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Chapter 3 Data Storage 23/05/2017 CIS110 1 Learning outcomes By the end of this Chapter you will know: • • • • • The difference between electronic, magnetic and optical memory How data are stored in these types memories The main memory is made up of logic gates The main memory is organised in terms of cells and addresses memory terms: • How the address decoder works • Memory capacity, access time, transfer rate, etc … 23/05/2017 CIS110 2 Additional Reading Essential Reading • Stalling (2003): Chapters 5 and 6 Further Reading • • • • Burrell (2004): Chapters 3 and 7 Schneider and Gersting (2004): Chapters 4 and 5 Tanenbaum (1990): Chapter 3 White (2002): Parts 3 and 4. 23/05/2017 CIS110 3 Introduction (1) Information can be stored in different ways: • • • Books, Films Paintings, It is not information if it could not used Information in computers must be able to able to be processed by computers: • • Information must be represented in appropriate format Information must be stored in appropriate places 23/05/2017 CIS110 4 Introduction (2) Breakthrough: • • The use of the binary system (Base 2) In the binary system: • There is only two types of values, 1s and 0s. • It is easy to store binary information/data in physical media • It is also easy to process binary information Different type of media storage • • • Electronic memory (main memory) Magnetic memory optical memory 23/05/2017 CIS110 5 Media Storage Main memory (Electronic Memory): Secondary Memory • Stores data currently being used • Is made of semiconductor chips. • magnetic (floppy discs, hard disc ) • Optical (CD-ROM, DVD) 23/05/2017 CIS110 6 Main Memory (Electronic Memory) Main memory stores data which are currently used by the CPU. • To run a program, it is first loaded in the main memory Main Memory is volatile • • Its content changes frequently Data is lost when the power is off It is also called electronic memory • • Based on electronic principles. Formed with logic gates • Group of transistors Cells • Sequence of one-bit memories Addresses • Each cell has a unique address 23/05/2017 CIS110 7 The physical principles of electronic memory Transistor • The smallest unit of an electronic memory Logic Gates Flip-Flops • Groups of transistors • Special type of circuit 23/05/2017 CIS110 8 Logic Gates (2) AND NOT OR ¬ a. a b a.b a b a.b a 00 01 10 11 00 01 10 11 0 1 1 0 0 0 0 1 23/05/2017 0 1 1 1 CIS110 9 Logic Gates (3) NAND XOR NOR a b a.b 00 01 10 11 1 1 1 0 a b a.b a b a.b 00 01 10 11 00 01 10 11 1 0 0 0 0 1 1 0 • For more details see •Schneider and Gersting (2004: 155-177) •Burrell (2004: 43-62) 23/05/2017 CIS110 10 Flip-Flop circuits Up to now the output of combinational circuits depends solely up the input Combinational circuits has no memory To build a sophisticated digital signal circuits, memory, we need: • We need circuits whose output depends upon both the • input of the circuit and its previous states. In other words, we need circuit that have memory. 23/05/2017 CIS110 11 A Simple Flip-flop Circuit • • • • As long as both inputs remain 0: output does not change Temporarily placing 1 on upper input => output = 1 Temporarily placing 1 on lower input => output = 0 So: output flip-flops between 2 values under external control 23/05/2017 CIS110 12 Setting the Output of a Flip-flop to 1 23/05/2017 CIS110 13 Setting the Output of a Flip-flop to 1 (cont’d) 23/05/2017 CIS110 14 Setting the Output of a Flip-flop to 1 (cont’d) 23/05/2017 CIS110 15 Controlled Flip-Flop • If control = 0 the the flip-flop does not change the state • If control = 1, then if D=0 then Q =1 else Q = 0 23/05/2017 CIS110 16 Clocked SR flip-flop • If CP = 0 the output of both AND gates is 0. • Regardless of the values of S and R. •If S=R=CP=1, then both outputs are set to 0 23/05/2017 CIS110 17 Main Memory Large collection of circuits, each capable of storing a single bit Arranged in small cells, typically of 8 bits each (a.k.a.: byte) 23/05/2017 CIS110 18 Arrangement of Memory Cells Each cell has a unique address Longer strings stored by using consecutive cells value = 01101101 23/05/2017 CIS110 RAM (random access memory) 19 One-bit Memory Q D CP •To write a datum (0 or 1) to this memory •send data to D, and at the same time •send a WRITE signal to CP •To read a datum from this memory •connect to Q by sending a READ signal 23/05/2017 CIS110 20 Main memory = linking many flip-flops See Burrell (2004: 111-112) and Tanenbaum (1990: 105-109) t 23/05/2017 CIS110 21 Memory cells n-bit cell Can hold m*n bits m cells In reality, most electronic memories have 8-bit cells. 23/05/2017 CIS110 22 Accessing Data in the Main Memory Instructions and data are stored in the main memory in a serial order. CPU executes instructions one by one top down. An instruction may tell the CPU • • to jump to particular cell and execute the instruction held in it, or fetch the data stored is that cell. How is this done? 23/05/2017 CIS110 23 System Bus Main memory and CPU are linked using a set of wire: • • Three wires: • • • address lines, data lines and control lines. Known as • • • address bus, data bus and control bus. 23/05/2017 System bus CIS110 24 CPU Main memory Add. bus Data bus Control bus 23/05/2017 CIS110 25 To read data from each cell To issue read or write signal To identify each memory cell CPU Main memory Add. bus Data bus Control bus 23/05/2017 CIS110 26 Address Bus Address Of the cell To activated CPU Main memory Address Of the cell To activated Address bus 23/05/2017 CIS110 27 Binary Address Representation Each cell has a unique address. I.e. using 4 digit binary representation we have: 0000 cell 0 0001 cell 1 0010 cell 2 0100 cell 3 How many bits are needed to represent an address? 23/05/2017 CIS110 28 Address Decoder Address Of the cell To activated Unique cell Has a unique Address. CPU Main memory Decoder Address bus 23/05/2017 CIS110 29 A Simple Address Decoder Q0 00 C0 2 ad-lines A1 A0 Q1 01 C1 Q2 10 C2 22 = 4 address cells Q3 11 C3 Decoder is a device between the Main Memory and the address lines. 23/05/2017 CIS110 30 Decoder with N Address Lines Main Memory a0 a1 0000…0000 0000…0001 0000…0010 2n add cell n add. lines an-1 23/05/2017 1111…1111 CIS110 31 Main Memory with 4 Chips decoder Main memory a0 a1. . . . . . . aN-1 Chip 1 Chip 2 Chip 3 Chip 4 23/05/2017 CIS110 32 The higher 2 bits of Address line to select The chip. a n-1 0 0 0 0 1 1 1 1 a n-2 0 0 1 1 0 0 1 1 23/05/2017 …..………..a0 0………….. 0 1………….. 1 0………….. 0 1………….. 1 0………….. 0 1………….. 1 0………….. 0 1………….. 1 CChip 1 h i Chip 2 p Chip 3 1 Chip 4 CIS110 33 Multiplexer Cells form rows and columns. Each cell can be identified by a row address and column address. Each cells address uses only n/2 address lines. This can be done using a multiplixed addresses. 23/05/2017 CIS110 34 Decoder with 4 Address Lines (non-multiplexed addresses) 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111 23/05/2017 CIS110 35 Decoder with 2 Address Lines (multiplexed addresses) 00 01 10 11 00 0000 0001 0010 0011 01 0100 0101 0110 0111 10 1000 1001 1010 1011 11 1100 1101 1110 1111 23/05/2017 CIS110 36 Two-Input Multiplexer A multiplexer is an electronic device that allows multiple logical signals to be transmitted simultaneously across a single physical channel (address line). 23/05/2017 CIS110 37 Example 1 Suppose computer’s Main Memory is linked to a decoder with 8 address lines. 1. 2. 3. Can 1000 memory cells be used? If no what is the maximum number of addresses that can generated? What is the maximum number of addresses that can be generated is multiplexed addresses are used? 23/05/2017 CIS110 38 Answer Suppose computer’s Main Memory is linked to a decoder with 8 address lines. 1. Can 1000 memory cells be used? 2. If no what is the maximum number of addresses that can generated? Answer: 1. NO 2. With 8 address lines, the maximum number of addresses 3. is 28=256 22*8 = 216 23/05/2017 CIS110 39 Example 2 Suppose that a computer’s Main Memory has 1013 cells. How many address lines are needed in order for all the cells to be useable? Explain your answer. 23/05/2017 CIS110 40 Answer Suppose that a computer’s Main Memory has 1013 cells. How many address lines are needed in order for all the cells to be useable? Explain your answer. Answer: • • • • • With N address lines a computer can have a maximum 2N usable cells. 29 = 512, 210 = 1024. 9 address lines would not generate enough addresses for 1013 cells to be used. 10 address lines would. Having more than 10 address lines would lead to too many addresses wasted. So the desired number of address lines is 10. N = ⌈log2(1050)⌉ can be used to find the number of address lines. If multiplexed addresses is used, then 5 address lines would be sufficient for 1013 cells to be useable. 23/05/2017 CIS110 41 What does a word mean? A word is the length of instructions the CPU can execute at one time. Some processor can handle 8-bit words others 16-bit, 32-bit, 64-bit. A cell does not necessarily store one word. A word can occupy more than one cell. 23/05/2017 CIS110 42 Address Space The address space of a computer is the maximum number of cells a computer can hold. The address space is determined by the number of address lines used in a computer. If each cell in a memory is 8-bit, then the memory is called byte addressable: 1 byte long has a unique address 23/05/2017 CIS110 43 Features of the Main Memory Memory Capacity. Access of information Access time Transfer rate 23/05/2017 CIS110 44 Memory Capacity Most computer’s memory have 8-bit (1-byte) cells. In this case we have: Address lines No of cells Capacity (byte) n 2n 2n x 1 32KB, 256MB and 20GB are used to describe the memory capacity. 23/05/2017 CIS110 45 Capacity Units 1kB = 210 = 1024 Byte. 1MB =1024 KB = 220 Bytes= 1, 048,576 B. 1GB =1024 MB = 230 kB=1, 073,741,824 Bytes. 23/05/2017 CIS110 46 Access Time Access time is taken between the moment when the CPU wants the read/write from/into a cell and the moment when the cell is activated. It is the moment that the CPU takes to activate a cell. 60ns (10-9 sec) 23/05/2017 CIS110 47 Transfer Rate Is the amount of information per second exchanged between the CPU and main memory. If the CPU can read n cells in a second and each cell has m bytes then transfer rate is n*m (bytes/s) Main memory • electronic signals • Implies fast transfer rate in the scale about 100MB/sec 23/05/2017 CIS110 48 Random Access If the CPU wants to activate particular cell. • It does not search for the target cell from top • • to bottom. It does put the address of the target cell in the address line, then the cell will be activated. This type of accessing information is called Random Access 23/05/2017 CIS110 49 The need for other type of memories. Main memory • Fast as all the exchange between CPU and • Main memory is done electronically. However, it is volatile. • Information lost when the machine is turned off. • The need for non-volatile memory: • Hold information when the machine is off. • i.e. Magnetic disk, optical disk, magnetic tape 23/05/2017 CIS110 50 Magnetic Memory Another way of storing information in the binary framework. Magnetic memory contains a number of spots. The information is stored by magnetising and demagnetising these spots. • Magnetised spot 1 • unmagnetised spot 0 • i.e. floppy disk 23/05/2017 CIS110 51 A Magnetic Disk Storage System • Each track contains same number of sectors • Location of tracks and sectors not permanent (formatting) • Examples: hard disks, floppy disks, ... 23/05/2017 CIS110 52 Magnetic Disk Terminology Platter: • • rigid metal or glass platter Coated with magnetic material. rotating at constant angular velocity Arm: • With movable magnetic read/write heads Track: • • A complete ring of data The disk surface is divided into tracks Sectors: • Each track is subdivided into sectors Cylinder (see slides 71-72): • A vertical collection of tracks at the same radial position 23/05/2017 CIS110 53 Data Organization and Formatting Concentric rings or tracks • Gaps between tracks • Reduce gap to increase capacity • Same number of bits per track (variable packing density) • Constant angular velocity Tracks divided into sectors • • • Minimum block size is one sector May have more than one sector per block See Stalling (2003) pages:167-168 23/05/2017 CIS110 54 Disk Data Layout spots sector 23/05/2017 CIS110 55 Magnetic Disks 23/05/2017 CIS110 56 Magnetic Disks Thus as the platter rotates under the head, a stream of bits can be written and later read back. 23/05/2017 CIS110 57 Read/write Head Coil of wire Iron former 23/05/2017 A coil of wire wound onto an iron former. gap. If a spot on the magnetic memory passes under the gap then an electrical current is induced in the coil. And the read/write head will know that there is a 1 stored on that spot. Otherwise it is 0. By passing an electric current on the wire we can magnetise and demagnetise spots. CIS110 58 Read and Write Mechanism (1) Recording and retrieval via conductive coil called a head May be single read/write head or separate ones During read/write, head is stationary, platter rotates Write • • • Electric Current through coil of wire produces magnetic field Magnetic Pulses sent to the head Magnetic pattern recorded on surface below Read • • Magnetised bit pattern • • Magnetic field induces an electrical current in the coil The bit pattern contains 1 Demagnetised bit pattern • • No Magnetic field induced, hence, no electrical current in the coil The bit pattern contains 0 23/05/2017 CIS110 59 Read and Write Mechanism (2) 1 CPU 01010 Add. bus 01010 1 Data bus 1 Control bus 23/05/2017 CIS110 60 Fixed/Movable Head Disk Fixed head Movable head • One read/write head per track • Heads mounted on fixed ridged arm • One read/write head per side • Mounted on a movable arm 23/05/2017 CIS110 61 Access Information on a Floppy disk To access information on a floppy: Head moves to the target track. waits for the desired sector to spin underneath it read/write begins. • Track number, and • Sector number. 23/05/2017 CIS110 62 Seek time and average seek time Seek time: • is the time it takes, the read/write head to move from one track to a particular track on a disk Average seek time: • is the average of seek time between every pair of tracks. 23/05/2017 CIS110 63 Example A disk has 5 tracks and the read/write head takes 1ms to move from a track to an adjacent one • • 1-2:1ms, 1-3:2ms, 1-4:3ms, 1-5:4ms, 2-3:1ms, 2-4:2ms, 2-5:3ms, 3-4:1ms, 3-5:2ms, 4-5:1ms Average seek time: 20/10= 2ms. 23/05/2017 CIS110 64 Average latency Average latency : Example: • is the time taken to make half a revolution. • Disk rotates at a speed of 100 rev/sec • Average latency is: 23/05/2017 1 / 200 sec. CIS110 65 Maximum data transfer rate It is the rate at which data passes under the read/write head (bytes/sec). • Number of bytes / track 23/05/2017 CIS110 * Number of rev / sec 66 Constant Angulair Velocity (CAV) Variable density 23/05/2017 CIS110 67 Multiple Platter (hard disk) Permanent storage that is inside of the computer, and NOT portable. Consists of several platters which spin very fast Heads are joined and aligned Aligned tracks on each platter form cylinders Data is striped by cylinder • • reduces head movement Increases speed (transfer rate) 23/05/2017 CIS110 68 Multiple Platters (2) • Disk platters speed (3600 to 10 000 rpm (rev/min). •floppy (360rpm). •The read data we need to specify cylinder, head, and sector numbers. Each cylinder represents a track number. 23/05/2017 CIS110 69 Cylinders 23/05/2017 CIS110 70 Magnetic Tape (1) Serial access Slow Very cheap High capacity Backup 23/05/2017 CIS110 71 Magnetic tape (2) • Serial access (slow) • Good choice for off-line data storage (archives) 23/05/2017 CIS110 72 Magnetic Tape column R/W head Blocks of data Track 1 Track 2 Track 9 A magnetic tape is a series of columns. Each column can store a word or two. Tapes offer a large storage capacity for backup. See Stalling (2003) pages:189-190. 23/05/2017 CIS110 73 Features of Magnetic Memory Memory capacity: Floppy can hold 700KB – 120MB. Hard disk can hold dozen of GB, 10, 20,.. Tapes can hold 100MB- 80GB. Access method • Floppy and hard disks is random as the main memory • Tape is serial Access time: • It is the average time taken to position the R/W head over the data to be read • For disk drives is about 10-3 sec when in MM 10-9 sec. Transfer rate: is slower. It is the transfer of data between MM and Mag/M. Floppy (500kB-2MB) and hard disc (4-12MB). 23/05/2017 CIS110 74 Optical Storage CD-ROM Originally for audio 650 Mbytes giving over 70 minutes audio Polycarbonate coated with highly reflective coat, usually aluminium Data stored as pits Read by reflecting laser Constant packing density (data/surface= constant) • • More data in outer edges Less data towards the centre of the disc Constant linear velocity • The drive must adjust the disc speed (495 to 212 rev/m) edges • Faster when reading data closer to the centre • Slower when reading data in outer edges 23/05/2017 CIS110 75 CD-ROMs In recent years, optical (as opposed to magnetic) disks have become available. They have much higher recording densities than conventional magnetic disks. Optical disks were originally developed for recording television programs, but they can be put to more esthetic use as computer storage devices. Due to their potentially enormous capacity, optical disks have been the subject of a great deal of research and have gone through an incredibly rapid evolution. 23/05/2017 CIS110 76 23/05/2017 CIS110 77 Optical Storage – CD-ROM Is a disc with highly reflective surface. Tiny areas flat and depressed: • Flat (land) strong reflection. • Depressed (pits) low reflection. Laser landstrong reflectionphoto-sensor generates electrical voltagestore 1s. • Laser: (light Amplification stimulated emission of radiation). Lightpitslow reflection no electrical voltage stores 0s. 23/05/2017 CIS110 78 CD-ROM Operation 23/05/2017 CIS110 79 Laser •Monochromatic light (single wave length) • Coherent (photons move in the same direction) • Directional (very tight beam, strong concentrated) 23/05/2017 CIS110 80 CD/DVD Storage Format • • • • Data stored by creating variations in the reflective surface Data retrieved by means of a laser beam Data stored uniformly (so CD rotation speed varies) Random access much slower than for magnetic disks 23/05/2017 CIS110 81 The pits and lands are written in a single continuous spiral starting near the hole and working out a distance of 32 mm toward the edge. The spiral makes 22,188 revolutions around the disk (about 600 per mm). If unwound, it would be 5.6 km long. 23/05/2017 CIS110 82 Constant linear velocity sector Constante density rev/m centre 23/05/2017 edges CIS110 83 Random Access on CD-ROM Difficult Move head to the right position Set correct speed Read address Adjust to required location 23/05/2017 CIS110 84 head Read and Write Mechanism (1) 1 CPU Head 01010 Add. bus 01010 1 Data bus 1 Control bus 23/05/2017 CIS110 85 CD-ROM for & against Large capacity Easy to mass produce Removable Expensive for small runs Slow Read only 23/05/2017 CIS110 86 Other Optical Storage CD-Recordable (CD-R) CD-RW • WORM(write once read many) • Compatible with CD-ROM drives • Erasable • Mostly CD-ROM drive compatible 23/05/2017 CIS110 87 DVD - what’s in a name? Digital Video Disk • Used to indicate a player for movies • Only plays video disks Digital Versatile Disk • Used to indicate a computer drive • Will read computer disks and play video disks 23/05/2017 CIS110 88 DVD - technology Multi-layer Very high capacity (4.7GB per layer) Full length movie on a single disk • Using MPEG (Moving Picture Expert Group) compression • Digital Compression of audio and video signals. • MPEG achieves high compression rate by storing only changes from one frame to another, instead of each entire frame. 23/05/2017 CIS110 89 Summary Main memory • • • • RAM Low storage capacity Fast (electrical signals) Volatile. Magnetic memory • • • Floppy disk Hard disk Magnetic tape Optical memory • • CD_ROM disk DVD 23/05/2017 CIS110 90