Why Mainframe cannot be Hacked ?

Windows is an operating system started off as being meant for one user only, and they could run only one program at a time, and it was written to manage real devices in the PC, that is, it was limited to what the PC had ( eg.,640K).

When it was realised that this approach meant that programs which required more hardware capacity (like memory) could not be written, special routines that virtualised hardware had to be written. However, the underlying operating system files etc were all open to anyone who wanted to install software, that is, any programmer could overwrite system software causing malicious or unintended damage.The operating system grew organically and tries to mimic multi- user environment, virtualisation and allowing multiple programs at the same time etc, but this has been developed as additions and not designed from ground up. Hence the stability issues and issues of hacking because the entire system software etc was open for anybody to overwrite with their own code.

Mainframe OZ (zOS) on the other hand was written ground up to be virtual,multi user system, multi program system. The hardware was always virtualised and programmers do not deal with physical limitations of the system.

Next, Operating system files and production code always was protected by the creation of a strict discipline and the creation of special libraries from which they could be run. This iscontrolled by system programmers.

Application programs never replace operating system files unlike in Windows or UNIX (to a lesser extent).

This created this environment never to be hacked to date.