9/24: Memory & Storage

• ROM

• RAM– different kinds &

uses inc. VRAM, SRAM

• Data storage– Magnetic– Optical

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ROM? RAM?

• Both refer to chip-based memory hardware – (not hard drives, CD-ROMs, etc.)

• ROM: Read-Only Memory– doesn’t change (much if at all)– BIOS stored here.

• RAM: Random Access Memory– changes frequently– short-term memory

ROM: Read-Only Memory

• Chips hardwired with start-up instructions

• Nonvolatile memory– doesn’t change

• Non-dynamic– not dependent on

electrical power to store information

• Necessary to start up the operating system (OS)

• Has a few options: disk drive boot order, etc.

RAM: Random Access Memory

• Synonyms: “main memory”, “primary memory”, “user memory”

• Volatile memory– contents can be changed

• Why “random access”?– sequential access vs. random access– analogy: cassette vs. CD

• Programs, files stored here while in use.

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How much RAM do you need?

• Current recommendation: 192 MB total RAM for a new computer

• Windows 98 uses ~ 20 MB of RAM

• Windows 2000 Pro uses ~ 95 MB of RAM

• McAfee Virus Scan & Task Manager ~ 7 MB

• MS Word uses ~ 7 MB

• MS Internet Explorer uses ~ 7 MB

• Each file you open is held in RAM, even after you close it

Varieties of RAM: DRAM

• DRAM: Dynamic RAM– dynamic: needs electricity to hold data. Must be

continually replenished with electricity.– FPM: Fast Page Mode– EDO: Extended Data Out

• can send data while receiving other data

– BEDO: Burst EDO• sends “pipelined” data, speeding up EDO

– SDRAM: Synchronous DRAM• runs at the CPU clock speed (faster than prev. types)

• today’s main DRAM.

Variety of RAM: SRAM

• SRAM: Static RAM– static: does not need to be continually refreshed.– more expensive, bigger, and faster than DRAM.– used in CPU chips (caches)– used in “flash memory”

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DRAM: Configurations

• Memory modules

• SIMMs - 32-bit, require pairing on Pentium systems

• DIMMs - 64-bit, do not require pairing on Pentium systems

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How RAM works

• RAM chips have millions of transistor/capacitor pairs.

• Capacitors can briefly hold an electric charge.

• Gates are opened and closed by the address lines.

• Data goes in andout on the data lines

Address Line

Data Line

Capacitor

Transistor

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Data storage: what is it?

• Long-term storage, not immediate memory

• Not RAM (short-term memory)

• Synonyms: secondary storage, auxiliary storage, mass storage

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Data storage: where is it?

• Storage involves 2 parts: the device & the media

• Removable vs. fixed media

• Where can you store data?– magnetic media– optical media– magneto-optical media– solid state media

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Storage characteristics• Random vs. sequential access• Tracks & sectors• Speed

– spin rate (RPM)– seek time & latency– access time– data transfer rate

• Storage Capacity– areal density– size

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Magnetic data storage

• Use magnets to store bits:– example: 0 = N , 1 = S

• Reading and writing

• Floppy disk drives

• Hard disk drives

PACE

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Floppy disk drives: types

• Low-density floppy drives: 5.25” floppy disks3.5” 1.44 MB floppy disks

• High-density floppy drives: Zip disks, LS-120 disks

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Floppy disk drives: how they work• Shutter opens with an arm.• Cookie spins on the spindle.• Read/write heads get close

to the cookie inside• Magnetic particles generate

electric currents in the heads.

• Electric currents are interpreted as a series of bits.

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Hard disk drives: overview

• Glass platters coated with magnetic particles

• Read/write heads like floppy drives but much smaller

• Multiple platters, multiple heads.

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Hard disk drives: how they work

• Platters spin on the spindle.

• Heads move over the platters.

• To read, magnetic particles generate currents in the heads.

• Electric currents are interpreted as a series of bits.

• To write, the electric currents manipulate the magnetic particles on the platters.

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Optical Storage• Instead of using magnetism to store bits, we use light• A laser is the light

– Laser: “light amplification by stimulated emission of radiation”

– Extremely focused, narrow beam of powerful light– Depending on the wavelength, the beam can be narrower or

wider

• Lands are flat areas on the disc– Reflect light like a mirror

• Pits are holes in the disc– Scatter light

Courtesy of http://www.microserve.net/~tpetchy/DVD.shtml

CD Drives: Types and Operation

• CD-ROM drive speeds: how many times faster than the original CD-ROM drives• 2x, 4x, 6x, 8x, 16x, 32x, 48x

• Operation:• Diode generates laser• Laser bounces off mirrors, reaching the disc• Disc reflects or scatters laser• Light-sensing diode sees the reflections, comparing

them to a timing circuit to generate 0s and 1s

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Types of Optical Discs

• Compact discs:– CD-ROM: read only memory (680 MB)– CD-R: Recordable (650 MB)– CD-RW: ReWritable (495 MB)

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Types of Optical Discs• Digital versatile discs – higher density storage

– DVD-ROM: read only memory (4.7 – 17 GB)– DVD-R: Recordable (3.9 GB?)– DVD-ram (5.2 GB) & DVD+RW (6.0 GB): Rewritable

(still fighting)

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