

GeoDsp1S is a one-sector variant containing all code in one sector that
is intended to test behavior with a typical MBR/partition table layout. 
A partition table should be written after writting an image.

GeoDspMS is a multi sector variant intended to look like Syslinux
installed on a file system on the raw device (as opposed to a file
system within a partition).

GeoDspMS can also be used to attempt to make the boot sector look like a
normal file system's boot sector (ie FAT12/FAT16/FAT32).  In order to do
this, you must first save a portion the existing boot sector (the
majority of the BIOS parameter block).

        dd bs=1 skip=3 count=87 if=/dev/sda1 of=sda1.bpb
        dd conv=notrunc if=geodspms.img of=/dev/sda1
        dd conv=notrunc bs=1 seek=3 count=87 if=sda1.bpb of=/dev/sda1

        dd bs=1 skip=3 count=87 if=/dev/fd0 of=fd0.bpb
        dd conv=notrunc if=geodspms.img of=/dev/fd0
        dd conv=notrunc bs=1 seek=3 count=87 if=fd0.bpb of=/dev/fd0