Questions about supervisor mode

Question

Reading OS from multiple resources has left be confused about supervisor mode. For example, on Wikipedia:

In kernel mode, the CPU may perform any operation allowed by its architecture ..................

In the other CPU modes, certain restrictions on CPU operations are enforced by the hardware. Typically, certain instructions are not permitted (especially those—including I/O operations—that could alter the global state of the machine), some memory areas cannot be accessed

Does it mean that instructions such as LOAD and STORE are prohibited? or does it mean something else?

I am asking this because on a pure RISC processor, the only instructions that should access IO/memory are LOAD and STORE. A simple program that evaluates some arithmetic expression will thus need supervisor mode to read its operands.

I apologize if it's vague. If possible, can anyone explain it with an example?

Answer 1

I see this question was asked few months back and this should have been answered long back.

I will try to set few things straight before talking about I/O part of your question.

CPU running in "kernel mode" means that OS has permitted CPU to be able to execute few extra instructions. This is done by setting some flag at an appropriate moment. One can think of it as if a digital switch enables or disables specific operations embedded inside a processor.

In RISC machines, LOAD and STORE are generally register related operations. In fact from processor's perspective, traffic to and from main-memory is not really considered an I/O operation. Data transfer between main memory and processor happens very much automatically, by virtue of a pre-programmed page table (unless the required data is NOT found in main memory as well in which case it generally has to do disk I/O). Obviously OS programs this page table well in advance and does its book keeping operations in it.

An I/O operation generally relates to those with other external devices which are reachable through interrupt controller. Whenever an I/O operation completes, the corresponding device raises an interrupt towards processor and this causes OS to immediately change the processor's privilege level appropriately. Processor in turn works out the request raised by interrupt. This interrupt is a program written by OS developers, which may contain certain privileged instructions. This raised privileged level is some times referred as "kernel mode".