Threading in MS DOS 7.1

I have here an old laptop and intend to send/receive data on the parallel port in set time intervals (20 kHz max).
The laptop has MS DOS 7.1 installed, as well as Turbo C. Is it possible to create threads in DOS via some lib, and if not, how would I implement ISRs in C? So far I have only done this in assembler, and I'm not quite sure how exactly to mix C and assembler without breaking everything.

_{I could install TinyCore, but I kinda want to keep MS DOS so I can play Commander Keen.}

>>12
That just leaves the problem to tell my ISR where my function is. I'd just load the ISR in a TSR program and then start the C program, writing the address in the memory, but those two programs would probably end up in different segments, and I don't really know what to do then.

>>11
It's a 233MHz CPU, so I figured I had enough cycles to spare for a waiting loop. At least that's what I hoped for. Thanks for the link though, it could really help if my documentation runs out of information.