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HowTo: Making a bootable ISO w/ miniroot.kmod etc.

I've asked how building a bootable ISO works now that we have kernel
modules and a bootloader that can load kernels before the kernel,
e.g. for a ramdisk. After some digging, I've managed to fit the pieces
together, get things going[1], and here is a write-up so others that
are intestested don't have to wade through that swamp again.

Old (pre-5.0/current):

 * build INSTALL kernel, which reserves fixed space for a ramdisk
   image (pseudo-device md, options MEMORY_DISK_HOOKS,
 * build ramdisk image (done in src/distrib/i386/ramdisks). This uses
   crunchgen(1) and makefs(8) to assemble things, configuration is in one
   or several "list" files.
 * insert ramdisk image into kernel to get an instkernel (done in
   src/distrib/i386/instkernel). This is done via mdsetimage(8).
 * build ISO with instkernel and bootloderer (done in
   src/distrib/i386/cdroms/bootcd). This uses makefs(8) again.

New (5.0/current):

 * build GENERIC kernel, which tells there will be a ramdisk, but size
   will be detected at load-time (pseudo-device md, options
 * build ramdisk image as before (in src/distrib/i386/ramdisks)
 * build the miniroot.kmod (in src/sys/modules/miniroot). This is an
   empty module for a start, which will get the ramdisk appended in
   the next step.
 * add the ramdisk image to the miniroot.kmod module (done in
   src/distrib/i386/kmod). This uses the GNU bintools objcopy(1) to
   append the ramdisk into a special ELF section.
 * prepare an instkernel that is basically just a compressed version
   of the GENERIC kernel (done in src/distrib/i386/instkernel). This
   is the same code as before, but for each kernel, variables can be
   set to prevent it from getting stripped and a ramdisk image
   inserted (MDSET_NOSTRIP.netbsd-GENERIC,
 * build ISO with the instkernel kernel, the miniroot.kmod file, the
   bootloader and a config file for the bootloader (done in


 - Hubert

P.S.: I wonder if/how module loading works in a PXE boot environment.
      anyone got experiences with that?

[1] g4u 2.4alpha3 is available for testing

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