When we try to argue about confinement from a mathematical perspective some strange issues arise. We can hardly hope to prove that a random number generator that you run will not produce the secrets that I had hoped to confine. At best we might hope to prove that your generator is no more likely to produce my secrets contingent on those secrets existing in a confined portion of the system.
The details of the individual implementations made most of the instructions determinate. The MVCL instruction was interruptible and the state of the registers upon resumption revealed that an interruption had taken place. This could reveal, in turn, information that the program should not have.
One edition of the 370 manual, I believe, was careful to say that the indeterminate state that was revealed to the user mode program did not depend on the state of other processes. This problem would be solved by modifying the hardware architecture by writing the four parameter registers with the final values upon completion of the MVCL. I recall the term “boundedly indeterminate” but that term has been used with other meanings among the CSP theorists.
Even when there is no possible exploitation of the noise, simplicity of arguments may justify the expense of determinism.