welcome to 236601 coding and algorithms to memories
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Welcome to 236601 - Coding and Algorithms to Memories. Overview. Lecturer : Eitan Yaakobi [email protected] , Taub 638 Lectures hours : Thur 12:30-14:30 @ Taub 8 Course website : http://webcourse.cs.technion.ac.il/236601/Spring2014/

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  • Lecturer: [email protected], Taub 638
  • Lectures hours: Thur 12:30-14:30 @ Taub 8
  • Course website: http://webcourse.cs.technion.ac.il/236601/Spring2014/
  • Office hours: Thur 14:30-15:30 and/or other times (please contact by email before)
  • Final grade:
    • Class participation (10%)
    • Homeworks (50%)
    • Take home exam/final Homework + project (40%)
what is this class about
What is this class about?

Coding and Algorithms to Memories

  • Memories – HDDs, flash memories, and other non-volatile memories
  • Coding and algorithms – how to manage the memory and handle the interface between the physical level and the operating system
  • Both from the theoretical and practical points of view
  • Q: What is the difference between theory and practice?
One of the focuses during this class: How to ask the right questions, both as a theorist and as a practical engineer
memory storage
Memory Storage
  • Computer data storage (from Wikipedia):

Computer components, devices, and recording media that retain digital data used for computing for some interval of time.

  • What kind of data?
    • Pictures, word files, movies, other computer files etc.
  • What kind of memories?
    • Many kinds…

5 Megabyte Hard Drive

A 2012 Terabyte Drive

  • Volatile Memories – need power to maintain the information
    • Ex: RAM memories, DRAM, SRAM
  • Non-Volatile Memories – do NOT need power to maintain the information
    • Ex: HDD, optical disc (CD, DVD), flash memories
  • Q: Examples of old non-volatile memories?
Some of the main goals in designing a computer storage:


Capacity (size)



Power Consumption

the evolution of memories1
The Evolution of Memories

One Song

14% of One Song

28% of One Song

140 Songs

960 Songs

5120 Songs

6553 Songs

209,715 Songs

optical storage
Optical Storage
  • Storage systems that use light for recording and retrieval of information
  • Types of optical storage
    • CD
    • DVD
    • Blu-Ray disc
    • Holographic storage
  • 1961,1969 - David Paul Gregg from Gauss Electrophysics has patented an analog optical disc for recording video
  • MCA acquires Gregg’s company and his patents
  • 1969 - a group of researchers at Philips Research in Eindhoven, The Netherlands, had optical videodisc experiments
  • 1975 – Philips and MCA joined forces in creating the laserdisc
  • 1978 – the laserdisc was first introduced but was a complete failure and this cooperation came to its end
  • 1983 – the successful Compact Disc was introduced by Philips and Sony
  • First generation – CD (Compact Disc), 700MB
  • Second generation – DVD (Digital Versatile Disc), 4.7GB, 1995
  • Third generation – BD (Blu-Ray Disc)
    • Blue ray laser (shorter wavelength)
    • A single layer can store 25GB, dual layer – 50GB
    • Supported by Sony, Apple, Dell, Panasonic, LG, Pioneer
optical disc
Optical Disc

Information is stored as pits and lands (corres. to –1,+1)

optical storage how does it work
Optical Storage – How does it work?
  • A light, emitted by a laser spot, is reflected from the disc
  • The light is transformed to a voltage signal and then to bits
the material of the cd
The Material of the CD
  • Most of the CD consists of an injection-molded piece of clear polycarbonate plastic, 1.2 mm thick
  • The plastic is impressed with microscopic pits arranged as a single, continuous, extremely long spiral track of data
  • A thin, reflective aluminum layer is sputtered onto the disc, covering the pits
  • A thin acrylic layer is sprayed over the aluminum to protect it
  • The label is then printed onto the acrylic
the laser
The Laser
  • The laser spot, emitted by the laser diode is reflected from the disc to the photodiode by the partially silvered mirror
  • When the spot is over the land:
    • The light is reflected and the received optical signal is high
  • When the spot is over a pit:
    • The light is reflected from both the bottom of the pit and the land
    • The reflected lights interfere destructively and the signal is low
the disc
The Disc
  • A CD has a single spiral track of data, circling from the inside of the disc to the outside
  • The track is approximately 0.5 microns width, with 1.6 microns separating one track from the next
  • The pits size is at least 0.83 microns and 125 nanometers high
  • The length of the track after stretching it is 3.5 miles!
  • Holds 74 minutes and 33 seconds of sound, enough for a complete mono recording of Beethoven’s ninth symphony
cd player components
CD Player Components
  • A drive motor -spins the disc and rotates it between 200 and 500 rpm depending on which track is being read
  • A laser and a lens system for focusing read the pits
  • A tracking mechanism moves the laser assembly so that the laser's beam can follow the spiral track
  • Similar to CD but has more capacity (4.7G Vs. 0.7G)
  • DVDs have the same diameter and thickness as CDs
  • They are made of the same materials and manufacturing methods
  • The data on a DVD is encoded in the form of small pits and lands
  • Similar to CD, a DVD is composed of several layers of plastic, totaling about 1.2 millimeters thick
  • A semi-reflective gold layer is used for the outer layers, allowing the laser to focus through the outer and onto the inner layers
the material of dvd
The material of DVD
  • Comparing to CD, the pits width is 320 nanometer, and at least 400 nanometer length
  • Only 740 nanometers separate between adjacent tracks
  • Therefore, the DVD supplies a higher density data storage
blu ray disc
Blu-Ray Disc
  • The wavelength of a blue-violet laser (405nm) is shorter than the one of a red laser (650nm)
  • It possible to focus the laser spot with greater precision
  • Data can be packed more tightly and stored in less space
  • Blu-ray Discs holds
    • 25 GB (one layer) 56%
    • 50 GB (dual layer) 44%
3 Generations of Optical Recording

Blu-Ray Disc




l = 650 nm

NA = 0.6

4.7 GBytes

l = 405 nm

NA = 0.85

22.5 GBytes

0.65 GByte

4.7 GByte

25 GByte

1.2 mm substrate

0.6 mm substrate

0.1 mm substrate

holographic storage
Holographic Storage
  • An optical technology that allows 1 million bits of data to be written and read out in single flashes of light
  • A stack of holograms can be stored in the same location
  • An entire page of information is stored at once as an optical interference pattern within a thick, photosensitive optical material
holographic storage1
Holographic Storage
  • Light from a coherent laser source is split into two beams: signal (data-carrying) and reference beams
  • The Digital data is encoded onto the signal beam via a spatial light modulator (SLM)
  • By changing the reference beam angle, wavelength, or media position many different holograms are recorded
data encoding
Data Encoding
  • The data is arranged into large arrays
  • The 0's and 1's are translated into pixels of the spatial light modulator that either block or transmit light
  • The light of the signal beam traverses through the modulator and is therefore encoded with the pattern of the data page
  • This encoded beam interferes with the reference beam through the volume of a photosensitive recording medium
  • The light pattern of the image is recorded as a hologram on the photopolymer disc using a chemical reaction
reading data
Reading Data
  • The reference beam is shined directly onto the hologram
  • When it reflects off the hologram, it holds the light pattern of the image stored there
  • The reconstruction beam is sent to a CMOS sensor to recreate the original image
the magnetic hard disk drive
The Magnetic Hard Disk Drive


Spindle motor

Read-Write Head



but what is this
But What is This?

A 1975 HDD Factory Floor

facts about this factory floor
Facts About This Factory Floor
  • The total capacity of all of the drives shown on this factory floor was less than 20 GB’s!
  • The total selling price of all of the drives shown on this floor was about $4,000,000!
1980 s ibm 3380 drive
1980’s: IBM 3380 Drive
  • The IBM 3380 was the first gigabyte drive.
  • The manufacturing cost was about $5000. The selling price was in the range $80,000- $150,000!
  • During the 1980’s, IBM sold billions of dollars of these drives each year.
  • It is the 2nd most profitable product ever manufactured by man.
1980 s ibm 3380 drive1
1980’s: IBM 3380 Drive

One Disk

From Drive

q what s inside an old 4gb nano
Q: What’s Inside an Old 4GB Nano?

A 4 GB 1”


disk drive basics
Disk Drive Basics



disk drive basics writing
Disk Drive Basics - Writing

Head on slider



MR Read Sensor

Magnetic flux leaking from the write-head gap records bits in the magnetic medium

Write Head


Recording Media


disk drive basics reading
Disk Drive Basics - Reading

Head on slider



Resistance of MR read sensor changes in response to fields produced by the recorded bits

MR Read Sensor

Write Head


Recording Media


magnetic write process
Magnetic Write Process

Gap is 100 nm but bits are 25 nm.

How can this be??

100 nm


100 nm

  • Shrink everything by factor s (including currents and microstructure).
  • Areal density of data increases by the factor s2.
  • Requires vastly improved head and disk materials.
  • Requires improved mechanical tolerances.
    • Scaling the flying height is a real challenge.
  • Requiresimproved signal processing schemes because the

SNR drops by a factor of s.

What is needed?

fundamental innovations
Fundamental Innovations

MR/GMR sensors (1991/1997)

AFC media (2001)

to 100 Gb/in2

GMR read


Perpendicular recording


to 500+ Gb/in2

Perpendicular media

longitudinal vs perpendicular
Longitudinal vs. Perpendicular

Longitudinal recording: horizontal orientation

Perpendicular recording: vertical orientation

(introduced commercially in 2005)

Areal Density Increase of Hard Disk Drives


* CAGR = Cumulative Annual Growth Rate

predicting the future of disk drives
Predicting the Future of Disk Drives
  • It looks like the present technology will max out in a few years
  • As the size of the stored bit shrinks, the present magnetic material will not hold it’s magnetization at room temperature. This is called the superparamagneticeffect
  • A radically new system may be required
the future of disk drives
The Future of Disk Drives
  • Two solutions are being pursued to overcome the superparamagneticeffect
    • One solution is to use a magnetic material with a much higher coercivity. The problem with this solution is that you cannot write on the material at room temperature so you need to heat the media to write
    • The second approach is called patterned media where bits are stored on physically separated magnetic elements
future technology
Future Technology?

HAMR-Heat Assisted

Magnetic Recording

Patterned Media

patterned media
Patterned Media

Ordinary Media Patterned Media Many grains/bit One grain/bit

In patterned media, the pattern of islands is defined by lithography.

An areal density of 1 Tb/in2 requires 25-nm bit cells. Presently, this is very difficult to achieve.

the history of flash memories
The History of Flash Memories
  • Flash memory was introduced in 1984 by Dr. FujioMasouka of Toshiba.
  • Why the name flash?
    • Because the erase operation is similar to the flash of the camera
  • There are two types: NOR and NAND flash.
  • NAND flash is used in most products because of its cost advantage.
  • Recently multi-level (MLC) NAND flash has been introduced because it can store more information.

Low Power


~105 P/E Cylces

solid state drives
Solid State Drives
  • What is a Solid State Drive (SSD)?

It is an “Hard Disk” with flash instead of a disk

  • Why to use a Solid State Drive?
    • Lower power consumption
    • Durability
    • Faster random access
  • Flash drives have not replaced HDDs in most large storage applications because:
    • They wear out
    • They are more temperature sensitive
    • Erasing is more difficult
    • They are more expensive
multi level flash memory model
Multi-Level Flash Memory Model
  • Array of cells, made of floating gate transistors
    • Each cell can store q different values.
    • Today, q typically ranges between 2 and 16.









multi level flash memory model1
Multi-Level Flash Memory Model
  • Array of cells, made of floating gate transistors
    • Each cell can store q different values.
    • Today, q typically ranges between 2 and 16.
    • The cell’s level is increased by pulsing electrons.
    • Reducing a cell level requires resetting all the cells in its containing block to level 0 – A VERY EXPENSIVE OPERATION
flash memory constraints
Flash Memory Constraints
  • The lifetime/endurance of flash memories corresponds to the number of times the blocks can be erased and still store reliable information
  • Usually a block can tolerate ~104-105 erasures before it becomes unreliable
  • The Goal: Representing the data efficiently such that block erasures are postponed as much as possible
slc mlc and tlc flash
SLC, MLC and TLC Flash

High Voltage

High Voltage

High Voltage

SLC Flash

MLC Flash

TLC Flash

1 Bit Per Cell

2 States

2 Bits Per Cell

4 States

3 Bits Per Cell

8 States

Low Voltage

Low Voltage

Low Voltage

flash memory structure
Flash Memory Structure
  • A group of cells constitute a page
  • A group of pages constitute a block
    • In SLC flash, a typical block layout is as follows
flash memory structure1
Flash Memory Structure


  • In MLC flash the two bits within a cell DO NOT belong to the same page – MSB page and LSB page
  • Given a group of cells, all the MSB’s constitute one page and all the LSB’s constitute another page





flash memory structure2
Flash Memory Structure

MSB Page CSB Page LSB Page MSB Page CSB Page LSB Page

ber per page for mlc block
BER per page for MLC block



Pages, colored the same, behave similarly






raw ber results1
Raw BER Results

High Voltage

Low Voltage