1. Lecture 5: 9/10/2002CS149D Fall 20021 CS149D Elements of Computer Science Ayman Abdel-Hamid Department of Computer Science Old Dominion University Lecture 5: 9/10/2002

2. CS149D Fall 20022 Outline Mass Storage Communication Errors Should cover sections 1.3 and 1.9

3. Lecture 5: 9/10/2002CS149D Fall 20023 Mass Storage Main Memory is limited in size and volatile We need secondary storage that is not volatile Additional memory devices called mass storage devices (magnetic disks, CDs, magnetic tapes) Mass storage devices required mechanical motion, hence have larger response time when compared to main memory (electronic operation)

4. Lecture 5: 9/10/2002CS149D Fall 20024 Magnetic Disk 1/3 Copyright © 2003 Pearson Education, Inc. Spinning disk with magnetic coating to hold data Read/Write heads above and below disk Tracks (concentric circles) Tracks divided into Sectors (why?) Common sector sizes are 512, 1024 bytes Implication of having the same number of bits per sector? Tracks and sectors created as result of formatting process

5. Lecture 5: 9/10/2002CS149D Fall 20025 Magnetic Disk 2/3 Floppy Disks (low capacity) Single disk with Read/Write heads for each side of the disk When disk is flexible it is called a floppy disk (5¼ diskette, extinct today) 3½ diskette holds 1.44 MB of data (rigid plastic case) Zip disks can hold 100 MB or 250 MB How a Floppy Disk Works (http://www.howstuffworks.com/floppy-disk-drive.htm)How a Floppy Disk Works Hard Disk (high capacity) Several rigid disks (platters) mounted on a common spindle Read/Write heads do not touch the disk but float above its surface How a Hard Disk Works (http://www.howstuffworks.com/hard-disk.htm)How a Hard Disk Works

6. Lecture 5: 9/10/2002CS149D Fall 20026 Magnetic Disk 3/3 Disk System Performance Seek time Time required to move read/write heads from one track to the other Latency time (rotation delay) average amount of time required for desired data to rotate around the read/write head once the head has been positioned over the desired track Access time Seek time + latency time Transfer rate Rate at which data can be transferred to or from the disk Why disk access slower than memory access?

7. Lecture 5: 9/10/2002CS149D Fall 20027 Compact Disk Copyright © 2003 Pearson Education, Inc. Reflective material covered with a clear protective coating Information recorded by creating variations on the reflective surface Information retrieved by means of a laser beam that monitors irregularities on the reflective surface of the CD as it spins Information recorded on a single track that spirals around the CD (from inside out) Track divided into sectors Distance around spiral track greater toward outer edge of disk than at inner portion Implication? More sectors will be read in a single revolution of the disk when the laser beam is scanning outer portion. To obtain a uniform rate of data transfer, CD players vary CD’s rotation speed depending on the location of the laser beam How a CD works (http://www.howstuffworks.com/cd.htm)

8. Lecture 5: 9/10/2002CS149D Fall 20028 Magnetic Tapes Thin plastic tape, wound on a reel for storage Sequential access Used mostly for archival purposes Tape divided into segments Segment contains several tracks than run // to one another lengthwise on the tape High capacity, reliability, cost efficiency Copyright © 2003 Pearson Education, Inc.

9. Lecture 5: 9/10/2002CS149D Fall 20029 Communication Errors When information transferred back and forth between various parts of a computer, bit pattern retrieved may not be identical to original one Coding techniques to allow detection and correction of errors Error detection Parity bit (add to sequence an additional bit, usually most significant bit) Odd parity: odd number of 1s Even parity: even number of 1s 01000001 (sequence is 8 bits, actually A in ASCII code) In odd parity sequence becomes101000001 In even parity sequence becomes001000001 Error correcting codes