The content of doc/mboot.txt (release 3.72):
mboot.c32 is a 32-bit comboot module that allows SYSLINUX and its variants to load and boot kernels that use the Multiboot standard (e.g. the Xen virtual machine monitor, and the Fiasco and GNU Mach microkernels).
To load a multiboot kernel and modules in SYSLINUX, put mboot.c32 (from com32/modules) in the boot directory, and load it as the "kernel" in the configuration file. The command-line to pass to mboot.c32 is the kernel command-line, followed by all the module command lines, separated with '---'. For example, to load a Xen VMM, xenlinux and an initrd:
DEFAULT mboot.c32 xen.gz dom0_mem=15000 nosmp noacpi --- linux.gz console=tty0 root=/dev/hda1 --- initrd.img
or, as a choice in a menu:
LABEL Xen KERNEL mboot.c32 APPEND xen.gz dom0_mem=15000 nosmp noacpi --- linux.gz console=tty0 root=/dev/ hda1 --- initrd.img
mboot.c32 requires version 2.12 or later of SYSLINUX.
Tim Deegan, May 2005