COS 3721theory. VERIFIED EXAM Q AND A.

COS 3721theory. VERIFIED EXAM Q AND A. Chapter 1 2018 S2 A1 What is the purpose of interrupts? How does an interrupt differ from a trap? Can traps be generated intentionally by a user program? If so, for what purpose?  An interrupt is a hardware-generated change of flow within the system. An interrupt handler is summoned to deal with the cause of the interrupt; control is then returned to the interrupted context and instruction.  A trap is a software-generated interrupt. An interrupt can be used to signal the completion of an I/O to obviate the need for device polling. A trap can be used to call operating system routines or to catch arithmetic errors. 2018 S2 A1 Describe some of the challenges of designing operating systems for mobile devices compared with designing operating systems for traditional PCs.  The greatest challenges in designing mobile operating systems include:  Less storage capacity means the operating system must manage memory carefully.  The operating system must also manage power consumption carefully.  Less processing power plus fewer processors mean the operating system must carefully apportion processors to applications. 2018 S2 A1 Identify several advantages and several disadvantages of open-source operating systems. Include the types of people who would find each aspect to be an advantage or a disadvantage.  Open source operating systems have the advantages of having many people working on them, many people debugging them, ease of access and distribution, and rapid update cycles.  Further, for students and programmers, there is certainly an advantage to being able to view and modify the source code.  Typically open source operating systems are free for some forms of use, usually just requiring payment for support services.  Commercial operating system companies usually do not like the competition that open source operating systems bring because these features are difficult to compete against.  Some open source operating systems do not offer paid support programs. Some companies avoid open source projects because they need paid support, so that they have some entity to hold accountable if there is a problem or they need help fixing an issue.  Finally, some complain that a lack of discipline in the coding of open source operating systems means that backward compatibility is lacking making upgrades difficult, and that the frequent release cycle exacerbates these issues by forcing users to upgrade frequently. 2018 S1 A1 Some early computers protected the operating system by placing it in a memory partition that could not be modified by either the user job or the operating system itself. Describe two difficulties that you think could arise with such a scheme. The data required by the operating system (passwords, access controls, accounting information, and so on) would have to be stored in or passed through unprotected memory and thus be accessible to unauthorized users. 2018 S1 A1 Distinguish between the client-server and peer-to-peer models of distributed systems.  The client-server model firmly distinguishes the roles of the client and server. Under this model, the client requests services that are provided by the server.  The peer-to-peer model doesn’t have such strict roles. In fact, all nodes in the system are considered peers and thus may act as either clients or servers—or both. A node may request a service from another peer, or the node may in fact provide such a service to other peers in the system.  For example, let’s consider a system of nodes that share cooking recipes. Under the client-server model, all recipes are stored with the server. If a client wishes to access a recipe, it must request the recipe from the specified server. Using the peer-to-peer model, a peer node could ask other peer nodes for the specified recipe. The node (or perhaps nodes) with the requested recipe could provide it to the requesting node. Notice how each peer may act as both a client (it may request recipes) and as a server (it may provide recipes). 2018 S1 A1 In a multiprogramming and time-sharing environment, several users share the system simultaneously. This situation can result in various security problems. a. What are two such problems? Stealing or copying one’s programs or data; using system resources (CPU, memory, disk space, peripherals) without proper accounting. b. Can we ensure the same degree of security in a time-shared machine as in a dedicated machine? Explain your answer. Probably not, since any protection scheme devised by humans can inevitably be broken by a human, and the more complex the scheme, the more difficult it is to feel confident of its correct implementation. 2017 S2 A1 What role do device controllers and device drivers play in a computer system?  A device controller provides for an interface between the peripheral(s) that it controls and the operating system.  It collaborates with the device driver, by means of interrupt, special-purpose registers and when necessary Direct Memory Access (DMA), to transfer data from/to the peripheral(s) to/from the buffer storage.  A device driver on the other side understands the device controller and hence provides the rest of the operating system with a uniform interface to de device.