OS Input Output Systems

Question 1

Which of the following is major part of time taken when accessing data on the disk?

• A. Settle time
• B. Rotational latency
• C. Seek time
• D. Waiting time

Answer: (C)

OS Input Output Systems
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Seek time is time taken by the head to travel to the track of the disk where the data to be accessed is stored.

Question 2

We describe a protocol of input device communication below. a. Each device has a distinct address b. The bus controller scans each device in sequence of increasing address value to determine if the entity wishes to communicate. c. The device ready to communicate leaves it data in IO register. d. The data is picked up and the controller moves to step-a above. Identify the form of communication best describes the IO mode amongst the following: Source: nptel

• A. Programmed mode of data transfer
• B. DMA
• C. Interrupt mode
• D. Polling

Answer: (D)

OS Input Output Systems
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See Polling

Question 3

From amongst the following given scenarios determine the right one to justify interrupt mode of data-transfer: Source: nptel

• A. Bulk transfer of several kilo-byte
• B. Moderately large data transfer but more that 1 KB
• C. Short events like mouse action
• D. Key board inputs

Answer: (C), (D)

OS Input Output Systems
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Both keyboard and mouse controllers typically use interrupt mode.

Question 4

Normally user programs are prevented from handling I/O directly by I/O instructions in them. For CPUs having explicit I/O instructions, such I/O protection is ensured by having the I/O instructions privileged. In a CPU with memory mapped I/O, there is no explicit I/O instruction. Which one of the following is true for a CPU with memory mapped I/O? (GATE CS 2005)

• A. I/O protection is ensured by operating system routine(s)
• B. I/O protection is ensured by a hardware trap
• C. I/O protection is ensured during system configuration
• D. I/O protection is not possible

Answer: (A)

OS Input Output Systems
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See question 1 of http://www.geeksforgeeks.org/operating-systems-set-16/

Question 5

Put the following disk scheduling policies results in minimum amount of head movement.

• A. FCFS
• B. Circular SCAN
• C. Elevator

Answer: (C)

OS Input Output Systems
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Circular scanning works just like the elevator to some extent. It begins its scan toward the nearest end and works its way all the way to the end of the system. Once it hits the bottom or top it jumps to the other end and moves in the same direction. Circular SCAN has more head movement than SCAN (elevator) because Circular SCAN has circular jump and it does count as a head movement. SCAN (elevator) is the best choice here.

Question 6

Consider a hard disk with 16 recording surfaces (0-15) having 16384 cylinders (0-16383) and each cylinder contains 64 sectors (0-63). Data storage capacity in each sector is 512 bytes. Data are organized cylinder-wise and the addressing format is . A file of size 42797 KB is stored in the disk and the starting disk location of the file is <1200, 9, 40>. What is the cylinder number of the last sector of the file, if it is stored in a contiguous manner?

• A. 1281
• B. 1282
• C. 1283
• D. 1284

Answer: (D)

GATE CS 2013 OS Input Output Systems
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File size is 42797KB= 4279*2^10B=85594*2^9B. Now one sector=512B so file will be stored in 85594 sectors i.e we need to cross 85594 sectors starting of the file is number of cylinders to cross=85594/16*64= 83 cylinders remaining sectors to cross=85594-(83*16*64)=602 number of surfaces to cross=9 so to cross 9 surface we need to cross on more cylinder as file has started at surface 9 and no of surface in cylinder is 16 so number of cylinder to cross=83+1=84 so cylinder no. 1200+84=1284

Question 7

A file system with 300 GByte disk uses a file descriptor with 8 direct block addresses, 1 indirect block address and 1 doubly indirect block address. The size of each disk block is 128 Bytes and the size of each disk block address is 8 Bytes. The maximum possible file size in this file system is

• A. 3 Kbytes
• B. 35 Kbytes
• C. 280 Bytes
• D. Dependent on the size of the disk

Answer: (B)

GATE CS 2012 OS Input Output Systems
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See http://www.geeksforgeeks.org/operating-systems-set-5/

Question 8

A computer handles several interrupt sources of which the following are relevant for this question.

. Interrupt from CPU temperature sensor (raises interrupt if 
  CPU temperature is too high)
. Interrupt from Mouse(raises interrupt if the mouse is moved 
  or a button is pressed)
. Interrupt from Keyboard(raises interrupt when a key is 
  pressed or released)
. Interrupt from Hard Disk(raises interrupt when a disk 
  read is completed)

Which one of these will be handled at the HIGHEST priority?

• A. Interrupt from Hard Disk
• B. Interrupt from Mouse
• C. Interrupt from Keyboard
• D. Interrupt from CPU temperature sensor

Answer: (D)

GATE CS 2011 OS Input Output Systems
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Higher priority interrupt levels are assigned to requests which, if delayed or interrupted, could have serious consequences. Devices with high speed transfer such as magnetic disks are given high priority, and slow devices such as keyboard receive low priority (Source: Computer System Architecture by Morris Mano) Interrupt from CPU temperature sensor would have serious consequences if ignored.

Question 9

An application loads 100 libraries at start-up. Loading each library requires exactly one disk access. The seek time of the disk to a random location is given as 10 ms. Rotational speed of disk is 6000 rpm. If all 100 libraries are loaded from random locations on the disk, how long does it take to load all libraries? (The time to transfer data from the disk block once the head has been positioned at the start of the block may be neglected)

• A. 0.50 s
• B. 1.50 s
• C. 1.25 s
• D. 1.00 s

Answer: (B)

GATE CS 2011 OS Input Output Systems
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See Question 3 of http://www.geeksforgeeks.org/operating-systems-set-6/

Question 10

A CPU generally handles an interrupt by executing an interrupt service routine

• A. As soon as an interrupt is raised
• B. By checking the interrupt register at the end of fetch cycle.
• C. By checking the interrupt register after finishing the execution of the current instruction.
• D. By checking the interrupt register at fixed time intervals.

Answer: (C)

GATE-CS-2009 OS Input Output Systems
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Hardware detects interrupt immediately, but CPU acts only after its current instruction. This is followed to ensure integrity of instructions.