Looking at the source, it looks like it includes the Z80 boot code from the MS-DOS diskette (first two tracks). ZFLIP may well be in play, even though we don't need the floppy code to boot from the hard disk, the boot loader appears to put it in place and the rest of the OS depends on it... executing the wrong code during this because ZFLIP is wonky would definitely cause weird behavior, including stack overflow. 8088 processors really hate 8080/z80 code smile