How to make bootable CDROM for BIOS-less computer?

Question

my Mac is the 2015 model that only supports UEFI. I'd like to create a bootable CDROM based on a bootable floppy disk image. Is this possible?

Answer 1

Yes, it's possible. Before proceeding, though, you should understand that CDs generally boot with the help of an El Torito image, which is a FAT filesystem stored on the CD alongside its normal ISO-9660 or UDF filesystem. Thus, you must prepare both an El Torito FAT filesystem and an ISO-9660 or UDF filesystem. That said, the ISO-9660/UDF part might conceivably be empty or nearly empty (and is likely to be so in your case). Broadly speaking, there are three steps to do this:

1. Prepare the OS to be booted by setting up a directory holding all the files. In your case, this image might be empty; or for convenience, you might copy out the contents of the disk image so that you can access it from the CD-R without using special tools. In a broader case, this might contain a more major OS installation to be run from the CD-R.
2. Create a bootable FAT filesystem image. Note that this image must be bootable in the EFI sense, not in the BIOS sense. That is, it should have an EFI boot loader at EFI/BOOT/bootx64.efi, and that boot loader should do whatever is required to boot the OS stored on the CD. In your case, this would be your existing floppy image. It's worth checking now to see if it has the EFI/BOOT/bootx64.efi file. (Note that filename case is irrelevant.)
3. Prepare the disc, storing the FAT filesystem image as an El Torito alternate image and the rest of the files (if any) in the main ISO-9660 or UDF filesystem.

Since you say you've got a bootable floppy disk image, much of your work is likely done for you -- at least if it's EFI-bootable (more on that shortly). I do something similar to this to prepare the CD-R image of my rEFInd boot manager. The command I use to prepare a .iso file is:

mkisofs -A "Bootable rEFInd" -V "rEFInd" -volset "rEFInd" \
    -J -r -v -x ./lost+found -o ../refind-cd.iso \
    -eltorito-alt-boot -efi-boot refind-bin.img -no-emul-boot ./

This example uses the mkisofs (or genisoimage) command, which comes with all major Linux distributions. I assume there's a version for OS X, but I haven't looked for it. This command prepares a disc image from the contents of the directory in which it's typed (the ./ at the end of the command). The key point for this discussion is the last line, which sets it up so that EFI systems will treat the refind-bin.img file as if it were an EFI System Partition (ESP). You'd specify your EFI-bootable floppy image rather than refind-bin.img. Once the command spits out an image file (../refind-cd.iso in this example), you can burn it to a CD-R just like any other such file.

The same thing is likely to be possible with other programs, but I don't happen to know the details for anything but mkisofs -- and I'm not really an expert even at that. (I'm just copying a command that I cobbled together and that works for me.)

Note that I've emphasized that the FAT disk image must be bootable in EFI mode. Many bootable floppy disk images are instead bootable in BIOS/CSM/legacy mode. Such images can be turned into bootable CD-Rs, too, but the details of what commands you'd use are different. Macs can boot BIOS-bootable CDs, and this boot mode may be perfectly acceptable to you, but your question emphasizes EFI-mode booting, so I assume you have a reason for this. (Maybe that's a bad assumption, though.)

If your floppy disk image lacks the EFI/BOOT/bootx64.efi file, then it cannot be booted in EFI mode, at least not as-is. (It might be modified with the addition of an EFI-mode boot loader, but the details depend on the OS the floppy contains. Not all OSes are bootable in EFI mode.) If this is the case, then something similar to the preceding command might work, but you'll need to modify it to make a BIOS-bootable image rather than an EFI-bootable one.

Answer 2

well it depends what the bootable floppy disk image is but if your motherboard can boot off the floppy image the yes. burn the contents of the image to the cd and insert the cd into the mac. restart and on the chime hold the option key and select the disk. if that doesn't work you can select the startup disk in System Preferences> Startup Disk select it from there and select restart from in the window.