Operating Systems Chapter 7 Quiz Study Guide

What is a buffer?

A temporary storage area in main memory, channels, or control units used to sync the fast CPU with slow I/O devices. (Waiting room).

(T/F) Using even parity on the 1s for a 4-bit codeword space, there will be 16 valid code words.

False

(T/F) Buffers are used to synchronize the movement of data between the relatively slow I/O devices and the very fast CPU.

True

____ peripheral devices are assigned to only one job at a time.

Dedicated

The ____ is a hardware flag that must be tested by the CPU to determine when a device has completed an operation.

CSW

When constructing the Hamming Code, which best describes the placement of the parity bits in the code?

Each 2^N bit position, where N=0,1,2,3,... are parity bits. The rest are data.

You are using even parity on the 1s for a 6-bit code system. Suppose the following code word is received. It is illegal since there are an odd number of 1s. The lowest-order bit is the parity bit. Which of the following represents the corrected code word?

Received Illegal Code: 110100

The answer cannot be determined

(T/F) When using DMA, the CPU controls the transfer of data to and from memory over the system bus.

False

(T/F) RAID Level 1 is not considered a true form of RAID because it cannot recover from hardware failure.

False

(T/F) The universal serial bus (USB) controller acts as an interface between the operating system, device drivers, applications, and the devices that are attached via the USB host.

True

The I/O ____ allocates the devices, control units, and channels.

Schedular

For a movable head drive, which of the following tends to be the slowest operation in the access time measurement?

Seek time

Single buffer compared to a double buffer?

The CPU must wait for the buffer to fill before processing.

While the CPU processes one record from the first buffer, the channel can simultaneously read/write the next record into the second buffer. (Overlapped processing).

Where are buffers often encountered in hardware?

In main memory, I/O channels, and control units.

What are dedicated devices?

Devices assigned to one job at a time for its entire duration (tape drives, plotters). It’s inefficient if it’s not used 100% of the time.

What are shared devices?

A device where multiple processes can access it at the same time via interleaving (hard drives, DASD).

What are virtual devices?

A dedicated device that acts like a shared device. To your programs it looks like the device is ready to be used. But in reality, the data is in a queue (spooling).

What is Raid 0?

Data striping. Spreads data across multiple disks. Fast, but there’s no redundancy or error correction.

What is Raid 1?

Mirroring. Provides a duplicate set of all data. It’s expensive (requires 2 disks) but it’s highly reliable.

What is Raid 10?

A combination of mirroring and striping (1 + 0 = 10). Has both speed and reliability.

What is the importance of Raid?

It improves I/O performance and enables data recovery incase of hardware failure.

What is a parity bit?

An extra bit added to the end of a binary string to make sure the data didn’t get corrupted.

What is the parity rule?

The total number of 1s in the entire message (including the parity bit) must be an even number (0,2,4,6).

What happens if a bit flips ilegally?

If a 0 becomes a 1, or 1 becomes a 0 during a transfer, the count of 1s will change from Even to Odd.

When the receiver counts the 1s, they will find an odd number of 1s when the rule was even, and know the data is illegal/corrupted and must be sent again.

What is the difference between hamming code and single parity bit?

A ___ can tell you if something went wrong.

The ___ can tell you exactly which bit is broken and fix it.

What is hamming code and how does it work?

It’s error control code that can detect and correct single bit errors. There are multiple parity bits throughout the data. Each parity bit checks a group of data bits. They overlap, allowing for easy detection and correction.

What is a I/O Traffic Controller?

Monitors the status of devices, control units, and channels. It finds all availible paths and maintains a status database.

What is an I/O Scheduler?

Similar to a process scheduler, but it decides which I/O request to satisfy first based on a schedule (predetermined).

What is an I/O Device Handler?

Performs the actual data transfer, handles interrupts, and manages error conditions.

Define Sequential Access

Records are stored and read in a specific linear order (in sequence). Good for massive backups/sequential files. Slow for random lookups. Example: Tape drive, VHS

Define Direct Access (DASD)

Can read/write directly to a specific area. Good for fast random access. Performance depends on record location. Example: HDDs, CDs, DVDs.

Between Sequential Access and Direct Access, which has faster access times, and longer/shorter time variance?

Direct access: faster access and shorter variance.

Sequential access: slower access, longer variance (can vary a little bit)

Data access for movable head?

Seek Time (move arm) + Search Time (rotate disk) + Transfer time (move data) = Access Time

Data access for fixed head?

Search time (rotate disk) + Transfer time (move data) = Access time
(No seek time)

What are channels?

Programmable units that act as “traffic cops” to sync the faster CPU with slow I/O devices.

What are control units?

They receive and intercept signals from the channel to manage specific groups of devices (each with their own disks/tapes).

First Come First Serve (FCFS)

Best with light loads. Service time is longer with heavy loads.

Shortest Seek Time First (SSTF)

Best with moderate loads, fast. Localization/starvation problems with heavy loads.

SCAN

Best with light-moderate loads. No starvation. Throughput and avg service time is similar to SSTF, but it favors the middle track.

Circular Scan (C-SCAN)

Best with moderate-heavy loads. Very small time variances.