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.}

>>1
I'm not quite sure how exactly to mix C and assembler without breaking everything.
Use the fucking ``asm'' keyword only if needed and if in doubt disassemble shit to look how it adds up with your program.

Also: shit dude you never heard of dosbox?