Anatomy of a Computer
RAM, ROM, CPU, etc.
A computer is
a person or thing that computes
to compute is to determine by arithmetic
means (The Randomhouse Dictionary)
so computing involves numbers
While typing papers, drawing pictures and
surfing the Net don’t seem to involve
numbers at first, numbers are lurking
beneath the surface
Representing numbers
Some attribute of the computer is
used to “represent” numbers (for
example: a child’s fingers)
two kinds of representation are:
analog the numbers represented take
on a continuous set of values
digital the numbers represented take
on a discrete set of values
Pros and Cons
the analog representation is fuller/richer
after all there are an infinite number of
values available
the digital representation is safer from
corruption by “noise;” there is a big
difference between the various discrete
values, and smaller, more subtle
differences do not affect the
representation
Digital signals
0 1 1 0 1 0 0 0
Our computers are
digital and electronic
(note that digital  electronic)
they are electronic because they use an
electronic means (e.g. voltage or current)
to represent numbers
Gives computers their speed and small size
they are digital because the numbers
represented are discrete
Noise resistant
Binary representation
the easiest distinction to make is between
low and high voltage
off and on
then we can only represent two
digits: 0 and 1
but we can represent any (whole) number
using 0’s and 1’s
Decimal vs. Binary
Decimal (base 10)
124 = 100 + 20 + 4
124 = 1  102 + 2  101 + 4  100
Binary (base 2)
1111100 = 64 + 32 + 16 + 8 + 4 + 0 + 0
1111100 = 1  26 + 1  25 + 1  24 + 1  23
+ 1  22 + 0  21 + 0  20
Bits and Bytes
A bit is a single binary digit (0 or 1).
The elementary unit of information
A byte is a group of eight bits.
A byte can be in 256 (28) distinct states
(which we might choose to represent the
numbers 0 through 255).
Note computer scientists like to start
counting with zero.
Realizing a bit
We need two “states,” e.g.
high or low voltage (e.g. computer chips)
why you should protect computer from power surges
north or south pole of a magnet (e.g. floppy
disks)
why you should keep floppies away from large magnets
light or dark (e.g. reading CD or DVD, also
laser printers)
hole or no hole (e.g. punch card or CD)
More, more, more
A kilobyte is 1,024 (210) bytes
approx. one thousand
A megabyte is 1,048,576 (220) bytes
approx. one million
A gigabyte is 1,073,741,824 (230) bytes
approx. one billion
A terabyte is 1,099,511,627,776 (240)
bytes
approx. one trillion
Storing it away
A standard 3.5 inch floppy disk holds 1.44
MB (megabytes)
An Iomega Zip disk holds approx. 100 MB
or 250 MB
A CD (compact disk) holds approx. 650 MB
A DVD (digital versatile [video?] disc)
holds several GB (gigabytes)
A typical hard drive holds several GB
Less portable, but faste
Transistors
When bits are represented using
voltage, the logical operators (gates)
can be constructed from transistors
The Pentium ® II has approximately
7.5 million transistors on it
The transistors have lengths
approximately 0.35 microns
(millionths of a meter)
A chip off the old block
Millions of transistors are connected into what is
called an integrated circuit or chip
Chips are made from silicon (a semiconductor, a
material halfway between a conductor and an
insulator)
“Silicon valley” is a nickname for the region
south of San Francisco that contains an
unusually high concentration of computer
companies.
CPU
The most important chip in a computer is the
microprocessor
The microprocessor houses the Central
Processing Unit (CPU), the “brain” of the
computer
Ex. Pentium IV is a microprocessor
Extending the brain analogy
Similar to dividing the brain into
Brain stem
Forebrain
We divide the CPU into
Control Unit
Arithmetic Logic Unit (ALU)
How good is your computer?
Computing means moving bits around, so an
important question is how many bits can be
handled at one time
Word size: how many bits are handled at a time
(8, 16, 32, 64) by memory or the processor
Bus size: how many bits can move around at
once
analogy: two-lane, four-lane or eight-lane highway
Bus speed: how fast they move
How fast is it?
Each of the computer’s manipulations
(instructions) begins with a “tick” of the clock
So the faster the clock ticks, the faster the
computer
Clock speed: a measure of how fast the
computer is, given in MHz (megahertz - millions
of cycles per second)
There are gigahertz machines now
Middle number written on LaSalle’s computers
Speed II
Sometimes one instruction can be started
before the previous one was complete
Like having a batter on deck
So another measure of speed is useful
instructions per second, given in MIPS
(millions of instructions per second)
(considered by some to be a misleading
measure of speed)
The mother of all circuit
boards
chips and other things are connected together
on what is called a circuit board
the mother board, a.k.a. the system board,
holds the main components of the computer
CPU
clock
connectors
expansion slots, ETC
If you can’t stand the heat
The chips, especially the microprocessor
can get hot
heat sink: the strangely shaped metal or
ceramic structure sitting on the processor
that serves to draw away the heat
there’s also a little fan near the processor;
that’s often what you hear whirring
away on old computers
A link to the outside world
The process of putting information into or
getting information out of a computer is
called interfacing or input/output (IO)
ports are sockets, typically in the back of
a computer, where one plugs in the cable
connecting the computer to the IO
devices
Newer computers have a port in
the front for convenience
Two types
serial
data sent one bit at a time
for modems and some printers
cable can be very long
ex. MIDI, USB
parallel
data sent eight bits at a time
limit on length of cable
ex. SCSI
SCSI port
Small computer system interface
pronounced “scuzzy”
allows more than one device to be
connected to a single port
daisy chain: getting the output for a
second output device from the first
(rather than directly from the computer),
the output for a third can come from the
second and so on
A connector in every port
Ports have connectors, as do cables
connectors come in two varieties
male: have pins sticking out
female: have holes to receive pins
Analog to Digital
Any measurement that can be converted
to an electronic signal (voltage or current)
can be directly fed into a computer
the original data is often continuous
(analog) and must be converted into
digital form
This signal can be fed in through a port so
long as the appropriate software is
installed
In the cards
Expansion Slot: A socket designed to hold
the circuit board for the device, such as a
sound or video card, that adds capability
to the computer system
Adapter cards: additional circuitry and
chips that extend your PC’s capabilities
allowing you to customize it
Some types of cards
video or graphics card: enhances computer’s
ability to convert output into video and send it
to the monitor
Sound card: improves your computer’s sound
capabilities, be it input (microphone) or output
(speakers)
internal modem: allows computer to connect
to networks via phone lines and such
Plug and play
refers to computer’s
capability to figure out what
to do when new expansion
cards and devices are added
this way the user does not
have to know how to
“configure” the system
Memories
Saving information we have entered (e.g. onto
floppies) is referred to as “storage;” it is long
term and slow by computer standards (storage
 memory)
Before we save the data, it is in the computer’s
memory, i.e. in memory chips, which hold the
information temporarily
Memory also holds the instructions a computer
needs to operate (“stored program concept”)
Reading and Writing
The basic actions involving memory are
WRITING: putting information into memory
READING: getting information from memory
The rest of the time memory just holds
onto information
ROM
Read Only Memory
This memory is loaded up by the
manufacturer (some is programmable)
contains low-level instructions for the
computer
Not lost when the computer is turned off
“nonvolatile” memory
RAM
Random Access Memory
The memory the user uses
The programs one loads and the data one
enters are here
Lost when the computer is turned off
“volatile” memory
Why is it called random?
Random Vs. Sequential
A cassette tape is sequential access; you
have to go through song one and two to
get to song three
A CD is random access; you can jump
directly to song three
Some Types of RAM
Dynamic RAM (DRAM): dynamic means
changing, which for memory is not
necessarily a good thing, so dynamic
memory must be continually refreshed
Synchronous DRAM: blocks of data (instead
of lines) read more efficiently
Static RAM (SRAM): doesn’t need
constant refreshing, is faster but more
expensive than dynamic
Cache
pronounced “cash”
The computer puts information it
anticipates (guesses) you will use soon in
a place which is accessed more quickly
A correct guess is called “a cache hit”, an
incorrect guess “a cache miss”
Caching improves speed
Some kinds of cache
Memory cache: put information from
DRAM into SRAM (high-speed memory)
L1 on the chip with the microprocessor
L2 on a different chip
Disk cache: put information from storage
(floppy or hard drive) into memory
Browser cache: put information from
internet into hard drive