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.}

>>6
An old laptop, because it was a spare part, and having a keyboard and screen makes a nice interface without having to pay for additional hardware for in-/output.
>>7
I have basic knowledge of x86 assembly; I know how to work with interrupts and all the basic instructions. I also have a ~250 pages document here for the times I'm in doubt. My problem is neither C or asm, I just need to call a function every 50µs (i.e. 20kHz).
Also: What benefit could dosbox do in my case?
>>8
Well, fuck. I already thought that'd be a problem. I was hoping to use C for the ISR, since I might have to use trigonometric and/or exponential functions, and doing that in asm is a pain.
As an alternative I was thinking about using clock() in a loop to determine when to call the function, since a little jitter is acceptable. The UI can lag a little for all that I care, I'm just not sure if it could work.