The previous PDP-11 Live CD that I made is great, but it only works with IDE CD-ROM drives, so you cannot use it if you have a SATA or USB CD-ROM. Which is a shame. So I wanted to make a version that doesn't rely on having CD-ROM drivers for DOS, since it's unlikely that I'd be able to support all of the different types of drive. I have tried to get my external USB CD-ROM drive to work under DR-DOS and found that I couldn't make it go.
So, for a few days I have been struggling to create an "El Torito" hard disk image, so that I could make a CD bootable version of a hard disk. I haven't gotten it to work yet. I even tried this (twice actually) by taking the HxD hex editor to an ISO file. I've given up for the time being, but I'll probably come back to it.
However, I can get floppy disk "El Torito" images to work perfectly, including the 2.88 Mb variety. That's how I got the previous Live CD to start (but I was only using a 1.44 Mb image that time). So with a bit of lateral thinking I realised that my PDP-11 disk compresses quite well. So I went off an got an UNZIP program for DOS, from here ... or to be specific, I downloaded unz600x3.exe from: ftp://ftp.info-zip.org/pub/infozip/msdos/.
This means that I can fit DR-DOS into a 2.88 Mb floppy image along with a compressed copy of the PDP-11 emulator and disk. Since everything is inside the 2.88 Mb image, we don't need to load CD-ROM drivers in DOS. It means that this bootable ISO will work with SATA and USB CD-ROM drives, it doesn't need the drivers since it's all done by the BIOS and the magic of El Torito. Cool.