Экзаменационный билет № 9

1) Intro to Operation systems: History of os

Operating Systems evolved tremendously in the last few decades. In fact, it would probably be impossible for someone coming directly from the 60s to perceive any resemblance between modern OSs and the OSs from those days.

The 1_rst approach for building Operating Systems, taken during the 40s and 60s, was to allow only one user and one task at a time. Users had to wait for a task to be finished before they could specify another task, or even interact with the computer. In other words, not only OSs were monouser and monotask, there was no overlapping between computation and IO.

The next step in the development of OSs was to allow batch processing. Now, multiple \jobs" could be executed in a batch mode, such that a program was loaded, executed, output was generated, and then the cycle restarted with the next job.

During the 70s, hardware became cheap, but humans (operators, users) were expensive. During this decade, interaction was done via terminals, in which a user could send commands to a mainframe. This was the Unix era. Response time and thrashing became problems to be dealt with; OSs started to treat programs and data in a more homogeneous way. During the 80s, hardware became even cheaper. It was then that PCs became widespread, and simple OSs, such as DOS and MacOS, were used. DOS, for example, was so simple that it didn't have any Multiprogramming features. From the 90s on (until today), hardware became even cheaper. Processing demands keep increasing since then, and \real" OSs, such as WindowsNT, MacOSX and Linux, finally became available for PCs.

If there is a lesson to be learnt from all this history, it is that it's very hard to outline trends for the future.

2) Memory allocation strategies: Contiguous allocation (first-fit and best-fit algorithms)

Memory Allocation Policies

• First-Fit: allocate the first one in the list in which the process fits. The search can start with the first hole, or where the previous firstfit search ended.

• Best-Fit: Allocate the smallest hole that is big enough to hold the process. The OS must search the entire list or store the list sorted by size hole list.

• Worst-Fit: Allocate the largest hole to the process. Again the OS must search the entire list or keep the list sorted.

• Simulations show first-fit and best-fit usually yield better storage utilization than worst-fit; first-fit is generally faster than best-fit.

3) Deadlock: Things you should be able to do: Given some code that might deadlock, describe how you might change the algorithm to prevent deadlock.

At first what does mean deadlock). Deadlock- is the set of blocked processes, each of which owns the some resource and waiting for a resource, which is owned by some other process in the set. What you’d do to prevent deadlocks.?there is exist some methods. The main idea is to limit the methods of resource requests from the processes. To limit the possibility of mutual exclusion ownership of resources (the first condition of deadlock), it should be noted that it is not required for all resources. For shared resources (eg, arrays, constants, codes, files) is not required. To limit the possibility of retention and waiting (second deadlock condition), you can require that the process requesting some resource, would have any resources. An alternative option is to require that all processes acquire all the resources which they need before the actual start of their execution.

If the process has a certain resource A and requests another resource B, which cannot be immediately selected to it, the process has to wait. At the same time resource A should be released immediately, which in the process. A resource A is added to the list of resources that the process is waiting. The process can be renewed only if it can be allocated at the same time all the old resources which he possessed and those new resources, which it expects. To prevent the situation of circular wait the simplest solution is to enter ordering of the numbers of all kinds of resources and to require that the process requests resources only in ascending order.

Экзаменационный билет № 10

1)intro to Operation systems: OS complexity

• Increased functionality & complexity

• First OS failures

  • Multics(GE&MIT): announced 1963, released 1969

  • OS/360 released with 1000 known bugs

• Need to treat OS design scientifically

The main complexity or some problems of OS is the requirements which need to construct a OS and develop it. They’re following:

1. expandability-here is we can easily enter spme changes or complete the OS.

2. Portability-easy transfer of code from one machine to another.

3. Reliability- protection from internal errors, failure, and rejections.

4. Compatibility-OS must have common applications.

5. Security-OS must possess protection of resources.

6. Performance-OS must have so good speed for performing actions so far as the hardware platform allows.