What are the basic features that form a real-time OS?

[matrixofdynamism]

When writing an operating system,
(1) what are the basic minimum features that an OS must have to be considered sufficient to be called a real-time OS?

On another note,
(2) what hardware features must a processor design have to be able to support such an OS? and also,
(3) what certain type of instructions in its instruction set must it also have?

 

[srizbf]

When writing an operating system,
(1) what are the basic minimum features that an OS must have to be considered sufficient to be called a real-time OS?

For such an OS , one should be able to predict the task execution time.

On another note,
(2) what hardware features must a processor design have to be able to support such an OS? and also,
(3) what certain type of instructions in its instruction set must it also have?

the processor must support task switching and context switching.

the instructions to enter into supervisor and user modes.