I was a software engineer at one of the 16-bit microcomputer companies that licensed DOS 1, 2 and 3 before and contemporary to the IBM PC, so I've seen the (assembler) source for DOS. It was impenetrable rubbish, even then, mostly due to the different groups using different sets of macros to "streamline" their work (MASM, you know!). Turned out the easiest way to figure out what it was trying to do was to run the output through a disassembler and crossref back to the source so you could see what all those stupid macros were actually doing. The different ways DOS could be configured were also a nightmare (we used to load COMMAND.COM in high memory for example, instead of just above the DOS core like everyone else).
On the plus side, letting everybody finally see all the undocumented stuff like the "inDos" flag in its original incarnation would be fun.