Transcript Week 6 and 7 - Portal UniMAP

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 Types
Memory Type
Random-access
memory (RAM)
Category
Read-write memory
Erasure
Electrically, byte-level
Read-only
memory (ROM)
Write Mechanism
Electrically
Volatility
Volatile
Masks
Read-only memory
Not possible
Programmable
ROM (PROM)
Erasable PROM
(EPROM)
UV light, chip-level
Nonvolatile
Electrically
Electrically Erasable
PROM (EEPROM)
Flash memory
Read-mostly memory
Electrically, byte-level
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
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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
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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
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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
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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
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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)
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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
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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
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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
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Read (traditional)
– Magnetic field moving relative to coil produces current
– Coil is the same for read and write
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Read (contemporary)
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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)
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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 Format
Seagate ST506
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Characteristics
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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
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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)
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Slow
Universal
Cheap
Obsolete?
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Winchester Hard Disk (1/2)
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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)
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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
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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
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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
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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
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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
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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
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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
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Difficult
Move head to rough position
Set correct speed
Read address
Adjust to required location
(Yawn!)
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CD-ROM for & against
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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
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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
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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 CDRW 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
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Serial access
Slow
Very cheap
Backup and archive
Linear Tape-Open (LTO) Tape Drives
– Developed late 1990s
– Open source alternative to proprietary tape
systems
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Linear Tape-Open (LTO) Tape Drives
LTO-1
LTO-2
LTO-3
LTO-4
LTO-5
LTO-6
2000
2003
2005
2007
TBA
TBA
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
Release date
Compressed capacity
Let's go through processing unit