GRUB on USB/Hard Disk is extensible in the sense that once you install to MBR/partition you can mount ISO using loop and boot of the ISO. Additionally it has many modules like ls and cat that are available to help you poke around and boot the right kernel/initrd image.

However with the Syslinux/ISOLINUX bootloader that comes pre-installed on Debian - I notice the following files (below). There are very few options to mess around and SYSLINUX commands like LABEL don't work at all. In fact the only thing I could do was type one of expert, install, etc - fixed labels stored in menu.cfg-->.cfg*

However there is a libcom32.c32 module and docs state that it can be used in Chainloading

COM32 chain.c32

However there is no chain.c32 module and grep of the files doesn't show COM32 and UTIL being used at all.

What's the purpose of these modules (libcom32, libutil) in the distribution DVD and can I do anything else besides load fixed labels (i know I can append kernel parameters by typing them after expert) but I can't load an ISO on the FS or chainload. I'm trying to figure out how useful the DVD-debian-stretch is in BIOS mode. I know with EFI systems you get the power of GRUB. What can I do with ISOLINUX in BIOS mode on Debian Stretch? (Can I file-presed or load an iso/different kernel stored on Hard disk/usb?)

ldlinux.c32 libcom32.c32 libutil.c32 vesamenu.c32 boot.cat isolinux.bin menu.cfg

The graphical boot menu is presented by vesamenu.c32, which depends on libutil.c32 and libcom32.c32 (you'll also find cat.c32 and ls.c32 on that page). Since stock Syslinux can use a single filesystem only, you won't be able to do much without adding files to the DVD. Besides remastering, it might be possible by writing a new session, too (I never tried using multisession CDs with Isolinux). For example, if you add other initramfs images, Isolinux can merge them. Or you can add other COM32 modules as well, just use the same version.

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