12/3/2004EE 42 fall 2004 lecture 391 Lecture #39: Magnetic
memory storage Last lecture: Dynamic Ram E 2 memory This lecture:
Future memory technologies Magnetic memory devices Hard drives,
tape drives, Optical disks
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12/3/2004EE 42 fall 2004 lecture 392 Future memory technologies
Memory speed, cost and density are among the chief bottlenecks on
compute power. Increasing CPU clock rates have only resulted in
small increases in speed of operation due to the memory system and
mass storage (disk) I/O bottleneck. A significant amount of
research effort is directed to improving memory technology
Slide 3
12/3/2004EE 42 fall 2004 lecture 393 Advanced memory
technologies Ferroelectric Random Access Memory (FRAMs)
Magnetoresistive Random Access Memories (MRAMs) Tunneling Magnetic
Junction RAM (TMJ-RAM): Experimental Memories Quantum-Mechanical
Switch Memories Single Electron Memory
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12/3/2004EE 42 fall 2004 lecture 394 FRAM
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12/3/2004EE 42 fall 2004 lecture 395 Ferroelectric
material
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12/3/2004EE 42 fall 2004 lecture 396 TMJ-Ram Tunneling Magnetic
Junction RAM (TMJ-RAM): Speed of SRAM, density of DRAM, non-
volatile (no refresh) Spintronics (electron spin affects transport)
Same technology used in the read heads of high-density disk-drives:
Giant magneto- resistive effect
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12/3/2004EE 42 fall 2004 lecture 397 Tunneling Magnetic
Junction
Slide 8
12/3/2004EE 42 fall 2004 lecture 398 Mass Storage For storage
of larger amounts of information, magnetic film storage dominates
Information is stored in the form of magnetic domains in a
Ferromagnetic film, written or read by a moving head
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12/3/2004EE 42 fall 2004 lecture 399 Magnetic domains
Ferromagnetic materials have a quantum interaction which makes
adjacent atoms line up their magnetic field in the same direction N
N N N N N N N N N N N N S S S S S S S S S S S S S
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12/3/2004EE 42 fall 2004 lecture 3910 Magnetic interactions On
a larger scale, magnets feel a force to line up in opposing
directions, reducing the total magnetic field. For example, if you
try to hold two magnets next to each other, there will be a strong
force which will rotate them to the configuration: N S S N
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12/3/2004EE 42 fall 2004 lecture 3911 Magnetic domains If you
look microscopically at a magnetic material, it forms domains, or
areas where the magnetic poles are aligned, adjacent to regions
where the magnetization is in the opposite direction. In a thin
film, the domains look like this:
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12/3/2004EE 42 fall 2004 lecture 3912 Moving magnetic domains
Magnetic domains dont move easily at room temperature, but they can
be changed by applying magnetic fields. If most of the domains in a
material are aligned in one direction, we call it a permanent
magnet. The core of an inductor or a transformer is made of a
ferromagnetic material where the domains line up easily, and then
randomize again when the external field is turned off
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12/3/2004EE 42 fall 2004 lecture 3913 Writing to magnetic media
Magnetic storage material is comprised of a thin film of
ferromagnetic material which is relatively magnetically hard. A
small electromagnet is used to create domains oriented in a
particular direction
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12/3/2004EE 42 fall 2004 lecture 3914 Reading magnetic material
Conventional read heads for magnetic media work just like the
secondary winding of a transformer. Instead of a primary winding
changing the magnetic field through a coil, and thus changing the
voltage, the magnetic media is moved next to the read coil. This
produces a voltage across the read coil which can be amplified and
translated as data
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12/3/2004EE 42 fall 2004 lecture 3915 Transformer +V1-+V1-
+V2-+V2-
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12/3/2004EE 42 fall 2004 lecture 3916 source: New York Times,
2/23/98, page C3, Makers of disk drives crowd even more data into
even smaller spaces 470 v. 3000 Mb/si 9 v. 22 Mb/si 0.2 v. 1.7
Mb/si Storage density for DRAM vs DISK
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12/3/2004EE 42 fall 2004 lecture 3917 SRAM vs. DRAM vs. Disk
Access latencies: DRAM ~10X slower than SRAM Successive bytes 4x
faster than first byte for DRAM Disk ~100,000X slower than DRAM
First byte is ~100,000X slower than successive bytes on disk
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12/3/2004EE 42 fall 2004 lecture 3918 Nano-layered Disk Heads
Recent large improvement in Disk capacity comes from Giant
Magneto-Resistive effect (GMR) read heads Coil for writing
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12/3/2004EE 42 fall 2004 lecture 3919 Typical Numbers of a
Magnetic Disk Rotational Latency: Most disks rotate at 3,600 to
15,000 RPM Approximately 16 ms to 4 ms per revolution, respectively
An average latency to the desired information is halfway around the
disk: 8 ms at 3600 RPM, 2 ms at 15,000 RPM Transfer Time is a
function of : Transfer size (usually a sector): 1 KB / sector
Rotation speed: 3600 RPM to 10000 RPM Recording density: bits per
inch on a track Diameter typical diameter ranges from 2.5 to 5.25
in Typical values: 2 to 80 MB per second Sector Track Cylinder Head
Platter
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12/3/2004EE 42 fall 2004 lecture 3920 Disk Device Terminology
Several platters, with information recorded magnetically on both
surfaces (usually) Actuator moves head (end of arm,1/surface) over
track (seek), select surface, wait for sector rotate under head,
then read or write Cylinder: all tracks under heads Bits recorded
in tracks, which in turn divided into sectors (e.g., 512 Bytes)
Platter Outer Track Inner Track Sector Actuator HeadArm
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12/3/2004EE 42 fall 2004 lecture 3921 Photo of Disk Head, Arm,
Actuator Actuator Arm Head Platters (12) { Spindle
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12/3/2004EE 42 fall 2004 lecture 3922 Disk Device Performance
Platter Arm Actuator HeadSector Inner Track Outer Track Disk
Latency = Seek Time + Rotation Time + Transfer Time + Controller
Overhead Seek Time? depends no. tracks move arm, seek speed of disk
Rotation Time? depends on speed disk rotates, how far sector is
from head Transfer Time? depends on data rate (bandwidth) of disk
(bit density), size of request Controller Spindle
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12/3/2004EE 42 fall 2004 lecture 3923 Disk Device Performance
Average distance sector from head? 1/2 time of a rotation 7200
Revolutions Per Minute 120 Rev/sec 1 revolution = 1/120 sec 8.33
milliseconds 1/2 rotation (revolution) 4.16 ms Average no. tracks
move arm? Sum all possible seek distances from all possible tracks
/ # possible Assumes average seek distance is random Disk industry
standard benchmark
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12/3/2004EE 42 fall 2004 lecture 3924 Devices: Magnetic Disks
Sector Track Cylinder Head Platter Purpose: Long-term, nonvolatile
storage Large, inexpensive, slow level in the storage hierarchy
Characteristics: Seek Time (~8 ms avg) positional latency
rotational latency Transfer rate 10-30 MByte/sec Blocks Capacity
Gigabytes Quadruples every 3 years (aerodynamics) 7200 RPM = 120
RPS => 8 ms per rev ave rot. latency = 4 ms 128 sectors per
track => 0.25 ms per sector 1 KB per sector => 16 MB / s
Response time = Queue + Controller + Seek + Rot + Xfer Service
time
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12/3/2004EE 42 fall 2004 lecture 3925 Areal Density Bits per
unit area changed slope from 30%/yr to 60%/yr about 1991
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12/3/2004EE 42 fall 2004 lecture 3926 Technology Trends Disk
Capacity now doubles every 12 months; before 1990 every 36 motnhs
Today: Processing Power Doubles Every 18 months Today: Memory Size
Doubles Every 18-24 months(4X/3yr) Today: Disk Capacity Doubles
Every 12-18 months Disk Positioning Rate (Seek + Rotate) Doubles
Every Ten Years! The I/O GAP The I/O GAP