If so, what are these OSes? Are they specially crafted? How difficult is it to apply this kind of program verification to the everyday OSes we use?

If not, why haven't people invented such OSes?

Package signature verification is quite common with today's package managers. What I'm asking about is signature verification at loading time.

iOS and Android both validates the signature of every single piece of code before loading them into memory.

Windows UWP apps are also all checked for signature before being loaded as well.


  Package signature verification is quite common with today's package managers. What I'm asking about is signature verification at loading time.


The difference is massive in terms of performances. A package signature is checked when it is installed and not afterward. 

To be effective, code signing must be checked for every binary before it is executed or loaded. 

Furthermore, special care must be taken by the OS (or runtime environment) in order to make sure a memory page marked as executable is signed (or, at least, that is has been loaded from something that was properly signed). That requirements is extremely hard to enforce on any environment that wasn't designed with code signing in mind because it tends to break a lot of legacy code.
