Hacker News Reader

(ms)dos has just a file system (it's the D in the name). It has no protected memory or scheduling, or any ability to "talk" between processes. It can load executable images .exe(MZ) or sub 64KB .com. ANd yes, it's possible to reserve memory and stay resident (TSR)

I have some experience repurposing 8259 (the interrupt controller), and if you hook on the timer (8), there is nothing else to return the execution back to the 'OS'. In short I'd disagree about the real-time.

You’d have to write your own scheduler.

With no memory protection a wayward process could easily overwrite the schedulers data structures, and probably hang the system.

FAT is not well suited to embedded use. The TFAT hack was patent encumbered and not widely supported outside of MS products.