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104.11.12 11/6 OS作業
名稱	11/6 OS作業
日期	104.11.12
課程名稱	作業系統
指導教師	劉艾華

1.Which situation is suitable to use message passing instead of shared-memory model for communication between processes? What are the pros and cons of using these two models?
A: Distributed memory machines use message passing to communicate data between processors at programmer-determined points in the program’s execution. Message passing offers the potential of high performance and allows easy and relatively inexpensive scaling the system by adding more workstations to the network. Unfortunately, the need to explicitly send data requires considerable care by the programmer. Shared memory multiprocessors present a common memory interface to all processors. As a result, a shared memory parallel program is typically easier to write than a message passing program that performs the same task. In the absence of heavy contention, shared memory machines also offer the potential for high performance. However, contention for shared resources typically limits the performance and scalability of these machines. Hardware solutions to this contention problem also increase the cost of shared memory machines significantly, particularly for machines with more than a few processors.
2.What is “system calls?” Please specify the functions and the purposes of a system call in a computer system.
A: In computing, a system call is how a program requests a service from an operating system's kernel. This may include hardware-related services (for example, accessing a hard disk drive), creation and execution of new processes, and communication with integral kernel services such as process scheduling. System calls provide an essential interface between a process and the operating system. In most systems, system calls are possible to be made only from user space processes, while in some systems, OS/360 and successors for example, privileged system code also issues system calls.
3.Please specify different ways for using the hardware configuration during the system generation (SYSGEN).What are the major differences between these different ways?
A: Hardware configuration information can be used in different ways. First, construct a system that is completely table driven. All the code is always part of the system, and selection occurs at execution time. Second , used to create tables and modules from a precompiled library. The needed modules are linked together to form the generated OS. Third, used to modify a copy of the source code of the OS and have it re-compiled. Hardware configuration references the details and system resource settings allotted for a specific device. Many computer specialists improve hardware performance by adjusting configurations, which may also include settings for the motherboard and the BIOS, as well as the bus speeds. System generation is a process that creates a particular and uniquely specified operating system; it combines user-specified options and parameters withmanufacturer-supplied general-purpose or nonspecialized program subsections to produce an operating system (or other complexsoftware) of the desired form and capacity. And the major differences among above is the size and generality of the generated system and the ease of modification as the hardware configuration changes.

更新日期：2015/11/12 下午 02:13:35