COS3721 Questions-and-answers.

Chapter 1 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. a) If the operating system cannot modify the memory, then some important and critical data such as protection and/or security data (e.g. passwords and access control information) necessary for the operating system would have to be passed through or stored in unprotected memory slots and would be accessible to unauthorised users. b) The operating system could never be updated or patched, since it is not modifiable or accessible by the user or the operating system itself. Distinguish between the client–server and peer-to-peer models of Distributed systems. The core benefits of Peer-to-Peer model (in opposition with the centralized model) stem from the fact that each participating computer in the model can be both the client and server. In a Peer-to-peer model, services are distributed across a collection of peers, rather than having a single, centralized server. Peer-to-peer provides fault tolerance and redundancy. Also, because peers constantly migrate, they can provide a level of security over a server that always exists at a known location on the Internet. Peer-to-peer systems can also potentially provide higher network bandwidth because you can collectively use all the bandwidth of peers, rather than the single bandwidth that is available to a single server. In a multiprogramming and time-sharing environment, several users share the system simultaneously. This situation can result in various security problems. What are two such problems? Stealing or copying one’s programs or data; using system resources (CPU, memory, disk space, peripherals) without proper accounting. Can we ensure the same degree of security in a time-shared machine as in a dedicated machine? Explain your answer. Probably not, since a human, and the more complex can inevitably break any protection scheme devised by humans the scheme, the more difficult it is to feel confident of its correct implementation. 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. 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: COS3721 Questions-and-answers.  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. 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. 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, specialpurpose 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. Direct memory access is used for high-speed I/O devices in order to avoid increasing the CPU’s execution load. How does the CPU interface with the device to coordinate the transfer? The CPU can initiate a DMA operation by writing values into special registers that can be independently accessed by the device. The device initiates the corresponding operation once it receives a command from the CPU. How does the CPU know when the memory operations are complete? When the device is finished with its operation, it interrupts the CPU to indicate the completion of the operation. Both the device and the CPU can be accessing memory simultaneously. The memory controller provides access to the memory bus in a fair manner to these two entities.