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.}

>>4
You can't use C functions as interrupt handlers without special compiler support, because it won't know to preserve all the registers and the stack.
It should be dead easy to mix C and assembly, though, you just need to know what ABI you are using.
Write a makefile to compile/assemble to object files, then link them together.