Memory models, FAT, interrupts, and the open-source continuation of the original PC OS.
Two genuinely different DOS executable formats, with real structural differences in how each loads into memory and why COM files had a strict 64K limit.
The clever reload-from-disk trick that let DOS's COMMAND.COM recover automatically after a large program overwrote its expendable transient portion.
Getting a USB flash drive to appear as a drive letter under FreeDOS by correctly configuring USBASPI's driver chain, or falling back to BIOS-level access.
FreeDOS's kernel is a from-scratch, clean-room reimplementation, and its authors' actual architecture choices differ deliberately from how MS-DOS was built.
Installing a C compiler and assembler on FreeDOS and building your first program, for anyone wanting to write DOS software rather than just run it.
Installing DOSLFN, understanding what it can and can't do, and verifying long filenames actually work with your specific FreeDOS utilities.
Getting sound, mouse, and memory configured correctly for DOS-era gaming - the three things almost every classic game setup guide assumes already work.
Configuring a RAM disk with the built-in RAM driver - a fast, volatile drive letter backed by memory, useful for temporary files and disk-heavy tasks.
Building a batch-file boot menu that manages memory-hungry TSRs - loading only what a chosen task needs, freeing conventional memory for everything else.
Configuring the USBASPI/ASPIDISK driver chain to give FreeDOS a working drive letter for a USB flash drive, bridging DOS-era drivers to a newer standard.