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 access mode 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 an operat-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 the new 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. Using four modes rather than two increases the security granularity for the operating system allowing greater access control. A disadvantage is that now when a user program wants to access memory or I/O functions it must traverse multiple layers rather than the normal two. This multi-level strategy is similar to the OS rings concept put forth by Dijkstra, which was first implemented in the MULTICS OS. The MULTICS OS later influenced the creation and design of Unix.

B. MULTICS used seven OS layers. Also many processors today allow for Hypervisor mode, which is known as layer 0 and grants direct hardware access. It is used in conjunction with virtualization software like VMware. Also recently the AMD-V processor has come out with a -1 level so that guest operating systems can run in level 0 natively without conflicting with other guest operating systems.

Note: guests operating systems = virtual machines