Oh, I understood - I've been a professional programmer for close to 30 years, and I've done a lot of operating systems work.

On this platform you have the burden of ensuring that the C runtime is thread safe, or putting locks around it. The same with BIOS calls. And DOS function calls too; you can't have those other CPU cores just blundering in on something that is not reentrant.

And besides, I target this at 16 bit DOS on vintage machines. I've never seen a PCjr or an AT class machine with multiple cores. There were some super expensive 386s at the time that were multi-processors, but those are a fairly rare item.