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PART 3: (2/2)

Computer Memory System

CHAPTER 5:Internal Memory

and CHAPTER 6:

External Memory

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CHAPTER 5:Internal Memory

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Semiconductor Memory TypesMemory Type Category Erasure Write Mechanism Volatility

Random-access memory (RAM)

Read-write memory Electrically, byte-level Electrically Volatile

Read-only memory (ROM)

Read-only memory Not possible

Masks

Nonvolatile

Programmable ROM (PROM)

Electrically

Erasable PROM (EPROM)

Read-mostly memory

UV light, chip-level

Electrically Erasable PROM (EEPROM)

Electrically, byte-level

Flash memory Electrically, block-level

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Semiconductor Memory

• RAM – Misnamed as all semiconductor memory is

random access– Read/Write– Volatile– Temporary storage– Static or dynamic

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Memory Cell Operation

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Dynamic RAM

• Bits stored as charge in capacitors• Charges leak• Need refreshing even when powered• Simpler construction• Smaller per bit• Less expensive• Need refresh circuits• Slower• Main memory• Essentially analogue

– Level of charge determines value

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Dynamic RAM Structure

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DRAM Operation

• Address line active when bit read or written– Transistor switch closed (current flows)

• Write– Voltage to bit line

• High for 1 low for 0– Then signal address line

• Transfers charge to capacitor

• Read– Address line selected

• transistor turns on– Charge from capacitor fed via bit line to sense amplifier

• Compares with reference value to determine 0 or 1– Capacitor charge must be restored

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Static RAM

• Bits stored as on/off switches• No charges to leak• No refreshing needed when powered• More complex construction• Larger per bit• More expensive• Does not need refresh circuits• Faster• Cache• Digital

– Uses flip-flops

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Stating RAM Structure

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Static RAM Operation

• Transistor arrangement gives stable logic state• State 1– C1 high, C2 low– T1 T4 off, T2 T3 on

• State 0– C2 high, C1 low– T2 T3 off, T1 T4 on

• Address line transistors T5 T6 is switch• Write – apply value to B & compliment to B• Read – value is on line B

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SRAM v DRAM(1/2)

• Both volatile– Power needed to preserve data

• Dynamic cell – Simpler to build, smaller– More dense– Less expensive– Needs refresh– Larger memory units

• Static– Faster– Cache

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SRAM v DRAM(2/2)

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Read Only Memory (ROM)

• Permanent storage– Nonvolatile

• Microprogramming• Library subroutines• Systems programs (BIOS)• Function tables

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Types of ROM

• Written during manufacture– Very expensive for small runs

• Programmable (once)– PROM– Needs special equipment to program

• Read “mostly”– Erasable Programmable (EPROM)

• Erased by UV

– Electrically Erasable (EEPROM)• Takes much longer to write than read

– Flash memory• Erase whole memory electrically

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Their Usages………

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Organisation in detail

• A 16Mbit chip can be organised as 1M of 16 bit words

• A bit per chip system has 16 lots of 1Mbit chip with bit 1 of each word in chip 1 and so on

• A 16Mbit chip can be organised as a 2048 x 2048 x 4bit array– Reduces number of address pins

• Multiplex row address and column address• 11 pins to address (211=2048)• Adding one more pin doubles range of values so x4 capacity

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Refreshing

• Refresh circuit included on chip• Disable chip• Count through rows• Read & Write back• Takes time• Slows down apparent performance

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Typical 16 Mb DRAM (4M x 4)

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Packaging

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256kByte Module

Organisation

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1MByte Module Organisation

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Interleaved Memory

• Collection of DRAM chips• Grouped into memory bank• Banks independently service read or write

requests• K banks can service k requests simultaneously

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Error Correction

• Hard Failure– Permanent defect

• Soft Error– Random, non-destructive– No permanent damage to memory

• Detected using Hamming error correcting code

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Error Correcting Code Function

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Hamming Code

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P1 P2 1 P4 1 0 0 P8 0 1 0 0

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P1 P2 1 P4 1 0 0 P8 0 1 0 0

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Advanced DRAM Organization

• Basic DRAM same since first RAM chips• Enhanced DRAM– Contains small SRAM as well– SRAM holds last line read (c.f. Cache!)

• Cache DRAM– Larger SRAM component– Use as cache or serial buffer

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Synchronous DRAM (SDRAM)

• Access is synchronized with an external clock• Address is presented to RAM• RAM finds data (CPU waits in conventional DRAM)• Since SDRAM moves data in time with system clock, CPU

knows when data will be ready• CPU does not have to wait, it can do something else• Burst mode allows SDRAM to set up stream of data and fire it

out in block• DDR-SDRAM sends data twice per clock cycle (leading &

trailing edge)

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SDRAM

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SDRAM Read Timing

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RAMBUS

• Adopted by Intel for Pentium & Itanium• Main competitor to SDRAM• Vertical package – all pins on one side• Data exchange over 28 wires < cm long• Bus addresses up to 320 RDRAM chips at

1.6Gbps• Asynchronous block protocol– 480ns access time– Then 1.6 Gbps

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RAMBUS Diagram

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DDR SDRAM

• SDRAM can only send data once per clock• Double-data-rate SDRAM can send data twice

per clock cycle– Rising edge and falling edge

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DDR SDRAM Read Timing

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Simplified DRAM Read Timing

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Cache DRAM

• Mitsubishi• Integrates small SRAM cache (16 kb) onto generic

DRAM chip• Used as true cache– 64-bit lines– Effective for ordinary random access

• To support serial access of block of data– E.g. refresh bit-mapped screen

• CDRAM can Pre-fetch data from DRAM into SRAM buffer• Subsequent accesses solely to SRAM

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CHAPTER 6:External Memory

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Types of External Memory

• Magnetic Disk– RAID– Removable

• Optical Discs– CD-ROM– CD-Recordable (CD-R)– CD-R/W– DVD

• Magnetic Tape

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Magnetic Disk

• Disk substrate coated with magnetisable material (iron oxide…rust)

• Substrate used to be aluminium• Now glass– Improved surface uniformity

• Increases reliability

– Reduction in surface defects• Reduced read/write errors

– Lower flight heights (See later)– Better stiffness– Better shock/damage resistance

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Read and Write Mechanisms

• Recording & 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

– Current through coil produces magnetic field– Pulses sent to head– Magnetic pattern recorded on surface below

• Read (traditional)– Magnetic field moving relative to coil produces current– Coil is the same for read and write

• Read (contemporary)– Separate read head, close to write head– Partially shielded magneto resistive (MR) sensor– Electrical resistance depends on direction of magnetic field– High frequency operation

• Higher storage density and speed

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Inductive Write MR Read

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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

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Disk Data Layout

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Disk Velocity

• Bit near centre of rotating disk passes fixed point slower than bit on outside of disk

• Increase spacing between bits in different tracks • Rotate disk at constant angular velocity (CAV)

– Gives pie shaped sectors and concentric tracks– Individual tracks and sectors addressable– Move head to given track and wait for given sector– Waste of space on outer tracks

• Lower data density

• Can use zones to increase capacity– Each zone has fixed bits per track– More complex circuitry

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Disk Layout Methods Diagram

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Finding Sectors

• Must be able to identify start of track and sector

• Format disk– Additional information not available to user– Marks tracks and sectors

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Winchester Disk FormatSeagate ST506

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Characteristics

• Fixed (rare) or movable head• Removable or fixed• Single or double (usually) sided• Single or multiple platter• Head mechanism– Contact (Floppy)– Fixed gap– Flying (Winchester)

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Fixed/Movable Head Disk

• Fixed head– One read write head per track– Heads mounted on fixed ridged arm

• Movable head– One read write head per side– Mounted on a movable arm

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Removable or Not

• Removable disk– Can be removed from drive and replaced with

another disk– Provides unlimited storage capacity– Easy data transfer between systems

• Non removable disk– Permanently mounted in the drive

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Multiple Platter

• One head per side• Heads are joined and aligned• Aligned tracks on each platter form cylinders• Data is striped by cylinder– reduces head movement– Increases speed (transfer rate)

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Multiple Platters

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Tracks and Cylinders

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Floppy Disk

• 8”, 5.25”, 3.5”• Small capacity– Up to 1.44Mbyte (2.88M never popular)

• Slow• Universal• Cheap• Obsolete?

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Winchester Hard Disk (1/2)

• Developed by IBM in Winchester (USA)• Sealed unit• One or more platters (disks)• Heads fly on boundary layer of air as disk spins• Very small head to disk gap• Getting more robust

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Winchester Hard Disk (2/2)

• Universal• Cheap• Fastest external storage• Getting larger all the time– 250 Gigabyte now easily available

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Speed

• Seek time– Moving head to correct track

• (Rotational) latency– Waiting for data to rotate under head

• Access time = Seek + Latency• Transfer rate

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Timing of Disk I/O Transfer

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RAID

• Redundant Array of Independent Disks • Redundant Array of Inexpensive Disks (Originally)• 6 levels in common use• Not a hierarchy• Set of physical disks viewed as single logical drive by O/S• Data distributed across physical drives• Can use redundant capacity to store parity information

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RAID 0

• No redundancy• Data striped across all disks• Round Robin striping• Increase speed– Multiple data requests probably not on same disk– Disks seek in parallel– A set of data is likely to be striped across multiple

disks

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RAID 1

• Mirrored Disks• Data is striped across disks• 2 copies of each stripe on separate disks• Read from either• Write to both• Recovery is simple– Swap faulty disk & re-mirror– No down time

• Expensive

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RAID 2

• Disks are synchronized• Very small stripes– Often single byte/word

• Error correction calculated across corresponding bits on disks

• Multiple parity disks store Hamming code error correction in corresponding positions

• Lots of redundancy– Expensive– Not used

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RAID 3

• Similar to RAID 2• Only one redundant disk, no matter how large

the array• Simple parity bit for each set of corresponding

bits• Data on failed drive can be reconstructed from

surviving data and parity info• Very high transfer rates

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RAID 4

• Each disk operates independently• Good for high I/O request rate• Large stripes• Bit by bit parity calculated across stripes on

each disk• Parity stored on parity disk

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RAID 5

• Like RAID 4• Parity striped across all disks• Round robin allocation for parity stripe• Avoids RAID 4 bottleneck at parity disk• Commonly used in network servers

• N.B. DOES NOT MEAN 5 DISKS!!!!!

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RAID 6

• Two parity calculations• Stored in separate blocks on different disks• User requirement of N disks needs N+2• High data availability– Three disks need to fail for data loss– Significant write penalty

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RAID 0, 1, 2

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RAID 3 & 4

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RAID 5 & 6

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Data Mapping For RAID 0

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Optical Storage CD-ROM

• Originally for audio• 650Mbytes giving over 70 minutes audio• Polycarbonate coated with highly reflective

coat, usually aluminium• Data stored as pits• Read by reflecting laser• Constant packing density• Constant linear velocity

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CD Operation

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CD-ROM Drive Speeds

• Audio is single speed– Constant linier velocity– 1.2 ms-1

– Track (spiral) is 5.27km long– Gives 4391 seconds = 73.2 minutes

• Other speeds are quoted as multiples• e.g. 24x• Quoted figure is maximum drive can achieve

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CD-ROM Format

• Mode 0=blank data field• Mode 1=2048 byte data+error correction• Mode 2=2336 byte data

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Random Access on CD-ROM

• Difficult• Move head to rough position• Set correct speed• Read address• Adjust to required location• (Yawn!)

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CD-ROM for & against

• Large capacity (?)• Easy to mass produce• Removable• Robust

• Expensive for small runs• Slow• Read only

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Other Optical Storage

• CD-Recordable (CD-R)– WORM– Now affordable– Compatible with CD-ROM drives

• CD-RW– Erasable– Getting cheaper– Mostly CD-ROM drive compatible– Phase change

• Material has two different reflectivities in different phase states

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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

• Dogs Veritable Dinner• Officially - nothing!!!

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DVD - technology

• Multi-layer• Very high capacity (4.7G per layer)• Full length movie on single disk– Using MPEG compression

• Finally standardized (honest!)• Movies carry regional coding• Players only play correct region films• Can be “fixed”

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DVD – Writable

• Loads of trouble with standards• First generation DVD drives may not read first

generation DVD-W disks• First generation DVD drives may not read CD-

RW disks• Wait for it to settle down before buying!

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CD and DVD

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High Definition Optical Disks

• Designed for high definition videos• Much higher capacity than DVD

– Shorter wavelength laser• Blue-violet range

– Smaller pits• HD-DVD

– 15GB single side single layer• Blue-ray

– Data layer closer to laser• Tighter focus, less distortion, smaller pits

– 25GB on single layer– Available read only (BD-ROM), Recordable once (BR-R) and re-

recordable (BR-RE)

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Optical Memory Characteristics

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Magnetic Tape

• Serial access• Slow• Very cheap• Backup and archive• Linear Tape-Open (LTO) Tape Drives– Developed late 1990s– Open source alternative to proprietary tape

systems

Linear Tape-Open (LTO) Tape DrivesLTO-1 LTO-2 LTO-3 LTO-4 LTO-5 LTO-6

Release date 2000 2003 2005 2007 TBA TBA

Compressed capacity 200 GB 400 GB 800 GB 1600 GB 3.2 TB 6.4 TB

Compressed transfer rate (MB/s)

40 80 160 240 360 540

Linear density (bits/mm) 4880 7398 9638 13300

Tape tracks 384 512 704 896

Tape length 609 m 609 m 680 m 820 m

Tape width (cm) 1.27 1.27 1.27 1.27

Write elements 8 8 16 16

Let's go through processing unit