Question

The VAX/ VMS operating system makes use of four processor access modes to facili-tate the protection and sharing of system resources among processes. The accessmode determines:

• Instruction execution privileges: What instructions the processor may execute
• Memory access privileges: Which locations in virtual memory the current instruc-tion may access

The four modes are as follows:
• Kernel: Executes the kernel of the VMS operating system, which includes memo-ry management, interrupt handling, and I/ O operations
• Executive: Executes many of the operating system service calls, including file and record ( disk and tape) management routines
• Supervisor: Executes other operating system services, such as responses to user commands
• User: Executes user programs, plus utilities such as compilers, editors, linkers, and debuggers

A process executing in a less- privileged mode often needs to call a procedure that executes in a more- privileged mode; for example, a user program requires anoperat-ing system service. This call is achieved by using a change- mode ( CHM) instruction, which causes an interrupt that transfers control to a routine at thenew access mode. A return is made by executing the REI ( return from exception or interrupt) instruction.

a. A number of operating systems have two modes, kernel and user. What are the advantages and disadvantages of providing four modes instead of two?
b. Can you make a case for even more than four modes?

Answer 1

a. The advantages and disadvantages of providing four modes instead of two:

Usually, many operating systems have two modes: kernel and user, instead of two, and some operating systems have four modes: the kernel, executive, supervisor, and user.

Advantage of having four modes instead of two modes:

• If the operating system has four modes, it can control access to the memory.

• It is allowing the finer tuning of memory protection.

The disadvantage of having four modes instead of two modes:

• If the operating system has four modes, then the system has more complexity and more system overhead.

• Consider the scenario, if the procedures are running on the access modes such as the kernel, executive, supervisor, and user, all require separate stacks with appropriate accessibility.

b. Case for more than four-nodes:

In principle, if the operating system has more modes, it has more flexibility, but it looks difficult to explain the system having more than four modes.