OS Input Output Systems

Question 11

A hard disk has 63 sectors per track, 10 platters each with 2 recording surfaces and 1000 cylinders. The address of a sector is given as a triple (c, h, s), where c is the cylinder number, h is the surface number and s is the sector number. Thus, the 0th sector is addressed as (0, 0, 0), the 1st sector as (0, 0, 1), and so on The address <400,16,29> corresponds to sector number:

• 505035
• 505036
• 505037
• 505038

Question 12

Consider the data given in previous question. The address of the 1039th sector is

• (0, 15, 31)
• (0, 16, 30)
• (0, 16, 31)
• (0, 17, 31)

Question 13

The data blocks of a very large file in the Unix file system are allocated using

• contiguous allocation
• linked allocation
• indexed allocation
• an extension of indexed allocation

Question 14

For a magnetic disk with concentric circular tracks, the seek latency is not linearly proportional to the seek distance due to

• non-uniform distribution of requests
• arm starting and stopping inertia
• higher capacity of tracks on the periphery of the platter
• use of unfair arm scheduling policies

Question 15

Which of the following statements about synchronous and asynchronous I/O is NOT true?

• An ISR is invoked on completion of I/O in synchronous I/O but not in asynchronous I/O
• In both synchronous and asynchronous I/O, an ISR (Interrupt Service Routine) is invoked after completion of the I/O
• A process making a synchronous I/O call waits until I/O is complete, but a process making an asynchronous I/O call does not wait for completion of the I/O
• In the case of synchronous I/O, the process waiting for the completion of I/O is woken up by the ISR that is invoked after the completion of I/O

Question 16

Consider a disk pack with 16 surfaces, 128 tracks per surface and 256 sectors per track. 512 bytes of data are stored in a bit serial manner in a sector. The capacity of the disk pack and the number of bits required to specify a particular sector in the disk are respectively:

• 256 Mbyte, 19 bits
• 256 Mbyte, 28 bits
• 512 Mbyte, 20 bits
• 64 Gbyte, 28 bit

Question 17

Suppose a disk has 201 cylinders, numbered from 0 to 200. At some time the disk arm is at cylinder 100, and there is a queue of disk access requests for cylinders 30, 85, 90, 100, 105, 110, 135 and 145. If Shortest-Seek Time First (SSTF) is being used for scheduling the disk access, the request for cylinder 90 is serviced after servicing ____________ number of requests.

• 1
• 2
• 3
• 4

Question 18

A device with data transfer rate 10 KB/sec is connected to a CPU. Data is transferred byte-wise. Let the interrupt overhead be 4 microsec. The byte transfer time between the device interface register and CPU or memory is negligible. What is the minimum performance gain of operating the device under interrupt mode over operating it under program controlled mode?

• 15
• 25
• 35
• 45

Question 19

Consider a disk drive with the following specifications: 16 surfaces, 512 tracks/surface, 512 sectors/track, 1 KB/sector, rotation speed 3000 rpm. The disk is operated in cycle stealing mode whereby whenever one byte word is ready it is sent to memory; similarly, for writing, the disk interface reads a 4 byte word from the memory in each DMA cycle. Memory cycle time is 40 nsec. The maximum percentage of time that the CPU gets blocked during DMA operation is:

• 10
• 25
• 40
• 50

Question 20

Consider an operating system capable of loading and executing a single sequential user process at a time. The disk head scheduling algorithm used is First Come First Served (FCFS). If FCFS is replaced by Shortest Seek Time First (SSTF), claimed by the vendor to give 50% better benchmark results, what is the expected improvement in the I/O performance of user programs?

• 50%
• 40%
• 25%
• 0%