1. 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

2. 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

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

4. 12/3/2004EE 42 fall 2004 lecture 394 FRAM

5. 12/3/2004EE 42 fall 2004 lecture 395 Ferroelectric material

6. 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

7. 12/3/2004EE 42 fall 2004 lecture 397 Tunneling Magnetic Junction

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

9. 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

10. 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

11. 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:

12. 12/3/2004EE 42 fall 2004 lecture 3912 Moving magnetic domains Magnetic domains don’t 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

13. 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

14. 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

15. 12/3/2004EE 42 fall 2004 lecture 3915 Transformer +V1-+V1- +V2-+V2-

16. 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

17. 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

18. 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

19. 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

20. 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

21. 12/3/2004EE 42 fall 2004 lecture 3921 Photo of Disk Head, Arm, Actuator Actuator Arm Head Platters (12) { Spindle

22. 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

23. 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

24. 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

25. 12/3/2004EE 42 fall 2004 lecture 3925 Areal Density –Bits per unit area changed slope from 30%/yr to 60%/yr about 1991

26. 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