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(3-1) The first task, managing the hardware and software resources, is very important, as various programs and input methods compete for the attention of the central processing unit (CPU) and demand memory, storage and input/output (I/O) bandwidth for their own purposes. In this capacity, the operating system plays the role of the good parent, making sure that each application gets the necessary resources while playing nicely with all the other applications, as well as husbanding the limited capacity of the system to the greatest good of all the users and applications.
(3-2) Interrupts are special signals sent by hardware or software to the CPU. It’s as if some part of the computer suddenly raised its hand to ask for the CPU’s attention in a lively meeting. Sometimes the operating system will schedule the priority of processes so that interrupts are masked — that is, the operating system will ignore the interrupts from some sources so that a particular job can be finished as quickly as possible.
(3-3) Trouble can begin if the user tries to have too many processes functioning at the same time. The operating system itself requires some CPU cycles to perform the saving and swapping of all the registers, queues and stacks of the application processes. If enough processes are started, and if the operating system hasn’t been carefully designed, the system can begin to use the vast majority of its available CPU cycles to swap between processes rather than run processes. When this happens, it’s called thrashing , and it usually requires some sort of direct user intervention to stop processes and bring order back to the system.
(3-4) One reason that drivers are separate from the operating system is so that new functions can be added to the driver — and thus to the hardware subsystems — without requiring the operating system itself to be modified, recompiled and redistributed. Through the development of new hardware device drivers, development often performed or paid for by the manufacturer of the subsystems rather than the publisher of the operating system, input/output capabilities of the overall system can be greatly enhanced.
(3-5) The kernel mode is a highly privileged mode of operation in which the program code has direct access to the virtual memory. This includes the address spaces of all user mode processes and applications and their hardware.
(3-6) UNIX users generally configure a system by editing the configuration files with any of the available text editors. The advantage of this mechanism is that the user does not need to learn how to use a large set of configuration tools, but must only be familiar with an editor and possibly a scripting language. The disadvantage is that the information in the files comes in various formats; hence the user must learn the various formats in order to change the settings. UNIX users often employ scripts to reduce the possibility of repetition and error.
(3-7) UNIX has several IPC mechanisms that have different characteristics and which are appropriate for different situations. Shared memory, pipes , and message queues are all suitable for processes running on a single computer. Shared memory and message queues are suitable for communicating among unrelated processes. Pipes are usually chosen for communicating with a child process through standard input and output. For communications across the network, sockets are usually the chosen technique.