Comment by LocalH

Kildall himself claimed he could input a "secret" command or keystroke to DOS, and have it print out a DRI copyright notice. He allegedly showed this to Jerry Pournelle, and neither man publicly disclosed the method.

I don't believe it, I think Kildall was lying. Enough source code is available to directly inspect this, even if Kildall's code was heavily obfuscated and supposedly translated to 8086 for 86-DOS