ISO 9660 discs still have bootable floppy images in them. The El Torito standard for bootable CDs specifies that a floppy image be incorporated into the disc image. The PC BIOS knows how to find this image, load it into memory, and boot it as a virtual floppy.
But of course, ugly hacks like this were only ever a peecee problem. Macs could boot CD-ROMs like any other drive for as long as they've had CD-ROMs.
> But of course, ugly hacks like this were only ever a peecee problem. Macs could boot CD-ROMs like any other drive for as long as they've had CD-ROMs.
This is a bit ridiculous, since in order to be bootable, a Mac requires the CD-ROM to contain a HFS filesystem.
PCs could also boot "like any other drive" from CD-ROM if they forced the CD-ROMs to contain a MBR.
Actually, that is exactly what El Torito is: putting a boot record on a CDROM, so that it can boot "like any other drive". The fact that the boot record is most commonly found in an image of a floppy disk is an implementation detail (you can have whatever you want, it's just that floppy is easier to work with), and hardly "a hack".
"A hack" would be to force the CD-ROM to contain a hybridized HFS filesystem, despite the entire raison d'être for ISO being to avoid having to put multiple filesystems in CDs.
ElTorito doesn't include normal boot record on the CD-ROM. Placing MBR or similar is a hack used among other things to support the ability to "burn" image to disk drive that is not CD/DVD.
El-Torito specifies a bunch of different possible images, among them floppy images (used by Win9x boot CDs) and native images (used by most linux bootloaders and by later WinNT versions), and there's IIRC separate type for UEFI bootloader.
There's still space left in front of the CD allowing for bootsector tricks, which is also how hybrid HFS+ disks work at all (there's space to fit necessary volume header, and Mac booting uses HFS+ "blessed file" pointer to find boot image).
But of course, ugly hacks like this were only ever a peecee problem. Macs could boot CD-ROMs like any other drive for as long as they've had CD-ROMs.