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[20090103] Creating a bootable disk image (for shark/arm and i386)
I've added a SD-Card to IDE adapter to my shark some time ago, and wanted to create an image for the SD-Card. Using Mac OS as development platform prevented the use of NetBSD's vnd(4), plus I wanted to do this without root privileges. NetBSD's makefs(8) seemed like a nice tool, but how to add bootblocks, partition tables, etc.? A script posted by Izumi Tsutsui got me on track, and after looking a bit at some hexdumps of various file system images, boot blocks, and BSD disk labels, I managed to adjust Izumi's script enough to give me bootable disk images for booth shar/arm and i386. The script can be found in the port-arm archive.

Lessons that I've learned by this (and that I'd like to share):

  • a MS-DOS MBR is 512 bytes (boring)
  • a BSD disklabel is some 300-400 bytes, apparently not really fixed in size. The interesting part: if you create a BSD disk label on a image, the first 512 bytes are left alone. Good to put a MBR in!
  • when you have a FFS file system image (e.g. created with mkfile), the actual file system data only starts at byte 8192 - that's plenty of space to put a BSD disk label in there. Oh, and a MBR, too, if needed! :-)
A followup by Izumi explains these things in more details.

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