A while ago I wrote about why IBM PC XENIX 1.0 can’t work on any CPU other than a 286. But not content to leave well enough alone, I attempted to patch this version of XENIX so that it would run on a post-286 processor.
While doing that, I had to change my original assessment of the problem. IBM PC XENIX does not put garbage into the reserved word of a descriptor table entry, it actually stores data there! That makes the behavior much harder to excuse, and also much harder to fix.
After some quality time spent with a disassembler, greatly aided by the symbol table included in the xenix and xenix.fd kernel files, I established that XENIX stores the segment size in the reserved descriptor word (this is the size in the file which may differ from the segment limit). Unfortunately, all my attempts to patch the XENIX kernel failed and I could never get the installation floppy to boot.
Or so I thought. A few days ago, I more or less by accident booted the patched XENIX floppy, and lo and behold:
It actually worked! It turned out that in my previous attempts, I assigned 8MB RAM to the XENIX VM. That was a mistake, because IBM PC XENIX 1.0 crashes with more than 4MB RAM! Duh… such a common problem which in various incarnations plagues many, many old operating systems (for example OS/2 1.0 crashes with more than 8MB RAM). Continue reading →
