Practice Question 1
(This is Problem 19 from Chapter 2 of the textbook).
Imagine you are writing the disk-management part of an operating system. Logically,you represent the disk as a sequence of blocks, from 0 on the inside to some maximum on the outside. As files are created, you have to allocate free sectors. You could do it from the outside in or the inside out. In terms of disk performance, does it matter which strategy you choose on a modern disk? Explain your answer.
Practice Question 2
(This is Problem 20 from Chapter 2 of the textbook).
How long does it take to read a disk with 10,000 cylinders, each containing four tracks of 2048 sectors? First, all the sectors of track 0 are to be read starting at sector 0, then all the sectors of track 1 starting at sector 0, and so on. The rotation time is 10 msec, and a seek takes 1 msec between adjacent cylinders and 20 msec for the worst case. Switching between tracks of a cylinder can be done instantaneously.
Practice Question 3
(This is Problem 27 from Chapter 2 of the textbook).
You are part of a top-secret international scientific team which has just been assigned the task of studying a being named Herb, an extra-terrestrial from Planet 10 who has recently arrived here on Earth. Herb has given you the following information about how his eyes work. His visual field consists of about 108
pixels. Each pixel is basically a superposition of five "colors" (i.e., infrared, red, green, blue, and ultraviolet), each of which has 32 intensities. The time resolution of Herb's visual field is 10 msec. Calculate the data rate, in GB/sec, of Herb's eyes.
Practice Question 4
Mr. Hacker does not like the fact that he has to buy preconfigured RAID systems. He decides to write software that makes a RAID-5 out of ANY set of disks, as long as each of those disks has the exact same data capacity. This way, the user can buy individual disks, and using Mr. Hacker's software they can use these disks as a RAID. Can Mr. Hacker succeed?
Mr. Hacker's enemy, Mr. Nemesis, predicts that Mr. Hacker will fail. He starts developing a rival software, that can also make a RAID-5 out of a set of disks that the user provides, but makes additional requirements that the set of disks must satisfy. What additional requirements does Mr. Nemesis need to make, to have a successful product?
Practice Question 5
A company has a number of RAID-5 arrays. Each array has 10 disks. On average, a disk fails every 20,000 hours. When a disk fails in a RAID-5 array, it takes one hour to fix the array. Fixing the array involves taking out the broken disk, putting in a new disk, and storing the right data on the new disk. If, during that hour, another disk fails on the array that is getting fixed, the data on the array is lost.
Under these assumptions: suppose that you are fixing an array that had a broken disk. What is the probability that, while you are fixing that array, the array's data will be lost?
Practice Question 6
Suppose that you are receiving Unicode characters over the network, one byte at a time. At some point, there is a network interruption, and you fail to receive some bytes, but you do not know how many bytes you have missed. Then, the network stars working again, and you receive more bytes. Can you reconstruct with certainty the text that you have received after the network interruption? Why, or why not?
What if instead of Unicode characters the text was transmitted in UTF-8 characters? In that case, would you be able to reconstruct with certainty the text that you have received after the network interruption? Why, or why not?
Practice Question 7
Suppose that a certain message needs 18KB of memory when stored in ASCII (one byte per ASCII code). How much memory, in kilobytes, will the same message take in:
Practice Question 8
Based on the description of keyboards given in the textbook, is there an upper limit on the amount of keys involved in a multikey combination that the operating system can recognize? In other words, can the operating system recognize and interpret multikey combinations where three, four, five, or more keys are pressed at the same time? Is there any limit to the number of keys pressed at the same time that the operating system can handle? Justify your answer.
Practice Question 9
Suppose we want to define a mouse motion that should automatically start the web browser window, and bring it to the foreground. This new mouse motion is defined (without getting into precise details) by the mouse making a square motion rapidly, moving first right, then down, then left, then up.
- Do we need a new mouse device to support this motion?
- Do we need to make changes to the operating system to support this motion?
Justify your answer.
Practice Question 10
A camera has a resolution of 1920x1080 pixels, and stores three bytes for each pixel (corresponding to the red, green, and blue components of the color of that pixel). That camera can also record a movie at 30 frames per second. Movies are saved in the MPEG format, with a compression factor of 100. How much memory is needed to store a one minute movie?
Practice Question 11
A commercial high-resolution movie has a resolution of 1920x1080 pixels, and stores three bytes for each pixel (corresponding to the red, green, and blue components of the color of that pixel). The movie has a frame rate of 30 frames per second.
What compression factor is needed to be able to store that movie on a single-sided Blu-ray disk? Justify your answer. You will need to make some assumptions about the size of the movie, you should state those assumptions explicitly.
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