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After text activity

VI. Reading Exercises:

Exercise 1.Read and memorize using a dictionary:

Multiplexing, several, claimed identity, initial, linearly addressed memory system, associated, unaware, rate, descriptor register, interposed, a base value, a bound value, embellish, supervisor program, privileged state bit

 

Exercise 2.Answer the questions:

1) What is an example of a complete isolation system?

2) What does a typical computer consist of?

3) What is the essence of a multiprogramming system?

4) What is a descriptor register?

5) What is the privileged state bit?

 

Exercise 4.Match the left part with the right:

1 It is relatively easy to use a single computer to a) simulate several, each of which is completely unaware of the existence of the others, except that each runs more slowly than usual.
2. Such a simulation is of interest, since during   b) a multiprogramming system.
3. Thus a single processor may be able to   c) the intervals when one of the simulated (commonly called virtual) processors is waiting for an input or output operation to finish, another virtual processor may be able to progress at its normal rate.
4. Such a scheme is the essence of d) take the place of several.

 

THE SPEAKING MODULE

II. Speaking Exercises:

Exercise 1.Define the terms using the suggested words and expressionsas in example:

Information science Computer science Data processing Multiplexing
discipline, deals with, processes, storing,transferring, information study, computers, computation, systems control, development, hardware, software, programming conversion, raw, data, machine-readable form     process, transmitting, multiple, signals, simultaneously, single, channel,

 

EXAMPLE: Information science is a discipline that deals with the processes of storing and transferring information.

 

Exercise 2.Ask questions to the given answers:

1)Question: ___________________________________________ ?

Answer: If any running program could load it with any arbitrary value, there would be no protection.

2) Question: ___________________________________________ ?

Answer: One program (named the supervisor--program S in Fig. 1) runs with the privileged state bit ON, and controls the simulation of the virtual processors for the other programs.

 

3)Question: ___________________________________________ ?

Answer: Later we will extend our model of guards and walls in the discussion of shared information.

 

 

THE WRITING MODULE

 

Writing exercises:

Exercise 1.Fill in the gaps with the suggested words:

Will, load, additional, to, state, with

The instruction that loads the descriptor register ____a new descriptor must have some special controls--either on the values it ____load or on who may use it. It is easier ____control who may use the descriptor, and a common scheme is to introduce an _______bit in the processor state. This bit is called the privileged state bit. All attempts to_____ the descriptor register are checked against the value of the privileged state bit; the privileged _____bit must be ON for the register to be changed.



 

Exercise 2. Compose a story on one of the topics (up to 100 words):

1) The multiplexing of a single computer system among several users

2) The authentication of a user's claimed identity

3) An isolated virtual machine

 

 

Lesson 7

The Reading Module

Read the text:

One might expect the supervisor program to maintain a table of values of descriptors, one for each virtual processor. When the privileged state bit is OFF, the index in this table of the program currently in control identifies exactly which program--and thus which virtual processor--is accountable for the activity of the real processor. For protection to be complete, a virtual processor must not be able to change arbitrarily the values in the table of descriptors. If we suppose the table to be stored inside the supervisor program, it will be inaccessible to the virtual processors. We have here an example of a common strategy and sometime cause of confusion: the protection mechanisms not only protect one user from another, they may also protect their own implementation. We shall encounter this strategy again.

So far, this virtual processor implementation contains three protection mechanisms that we can associate with our abstractions. For the first, the information being protected is the distinct programs of Fig. 1. The guard is represented by the extra piece of hardware that enforces the descriptor restriction. The impenetrable wall with a door is the hardware that forces all references to memory through the descriptor mechanism. The authority check on a request to access memory is very simple. The requesting virtual processor is identified by the base and bound values in the descriptor register, and the guard checks that the memory location to which access is requested lies within the indicated area of memory.

The second mechanism protects the contents of the descriptor register. The wall, door, and guard are implemented in hardware, as with the first mechanism. An executing program requesting to load the descriptor register is identified by the privileged state bit. If this bit is OFF, indicating that the requester is a user program, then the guard does not allow the register to be loaded. If this bit is ON, indicating that the requester is the supervisor program, then the guard does allow it.

The third mechanism protects the privileged state bit. It allows an executing program identified by the privileged state bit being OFF (a user program) to perform the single operation "turn privileged state bit ON and transfer to the supervisor program." An executing program identified by the privileged state bit being ON is allowed to turn the bit OFF. This third mechanism is an embryonic form of the sophisticated protection mechanisms required to implement protected subsystems. The supervisor program is an example of a protected subsystem, of which more will be said later.

The supervisor program is part of all three protection mechanisms, for it is responsible for maintaining the integrity of the identifications manifest in the descriptor register and the privileged state bit. If the supervisor does not do its job correctly, virtual processors could become labeled with the wrong base and bound values, or user programs could become labeled with a privileged state bit that is ON, The supervisor protects itself from the user programs with the same isolation hardware that separates users, an example of the "economy of mechanism" design principle.

 


Date: 2016-01-03; view: 833


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