Transcript 9/20: Memory

9/24: Memory & Storage
• ROM
• RAM
– different kinds &
uses inc. VRAM,
SRAM
• Data storage
– Magnetic
– Optical
image courtesy of How Computers Work CD
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.
image courtesy of How Computers Work CD
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”
image courtesy of Sonystyle.com
DRAM: Configurations
• Memory modules
• SIMMs - 32-bit, require pairing on Pentium
systems
• DIMMs - 64-bit, do not require pairing on
Pentium systems
image courtesy of gamespot.com
How RAM works
• RAM chips have millions of transistor/capacitor
pairs.
• Capacitors can briefly hold an electric charge.
• Gates are opened and
Address Line
closed by the address
lines.
Transistor
• Data goes in and
out on the data lines
Capacitor
Data Line
image courtesy of How Computers Work CD
Data storage: what is it?
• Long-term storage, not immediate memory
• Not RAM (short-term memory)
• Synonyms: secondary storage, auxiliary storage,
mass storage
images courtesy of dalelabs.com,, iomega.com, imation.com,
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
image courtesy of How Computers Work
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
Image courtesy of How Computers Work
Magnetic data storage
• Use magnets to store bits:
– example: 0 = N , 1 = S
• Reading and writing
• Floppy disk drives
• Hard disk drives
image courtesy of How Computers Work
PACE
Floppy disk drives: types
• Low-density floppy drives:
5.25” floppy disks
3.5” 1.44 MB floppy disks
• High-density floppy drives:
Zip disks, LS-120 disks
Images courtesy of How Computers Work,
Imation.com, and Iomega.com
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.
Images courtesy of How Computers Work
Hard disk drives: overview
• Glass platters coated
with magnetic
particles
• Read/write heads
like floppy drives
but much smaller
• Multiple platters,
multiple heads.
Image courtesy of How Computers Work
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.
Images courtesy of How Computers Work
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
Image courtesy of c-cube.com
Types of Optical Discs
• Compact discs:
– CD-ROM: read only memory (680 MB)
– CD-R: Recordable (650 MB)
– CD-RW: ReWritable (495 MB)
Image courtesy of c-cube.com
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)
Image courtesy of c-cube.com