The DOS 3.3 SYS.COM bug hunt!

Last year somebody reported a problem with the DOS 3.3 SYS.COM command when used with NetDrive. They started with a valid FAT12 image, ran SYS.COM to make it bootable, and then they were not able to mount the image using NetDrive again. Running SYS.COM against the image had broken something.

Besides copying the operating system’s hidden files to the target drive letter, SYS.COM also copies some boot code into the first sector of the disk. In general it does not make sense to run it against a NetDrive image because you already had to boot DOS to mount the image, but it should not hurt anything. So I decided to have a look at what was going on.
↫ Michael Brutman

A good old classic bug hunt in some retro DOS code from roughly 1987. This one’s a bit more technical and in-depth than these things usually are, and quite a bit of it goes over my head, but I’m sure since most of you are much smarter than I am, you’ll do a better job understanding what’s going on.