All of the information contained inside the machine’s storage array (see the Third Law), whether executable or not, shall be accessible at all times for inspection and modification by the user/programmer, in the form preferred for modification. The user/programmer shall have the ability to modify the functionality of any executable code within the system without cycling power or performing any similar ritual. No modification of any kind shall be able to render the means for undoing said modification (the keyboard and a spare instance of the built-in debugger) inoperable.
Personal computer operating systems known to obey this law:
1 (and many others) [*]
Notes and observations:
Any 1980s micro equipped with a ROM debugger technically qualifies, though not usefully.
To be continued.
[*] Corrections are welcome.