Writing Assignment #2 due Today (5:00pm) - Post on your CSC101 webpage - Ask if you have questions! Lab #2 Today. Quiz #1 Tomorrow (Lectures 1-7)

Transcription

1 Overview of Computer Science CSC 101 Summer 2011 Main Memory vs. Auxiliary Storage Lecture 7 July 14, 2011 Announcements Writing Assignment #2 due Today (5:00pm) - Post on your CSC101 webpage - Ask if you have questions! Lab #2 Today Quiz #1 Tomorrow (Lectures 1-7) 2 Objectives Important points from yesterday s lecture How main memory is constructed Differences between main memory and auxiliary storage Look at how various storage devices work Magnetic Tape Hard Drives CD-ROM / CD-RW / DVD 3 1

2 The von Neumann Architecture All modern computers follow the logical model of computing called the von Neumann Architecture Computer organized into four main sections: Central Processing Unit (CPU) All of the computer s operations and calculations Main Memory Active data and programs Auxiliary Storage Disks and other storage devices I/O Subsystem Input and output peripherals 4 Program Interpretation Cycle Programs exist in main memory while they are running Everything in memory is just binary bits Bit patterns can represent many types of information Certain bit patterns represent specific computer instructions The CPU executes programs one instruction i at a time Fetch a binary word from main memory Decode the instruction in that word Execute the instruction Repeat until done, a billion times per second Computers are very dumb They can only do very simple things (machine language instructions) next week Execute Fetch Decode 5 Main Memory Main memory generally uses Random Access Memory (RAM) General use For programs and data Very fast Can keep up with fast CPUs Expensive Volatile Data disappears when powered off Comprised of flip-flop circuits 6 2

3 Boolean Logic Review The basic values in Boolean logic are TRUE FALSE There are two possible values (just like binary data!) TRUE can be represented by a 1 bit FALSE can be represented by a 0 bit 7 Boolean Logic Review NOT X = A If A is FALSE, then A is TRUE If A is TRUE,, then A is FALSE X is always the opposite of A 8 Boolean Logic Review AND X = A B (or just X = AB) If A is TRUE, AND B is TRUE, then A B Bis TRUE; otherwise A B is FALSE 9 3

4 Boolean Logic Review OR X = A + B If either A OR B is TRUE, then A+B is TRUE; otherwise A+B is FALSE 10 Flip-Flop Flop Circuit A circuit that can remember a value is called a flip-flop circuit Also called an S-R latch If Q = 1, the circuit is storing a 1 bit If Q = 0, the circuit is storing a 0 bit As long as S and R are both h0, the Q output remains unchanged If S ( set ) is changed momentarily to 1, Q becomes 1 (regardless of its previous value) If R ( reset ) is changed momentarily to 1, Q becomes 0 (regardless of its previous value) A flip-flop is one bit of memory There are billions of these flip-flops in your laptop s memory 11 Auxiliary Storage Main memory contains data and instructions that are in active use Auxiliary storage is for data and programs that aren t in active use Usually disk drives or flash memory 12 4

5 Auxiliary Storage vs. Main Memory Secondary Auxiliary Storage Non-volatile ( permanent ) Much greater capacity possible ( infinite expansion) Much cheaper per MB than RAM Much slower than RAM Main Memory Volatile Limited capacity and expansion possibilities More expensive Very high speed access Auxiliary storage devices and media choices based on Capacity Speed Cost 13 Types of Auxiliary Storage Auxiliary storage that is always available (like your laptop s hard disk) is called online storage Removable storage devices (like a CD-ROM or a USB jump drive) are called offline storage Three broad types of auxiliary storage Sequential Access (Magnetic Tape) Direct Access (Hard Drives / CDs / DVDs) Random Access (Jump Drives / Memory Cards) 14 Types of Auxiliary Storage Sequential Access Storage Devices (SASD) Data items are organized in a linear sequence Access time is highly variable Items near the beginning of the sequence are accessed quickly, but accessing items near the end may take a long time Off-line storage Example Magnetic tape 15 5

6 Magnetic Tape UNIVAC 1 IBM System/360 (1952) (1960 s 1980 s) Tape drives provided online storage when hard disks were too expensive or not yet invented Now hard disks usually used for archive storage (offline) 16 Archival Data Storage Because tape is sequential access, it is no longer appropriate for online storage But, because it is very cheap and compact per GB, is is very appropriate for archival storage (data backup) For lots of data, need lots of tape cartridges Need to be able to automatically change tapes IBM GB (100 billion bytes) 17 Archival Data Storage Automated tape libraries or robots can manage very large numbers of tape cartridges IBM ~1 PB (1 quadrillion bytes) 18 6

7 How Magnetic Tape Works Remember that binary data is a collection of bits Bits can be 1 0, on off, yes no, left right any two-state information Bits can be represented by magnetization of tiny metallic particles on the surface of a strip of tape Each metallic particle is like a small magnet Aligned particles pointing one way represent a 0 Aligned particles pointing the other way represent a 1 19 How Magnetic Tape Works Magnetic tape is composed of a very large number of microscopic magnetic particles These tiny particles are initially aligned randomly If a magnet is held up to the tape, the particles all become aligned to the magnet S N Different sections of the tape can be magnetized oppositely. The sections of tape then have one of two states, and so can represent binary information Types of Auxiliary Storage Direct Access Storage Devices (DASD) Data items are independently addressed in regions Access time is mostly constant Most common form of auxiliary storage used in PCs Examples Hard disks On-line storage Optical disks and floppy disks Removable, off-line storage 21 7

8 Direct Access Storage Devices Magnetic hard disks and floppy disks Optical disks (CD-ROM, CD-R, DVD-R, DVD-RW RW, DVD+RW, etc.) Common geometry all arranged in tracks and sectors Each sector contains many words of data 22 DASD Media While the disk is spinning: Seek time: the read/write head advances to proper track Latency time: wait for the proper sector to rotate under the read/write head Read/write time: the read/write head scans the sector for read or write Data transfer rate: the average rate of sending or retrieving data from the disk (MB/sec) 23 Magnetic Disks Floppy Disks Laptop Hard Disks Highperformance Hard Disks Diameter (in) 3 ½ 2 ½ 3 ½ Typical Capacity (GB) Rotation speed (RPM) up to 10,000 Avg. data transfer rate (MB/sec) ½ Avg. seek time (ms) Avg. latency time (ms)

9 Optical Disks CD-ROM (Compact Disk Read-Only Memory) Factory-produced (pressed) for published information CD-R (Compact Disk Recordable) Can be recorded ( burned )by by desktop devices Readable by CD-ROM readers CD-RW (Compact Disk Re-writable) Writable and erasable by desktop devices Can be used as on-line storage DVD (D V D) Similar to CD, but higher storage capacity Thousands of tracks 25 CD-ROM Disks are mastered and pressed in a factory Binary data is stored as depressions ( pits ) on a metallic surface ( land ) Pit = 1, Land = 0 Nonerasable, nonwritable Laser beam reads data by reflecting off the pits and land areas 26 CD-R Disks are individually written ( burned ) High-intensity laser burns darkened dye spots on grooved tracks Spot = 1, no spot = 0 Writable, but not erasable Laser reads data by reflecting off dye spots grooved tracks burned dye spots 27 9

10 CD-RW Disks are individually written Laser aligns microscopic crystals in grooved tracks Aligned crystals look dark; unaligned crystals look bright Dark = 1, bright = 0 Writable, and erasable Laser reads data by reflecting off spots of aligned or unaligned crystals grooved tracks unaligned crystals aligned crystals 28 Optical Disk Technology Electron micrographs: CD-ROM (pits and lands) CD-R (burned dye spots) 29 Optical Disk Technology Electron micrographs: CD-RW (aligned liquid crystals) Source: p

11 CD vs. DVD CD, DVD and Blu-ray all use similar technologies, just higher densities (more bits per square inch) Format CD-ROM DVD-ROM Blu-ray (ROM) Capacity 650 MB 4.7 GB 18 GB 25 GB 50 GB 31 Source: Blu-ray Disc Association 32 Types of Auxiliary Storage Random Access Storage Data words are addressed individually Access time is constant (same for all data words, regardless of location) Usually off-line storage Examples USB jump drives Memory cards 33 11

12 Storage Tradeoffs Tradeoffs between access speed, convenience, and cost of memory Faster access is usually more expensive Large, archival storage systems usually use magnetic tape because of the low cost 34 12