• A computer system has three major
 Memories (primary and secondary)
 I/O (Input/Output) equipment
 Physically, most PCs have a structure similar to
that shown on the next slide.
 The PC contains a metal box with a large
printed circuit board (the motherboard) at the
bottom. The motherboard contains:
CPU chip
Slots for DIMM modules
Various support chips
A bus etched along its length (maybe two)
Sockets into which the edge connectors of I/O
boards can be inserted.
Physical Structure of a PC
 Each I/O device consists of two parts:
• the controller, which contains most of the
• the I/O device itself
 The controller is usually on a board plugged
into a free slot, except for those controllers
which are not optional which are sometimes
located on the motherboard.
• The monitor is not optional, but the video controller
is sometimes located on a plug-in board to allow the
user to choose the type of display.
Logical Structure of a PC
 The job of a controller is to control its I/O
device and handle bus access for it.
 A controller that reads or writes data to or from
memory without CPU intervention is said to be
performing Direct Memory Access (DMA).
 When the transfer is finished, the controller
issues an interrupt, forcing the CPU to switch
from the running program to an interrupt
handler. After this, the OS can resume the
suspended job.
 The bus is used by both I/O controllers and the
CPU. When both want to use the bus, a bus
arbiter decides who will go next. In general,
I/O devices are given precedence since disks
and other moving devices cannot be stopped
without losing data.
 When no I/O is in progress, the CPU has all the
bus cycles for itself to reference memory. When
some I/O device is also running, it will request
and be granted the bus. This is called cycle
stealing and it slows down the computer.
 As CPUs, memories and I/O devices became
faster, the bus could no longer handle the load.
 Replacing the bus every time it became
overloaded with a higher capacity one would
cause the old peripherals to become useless.
• Therefore many manufacturers stayed with the old
ISA (Industry Standard Architecture) bus.
Eventually companies started developing computers
with multiple buses including the ISA, the
backward-compatible EISA (Extended ISA), and
now PCI (Peripheral Component Interface).
PC with PCI and ISA Buses
 On PCs, when a key is depressed, an interrupt
is generated and the keyboard interrupt handler
is started.
 The interrupt handler reads a hardware register
inside the keyboard controller to get the number
of the key depressed.
 When a key is released, a second interrupt is
generated. This allow for multikey sequences
(including SHIFT, CTRL, and ALT), which are
handled entirely in software.
CRT Monitors
 A monitor is a box containing a CRT (Cathode
Ray Tube) and its power supplies. The CRT
contains a gun that can shoot an electron beam
against a phosphorescent screen near the front
of the tube.
• Color monitors have three electron guns for read,
blue and green.
• The CRT is a raster scan device since it produces
an image line by line.
• A full-screen image is repainted 30 to 60 times a
CRT Monitors
Flat Panel Displays
 CRTs are too bulky and heavy to be used in
notebook computers, so a technology called
LCD (Liquid Crystal Display) is used.
 Using an applied electric field, the molecular
alignment of the liquid crystals, and hence the
optical properties, can be changed. The crystal
is sealed between two glass plates.
• Passive matrix displays use a two-dimensional grid
of wires to generate the voltage required.
• Active matrix displays contain a tiny switching
element at each pixel position.
LCD Screens
Character-Map Terminals
 Three kinds of terminals are in common use:
• Character-map terminals
• Bit-map terminals
• RS-232-C terminals
 A character-map terminal is shown on the
following slide.
 To display characters, the CPU copies them to
the video memory in alternate bytes. Associated
with each character is an attribute byte that
describes how that character is to be displayed.
Character-Map Terminals
• Attributes include color, intensity, blinking, etc.
 A screen of 25 by 80 characters requires 4000
bytes of video memory, 2000 for the characters
and 2000 for the attributes.
 The video board fetches characters from the
video RAM, a line at a time, and generates the
necessary signal to drive the monitor.
 Because the board outputs a video signal, the
monitor must be within a few meters of the
computer to prevent distortion.
PC Video Output
Bit-map Terminals
 A variation on this idea is to have the screen
regarded not as a 25 by 80 array of characters,
but as an array of picture elements called,
 The general idea is the same, except that the
video RAM is seen as a big bit array.
 To draw characters, for example, the software
might allocate a 9 by 14 rectangle for each
character and fill in the necessary bits to make
the character appear.
Bit-map Terminals
 Bit-map terminals are typically used to support
displays containing several windows.
 Bit-map terminals require a considerable
amount of video RAM. The most common sizes
are 640x480 (VGA), 800x600 (SVGA),
1024x768 (XVGA), and 1280x960. Each of
these has an aspect ratio of 4:3.
 To get true color, 8 bits are needed for each of
the three primary colors, or 3 bytes/pixel. Thus,
1024x768 requires 2.3 MB of video RAM.
Bit-map Terminals
 To lessen this requirement, some computers use
an 8-bit number to indicate the desired color.
This number is then used as an index into a
hardware table called the color palette that
contains 256 entries, each holding a 24-bit RGB
value. This is called indexed color. It reduces
the required RAM by 2/3, but allows only 256
 Usually each window on the screen has its own
mapping. The palette is changed when a new
window gains focus.
Bit-map Terminals
 To display full-screen full-color multimedia on
a 1024x768 display requires copying 2.3 MB of
data to the video RAM for every frame. For
full-motion video, 25 frame/sec is needed for a
total data rate of 57.6 MB/sec.
• This is too much for an (E)ISA bus, so highperformance video cards need to be PCI cards.
RS-232-C Terminals
 To allow terminals to be used with many
computer systems, a standard computerterminal interface called RS-232-C has been
 When the computer and terminal are far apart,
the only practical way to connect them is over
the telephone system. The telephone system,
however, is not capable of transmitting the
signals required by RS-232-C, so a device
known as a modem (modulator-demodulator)
has to be used.
RS-232-C Terminals
 To communicate, the terminal and computer
each contain a chip called a UART (Universal
Asynchronous Receiver Transmitter), as well
logic to access the bus.
 To display a character, the computer takes a
word from memory and presents it to the UART
which shifts it onto the RS-232-C cable 1 bit at
a time. It also adds a start bit and a stop bit to
each character to delimit the character.
RS-232-C Terminal
• A mouse is used as a pointing device. The mouse
can have one, two, or three buttons.
• Three kinds of mice have been produced:
 mechanical mice
 optical mice
 electromechanical mice
• The optical mouse is used on top of a special plastic
pad containing a rectangular grid of lines.
 A common arrangement is to have the mouse send a
sequence of three bytes every time the mouse moves a
certain minimum distance (e.g. 0.01 inch). Byte 1 gives
movement in the x direction, 2 in the y direction and 3
gives the state of the mouse buttons.
 The cheapest kind of printer is the matrix
printer, in which a print head of between 7 and
24 lines is scanned across each page.
• Print quality can be increased by two techniques:
 use more needles
 have the dots overlap
 Inkjet printers have a movable print head
which holds an ink cartridge and is swept
horizontally across the paper
• An ink droplet inside a nozzle is heated to the
boiling point, explodes out the front of the nozzle.
Matrix Printers
Laser Printers
 The heart of a laser printer is a rotating
precision drum.
• The drum is charged to 1000 volts and coated with a
photosensitive material.
• Light from a laser is scanned along the length of the
drum. The light beam is modulated to produce a
pattern of light and dark spots. The spots where the
beam hits lose their electrical charge.
• The drum rotates until the first line of dots reaches
the toner, a reservoir of electrostatically sensitive
black powder. The toner is attracted to the charged
dots, forming an image of that line.
Laser Printers
Laser Printers
Color Printers
 Nearly all color printing systems use four inks:
cyan, magenta, yellow, and black. These
systems are called CMYK printers.
• In theory, CMY should be enough, but due to
impurities in the ink, it is difficult to produce a pure
• The lowest cost color printers are color ink jet
printers which work with four ink cartridges.
• Solid ink jet printers use a waxy ink which is
melted into hot ink reservoirs. The ink is sprayed
onto the paper where is solidifies and is fused with
the paper by forcing it between two hard rollers.
Color Printers
• The color laser printer has four different toners (for
C, M, Y, and K) on which separate images are laid
• The wax printer has a wide ribbon of of four-color
wax that is segmented into page-size bands.
Thousands of heating elements melt the wax as the
paper moves under it. The wax is fused to the paper.
• In dye sublimation printers, a carrier containing
the CMYK dyes passes over the thermal print head
containing thousands of programmable heating
elements. The dyes are vaporized instantly and are
absorbed by a special paper close by.
 A raw telephone line is not suitable for
transmitting computer signals since two-level
signals suffer distortion leading to errors.
 A pure sine wave called a carrier can be
transmitted with little distortion, however.
 By varying the amplitude, frequency, or or
phase of the sine wave, a sequence of 0s or 1s
can be transmitted. This process is called
• In amplitude modulation, two different voltage
levels are used for 0 and 1.
• In frequency modulation, the voltage level is
constant, but the carrier frequency is different for 1
and 0.
• In phase modulation, the phase of the carrier is
reversed 180 degrees when the data switch from 0 to
1, or 1 to 0. More sophisticated phase-modulated
schemes are also possible.
• Modems accept 8-bit characters and serialize them
for transmission on the phone lines in amplitude-,
frequency-, or phase-modulated form.
 ISDN (Integrated Services Digital Network)
is a standard for digital telephony.
 A digital ISDN line holds two independent
digital channels at 64,000 bits/sec each plus a
signaling channel at 16,000 bits/sec.
 ISDN allows quicker connection, requires no
modem, and is more reliable.
 Specialized equipment must be installed to use
Character Codes
 Each computer has a set of characters that it
uses. In order to store the characters in the
computer, each one is assigned a number. The
mapping of characters onto integers is called a
character code.
• One widely used code is ASCII (American
Standard Code for International Interchange).
Each ASCII character has 7 bits, allowing for 128
• Non-English languages work less well with ASCII.
For this reason, a new system called UNICODE is
being introduced. UNICODE uses 16 bits/character.

Computer Systems Organization