I will skip right to the point: The magic command for creating an ISO9660 image for a disc that is bootable both on a PC BIOS and on a modded Xbox goes something like this:

$ mkisofs -udf -r -joliet -eltorito-catalog isolinux/boot.cat -boot-info-table -eltorito-boot isolinux/isolinux.bin -no-emul-boot -boot-load-size 4 -o basic.iso basic/

Explanations follow

  • -udf creates a UDF filesystem in addition to the standard ISO9660. Apparently this is required for the Xbox to recognize the disc as bootable.
  • -r enables the creation of Rock Ridge file metadata. This allows for longer filenames, permissions and other improvements in POSIX systems. Probably used once the Linux kernel is booted from the disc. Unlike the -rock parameter, -r overrides some of the metadata with default values that make more sense for removable media.
  • -joliet enables the creation of Joliet file metadata. Same purpose as Rock Ridge but for Windows based systems. Not sure if strictly required here (but it seems related to UDF according to genisoimage's man page).
  • -eltorito-catalog specifies the location of the "boot catalog" that is supposedly ignored by everything but is still required according to the spec. Note that isolinux/boot.cat specifies a file to be created inside the image file. It does not (need to) exist in the source directory basic/.
  • -eltorito-boot specifies a boot image to be embedded in the image. This is the path to an existing file inside the CD root directory specified last on the command line. The boot image is an executable file to be run without an operating system, typically a boot loader. As I understand it this is comparable to the code inside the MBR of a hard drive.
  • -boot-info-table specifies that the "partition table" inside the boot image should indeed be updated with the actual CD layout before embedding it in the image file. Note that this makes permanent changes to the source file.
  • -no-emul-boot specifies that the boot image should be run "as is". Default is to emulate a floppy drive, in which case the boot image must be an actual floppy image.
  • -boot-load-size specifies the size of the boot image in fake "sectors" (512 byte). This should typically be the size of a CD sector, which is 2048 bytes in "mode 1". If this is smaller than the boot image (as in this case) it is truncated, which ISOLINUX is designed to handle. If unspecified the entire image is embedded.
  • Finally -o specifies the filename to write and basic/ is the directory which will become the CD root directory in the image.

Further details

The ISO9660 format, due to being very old, is fairly limited in its vanilla shape and has therefore been extended several times during the years. That's why there are now four ways of encoding the filename and other metadata on a CD (ISO9660, Rock Ridge Interchange Protocol, Joliet and UDF).

Vanilla ISO9660 supports filenames of up to 31 characters, and the allowed list of characters is short. But for compatibility with DOS, typically only 8+3 characters are allowed in implementations (like genisoimage/mkisofs). -l unlocks the full length of 31 chars, but I'm not sure if this is supported by the Xbox BIOSes.

As eluded to earlier, -r overrides some of the file metadata with sensible values. Examples are squashing uid/gid to 0 (since the user who inserted the disc is usually treated as the "owner" of its contents), removing write permissions for everyone (read-only media), and allowing read access to both user/group/others (CD's are not really multi-user systems).

El Torito is the name of the ISO9660 extension for bootable optical media. This is used by all PC's in BIOS mode. (U)EFI was just a baby when the Xbox was new, works differently and is outside the scope of this article.

The Xbox BIOS does not consider El Torito however. It simply looks for a default.xbe file and executes it if found (and the disc is valid, unless those checks have been defeated like in a modded Xbox). The gotcha is that the Xbox original BIOS (and hacked versions thereof) only supports UDF, none of the other file metadata formats. It makes some sense since games are normally only stored on DVD's, and DVD's typically (but not always) use UDF.

The free BIOS Cromwell however, does support one or more of the other formats (I haven't checked which). And as you may know it does not work with XBE's at all, instead Cromwell is typically used to directly load a Linux kernel as specified in the linuxboot.cfg text file.

Test case

This was tested using Ed's Xebian 1.1.4 basic edition, available from the Xbox-Linux Sourceforge project page.

References

See messages 20040714-Re_dual_boot_iso-4805 and 20040714-Re_dual_boot_iso-4807 on the xbox-linux-user mailing list.

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