CSC103 How Computers Work

Notes5

Memory

1. RAM

   • Random Access Memory
   • Volatile: contents lost without power
   • Two types:
     • SRAM: Static RAM
       • 1 bit stored in flip-flop of 4 transistors
       • two stable states represent 0/1
     • DRAM : Dynamic RAM
       • 1 bit stored by capacitor, just 1 transitor
       • but capacitors discharge, so needs dynamic updating.
   • Both are "gated" by an AND-gate, so their content only comes out if other line is turned on. (Remember: AND gate essentially two serial switches.)
   • Large grid of bit cells
   • RAM vs. ROM (Read-only Memory)
     • ROM: BIOS (basic input/output system): ROM: character shapes

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2. Magnetic Memory

   
   • Review: KB, MB, GB, TB
   • History: magnetic drums developed in Britian in the 1940's.
   • Magnetic materials
   • Moving magnetic field == Electric field (cf. relays)
   • Storing a bit:
   • Tapes
     • Plastic, coated
     • Tracks: 9, 8, ...
     • R/W head: coils of wires
     • Sequential (vs. random) access
     • Backup tapes: 100 GB
   • Disks
     • Structure: platters, cylinders, sectors, tracks.
     • R/W heads: 15 nm; 2 millionths inch (smaller than smoke)
     • Spin @ 10,000 RPM
     • Seek time computation: 10 ms
     • Transfer: 10 MB per sec
     • Speed computation: 200 km/hr?
     • Superparamagnetic effect: 100 GB/in^2?
   • Disk Fragmentation
     • How it happens
     • What is disk defragmentation
   

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3. Optical Storage

   • CR-ROM
     • 2 x 10^9 pits
     • Laser, reflected from aluminum, photodetector
     • 3 mile spiral !
     • 700 MB (~1GB)
     • Spin@ 500rpm(1x)-4000 rpm(8x)
     • Tracking
   • DVD
     • Glue two disks together, gold & silver
     • Laser focusses through one to other
     • 17 GB: 8 hrs.
   • CD-RW Technology
   • silver, indium, antimony, tellurium crystal structure

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4. Flash Memory

   • Invented in 1980, but only widespread in the 1990s.
   • Floating Gate (FG) insulated by oxide from surroundings.
   • Electrons in FG will stay there for many years.
   • Can be sensed from nearby gate without removing electrons.
   • To change, high enough voltage to induce quantum tunneling: electrons jump through barrier
   • Capacity now reaching 4, 8, 16, 32GB

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5. What's Next?

   • Photonic Storage: Peter Rentzepis
     • polymer sugar cube
     • 6.5 Tb ~= 1 TB; 100 Tb ~= 10 TB possible
     • Read out all in 20 ns
     • Storage needed for all knowledge?