DOS is small relative to modern OSs, but don't let the binary size (few dozen KB) of the kernel fool you --- being written in Asm, it is very dense code, basically no bytes wasted. There's probably more dead code in a "Hello World" binary compiled with a modern language than there is live code in the DOS kernel.
Sure but it's still tiny. If every single byte of it were a full line in a high level language it would still be still a tiny, fully externally-analyzable program. I don't need to make that kind of theoretical argument, though - the entire late stage of serious DOS software was completely dependent on DOS having been taken apart and understood, bit by individual bit.
