UNIT 1 – THE SYSTEM UNIT
How Computer Processors Communicate
Before discussing what a computer or an operating system is, let’s look at the type of numeration systems used to make
notations in setting up operating systems, since it is impossible to avoid in this discussion.
The basic memory cell is composed of a transistor that acts as a switch and a capacitor that holds an electrical charge.
Thus, a cell can have only two possible states, that of being charged or discharged. Digital computers store information as
electrical charges, all information must be broken down into “on” and “off” states. “0” represents a discharged memory
cell (low voltage) and may be referred to in the following ways:
0
LO
OFF
FALSE
NO
LOW VOLTAGE
1
HI
ON
TRUE
YES
HIGH VOLTAGE
Since the real world requires the expression of many values and ideas expressed in the alpha-numerics of human
language, a method must be found to express these quantities using only two states.






How do we get two numerals, “0” and “1” to express all necessary values?
How do we create graphical images?
How do we make comparisons like equalities, inequalities, greater and lesser states?
How do we express characters of the alphabet, punctuation and special characters?
How do we locate information in memory?
How do we issue instructions to the computer?
Using Binary Notation to Express Values other than “0” or “1”
So, how do we express base 10 numbers and characters as “on” and “off”? First, we have assigned a standard number
to each character. This universal system is called the ASCII Code system. Then we use place value. In our numeration
system, a digit from “1” through “9” can represent different values according to position in the numeric string. Example: 9
= 9 units. 90 = 9 x 10 units. 900 = 9 x 100 units. This is because of place value. Place value is computed using the
base, in this case 10 since we use 10 different symbols (0,1,2,3,4,5,6,7,8,9). Place value takes this base and raises it to
consecutive powers from right to left.
Example: 10 to the zero power (10/10) = 1.
10 to the first power (10) =10.
10 to the second power (10 x 10) = 100.
10 to the third power (10 x 10 x 10) = 1000, etc. Now let’s apply the same rules to base 2 (binary).
We use two since we must express two states: “on” or “off” expressed as “1” or “0”. Thus, we have two different symbols.
That’s why we call it base 2. Let’s do an 8-bit example using the following table. Using the base 10 ASCII values for the
characters: “A”, “B”, and “C” which are 65, 66, 67 respectively.
Powers of 2 >
Base 10 Value >
65
66
67
27
128
0
0
0
26
64
1
1
1
25
32
0
0
0
24
16
0
0
0
23
8
0
0
0
22
4
0
0
0
21
2
0
1
1
Programming in Low-Level Programming Language
Binary code can be used:
 To indicate a register or location in memory to store something.
 To express a numeric value.
 To express an alpha-numeric character code.
 To express a control character like a return or page break.
 To indicate the presence of a dot in a graphic; to express its intensity and color.
 To issue an instruction or op-code to a particular chipset.
20
1
1
0
1
Sorting Things Out
If numeric values, ASCII character codes, Instructional Op-Codes and addresses are all binary codes, how does the
processor and chipset tell which binary sequence in an instruction is a value, instruction, character code or address? The
chipset is hard-wired to look for a specific sequence. This comprises the “grammar” and “Syntax” of the low-level chipset
language. Just as each processor and chipset has a unique instruction set, they also have a specific sequence. In order
for a chipset to be compatible, it must be designed to accept both the instruction set and its sequence. This is why
Windows, designed for the Intel and compatible chips, cannot run on the Mac’s Motorola chip. The following are some
examples of how things are done.
Using Binary Bit-Mapping to Draw Graphics
Using binary values, we can map any graphical character or alphanumeric font as seen below. We are using an 8 bit (1
byte) word length to keep things simple. A 32 bit word would yield a smoother graphic.
Address
00000000
00000001
00000010
00000011
00000100
00000101
00000110
00000111
Base 10
120
36
36
56
32
32
32
112
Binary
01111000
00100100
00100100
00111000
00100000
00100000
00100000
01110000
27
26
25
24
23
22
21
20
Using Hexadecimal Notation to Simplify Notation
Even though computers always use binary internally and seldom get confused, humans are highly prone to errors when
long binary numbers are used. Imagine what a 16 or 32 bit notation would be to enter directly. Computer scientists often
translate these into hexadecimal numbers (base 16), often referred to as “hex”. Using the same logarithmic mathematics
which we have applies to both base 10 and base 2, let’s take a look at some hex expressions. Since base 16 needs 16
different single-digit symbols and our numeration systems has only 10 (0 to 9) we must invent additional symbols.
In base 16 we count (0 to 15) as: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F.
Powers of 16>
Base 10 Value>
65
66
67
255
4000
65535
163
4096
0
0
0
0
0
F
162
256
0
0
0
0
F
F
161
16
4
4
4
F
A
F
160
1
1
2
3
F
0
F
Note: In order to avoid confusion with base 10 numbers, hex notations generally are written as follows:
$ 0041 or & 0041 or
x 0041 or 0041 h
Expressing Equality, Inequality, Values Greater and Values Lesser
Computers make decisions based upon evaluations. Things are either the same (equal) or different (unequal). If they are
unequal, one thing is either greater or lesser.
When comparing two values where X=5 and Y=6 mathematically, the following takes place.
Expression
X=Y
X <> Y
X>Y
X<Y
Condition
False
True
False
True
Yield
0
1
0
1
Expressing Characters as Binary Values
By standard, each character of the alphabet, punctuation mark, numeral and control character (unprintable characters
used to control cursor position, lineation, etc.) is given a sequential 8 bit binary value called an ASCII Code. The following
table gives a few examples.
Character
“
0
1
2
=
@
A
B
C
a
b
c
Function
Null
Line Feed
Form Feed
Return
Punctuation
Numeral
Numeral
Numeral
Operator
Character
Character
Character
Character
Character
Character
Character
Binary Code
00000000
00001010
00001100
00001101
00010010
00110000
00110001
00110010
00110011
01000000
01000001
01000010
01000011
01100001
01100010
01100011
Hex Code
$00
$0A
$0C
$0D
$22
$30
$31
$32
$3D
$40
$41
$42
$43
$61
$62
$63
Decimal Code
0
10
12
13
34
48
49
50
61
64
65
66
67
97
98
99
Instructing the Computer Using Binary Operational Codes (OPCODES)
Just as each character is assigned a code, each chipset (Processor and motherboard chips) have unique binary codes for
every action that can take place. These Low-level OPCODES instruct the processor to carry on computations and data
processing. Each brand of processor has its own low-level instruction set.
Locating Information Using Binary Addresses
Random Access Memory (RAM) is a matrix of millions of transistor-capacitor units forming each bit cell of memory
depending on the architecture of the memory and the required number of bits per word of data. For discussion, lets
suppose we use 8 bits or one byte, since this is the minimum required length of ASCII and Op-Codes. Each byte would
have a unique sequential binary address. The use a familiar metaphor, if the byte were your locker, the address would be
the locker number. Your locker number allows you to locate your locker and its contents among all the lockers in the
school. Likewise data and instructions are stored using blocks of addresses. Take another look at the bit-map table on
the first page of this unit for an example.
High-Level Languages
Yes, programmers who design components need to code in binary sometimes. In reality, most use a language called
“Assembly”. This language uses an abbreviated symbolic language to program. It is then translated into straight binary
by another program. The programmer must still decide on actions, addresses and locations to be used and expresses
quantities in hexadecimal. Careful attention must be taken to sequence of actions and the locations used to avoid
destroying critical system instructions.
Most programming is done in high-level coding using a language like C, C++ or C#, where English-like sentences and
instructions are used to produce “source code” for each part of the program. A “compiler”, translator program “reads” this
source code and changes it into binary code and a “linker” program patches the code of the different parts of the program
into a continuous binary instruction set called an executable (.EXE) file. In programming for the Windows Operating
System, this program must produce Dynamic Link Libraries (.DLL) files. Windows programming is a different technique,
since most of the programming requires “calls” to already-existing instructions residing in the system itself.
THE SYSTEM UNIT HARDWARE
The Power Supply
The system unit starts with a source of power. Power supplies convert line power 120 VAC 60 Hz. as used in the U.S.,
into the 3.5 VDC, 5.0 VDC and 12 VDC needed by the system components. This is accomplished through the use of
step-down transformers, rectifiers, capacitors and voltage regulator circuits. In most cases, the power supply exhaust fan
also removes heat from the system case. The ATX power supply communicates with the motherboard and disk drives to
control temperature and power consumption. Software can report the state of the processor and speed of cooling fans.
The Motherboard
The main system board or “motherboard” supplies mechanical mounting and support circuitry for all of the system
components: The clock chip, processor, ROM BIOS chip, CMOS chip, RAM, cache, bus connectors and some ports. The
motherboard is designed around its clock, processor and BIOS as a total unit.
System Clock
The system clock chip is a precise oscillator circuit that provides a steady reference signal by which instructions are
executed by the processor and by synchronous data moving from one component to another. It also provides a reference
for the calendar clock.
Read Only Memory (ROM)
The ROM chip has permanently burned low-level instructions and data used by the system during “boot” or initial startup
to identify its motherboard components until the operating system is loaded. These cannot be changed without changing
the chip itself. This chip holds the Basic Input-Output System or BIOS instructions.
CMOS Chip
This chip is a low-power consumption memory chip. The chip contains the system configuration as entered by the user
during the BIOS setup. Information as date and time, drives, memory configuration, port configuration and security is
stored here. A battery provides this chip with power while the computer is turned off. This battery must be replaced
occasionally. CMOS Chips are being replaced with flash memory, requiring no battery power.
The Central Processor (CPU)
The Central Processor (CPU) and Arithmetic Logic Unit (ALU) or Coprocessor Usually now built into one package, these
comprise the “brain” of the computer. The processor chip, containing the transistor logic gates and pathways necessary
for processing as well as an internal bus, cache and registers for temporary storage of data during computation and
processing. Since the chip is large and runs hot, it usually can be seen with heat sink fins and a small fan.
Random Access Memory (RAM)
These chips, located on little circuit strips, plug into sockets on the motherboard. They are comprised of millions of tiny
transistors and capacitors that hold binary information. This information may be data or instructional code. Since this
memory is volatile (held for a fraction of a moment) it must be refreshed constantly and loses it when power is lost.
Cache
Cache is comprised of very high-speed memory chips on the motherboard. These chips are faster than RAM and more
costly. However, they provide speed during processing by holding information that is needed over and over saving the
processor from fetching it from RAM. Cache memory located on the motherboard is called external cache, while cache
memory located within the processor chip is called internal cache.
Bus
The internal bus, contained within the processor chip, connects all its internal circuits together and terminates at the chip’s
socket prongs. The external bus is located on the motherboard. A bus is a set of parallel connectors all attached to the
same set of conductors. It serves as sort of an “electrical highway” over which signal travel. The main external bus has
evolved from the 8 bit PC but to the 16 bit Industry Standard Architecture (ISA). A set of edge connectors that receive
adapter circuit boards. Currently, the 32-bit Peripheral Component Interconnect (PCI) bus-mastered bus is the main
support of most adapters. The bus connectors contain a standard pin-out to which all adapter boards must be made,
whether ISA, PCI. These connections carry the 8 bit or 16 bit (ISA) or 32 bit (PCI) binary signals, a signal ground, a
protective ground, handshaking lines, lines for required voltages to power the adapter, the strobe (clock) signal for
synchronous data transmission and connections for Plug and Play (PnP) detection. Because transmission speeds are
slower at the bus than the processor, the bus presents the greatest opportunity for data to “bottleneck” or slow up. The
Small Computer System Interface (SCSI) also functions as an auxiliary bus, as are the Universal Serial Bus (USB), HighSpeed USB2 and Firewire (IEEE-1394).
Ports
A port is a point of connection between the computer and an external device. Ports are both physical and logical.
Physical ports are the hardware actual connectors where devices are attached. Logical ports are entities by which the
operating system addresses the devices. Each physical port has a corresponding logical port name (e.g. C0, COM1:) to
which are attached the memory addresses and interrupts flag by which the operating system communicates. Logical
ports can be used that have no corresponding physical ports. Ports may be internal or in the case or external on the
outside of the case. Ports must be assigned their own base address, interrupt (IRQ) and sometimes a direct memory
address (DMA). Ports may be internal (on the motherboard) or external (on the adapter board tab in back of the system
unit).
Internal ports
Internal ports may be located on the motherboard itself or on an adapter board. They are usually header connectors that
receive flat cables connecting to devices attached to the computer’s case. The 8 bit Floppy Disk (FDD) port; the Primary
and Secondary EIDE and internal SCSI are examples. Other ports may also be available on the motherboard.
SHORT
NAME
FDD
LONG NAME
USE
LOCATION
Floppy Disk Drive
PCI
Peripheral
Component
Interconnect
Accelerated Graphics Port
Floppy Disk
Tape Drive
Adapter
Boards
Graphics
Board
Hard Drives
CD-ROM
Drives
Zip Drive
Super Disk
Motherboard
Internal
Motherboard
Internal
Motherboard
Internal
Motherboard
Internal
Hard Drives
RAID Hard
Drives
Motherboard
Internal
AGP
EIDE
Enhanced
Electronics
ATA
Advanced Technology Attachment
SATA
Serial
Advanced Technology Attachment
SCSI
Integrated
Drive
Bus Port supporting up to 8 Various
devices in a chain using a single Devices
IRQ.
SCSI
Max
Bus
Max Cable
Interfaces
Transfer
Width
Length((met
Rate
(bits)
ers)
(MBps/s)
SCSI-1
5
8
6
Fast SCSI
10
8
3
Fast/Wide SCSI
20
16
3
Ultra SCSI
20
8
1.5
Wide Ultra SCSI
40
16
1.5
Ultra2 SCSI
40
8
N/A
Wide Ultra2 SCSI
80
16
N/A
Ultra 160 SCSI
160
16
N/A
Ultra320 SCSI
320
16
N/A
PORT
CONNECTOR
Header (Male)
Edge (Female)
Edge
(Female)
Header
(Male)
40 Pin Cable with
Color-coded
headers
Four pin SATA
cables.
Motherboard
Header (Male)
Internal/Externa
l
Differenti
Low
Max # of
al
Voltage
Devices
2 wires/
Dif. LVD
bit
25
N/A
8
25
N/A
8
25
N/A
16
N/A
N/A
8
25
N/A
8/(16diff)
25
12
8
25
12
16
N/A
12
16
N/A
12
16
External Ports
External ports are available from the outside of the computer’s case. They are either mounted the case or to metal tabs in
the accessory slots and fed by flat cables plugged into the motherboard. Other external ports are attached to the adapter
boards in the computer’s bus connectors. External ports can be recognized by their unique connectors. We will discuss
several of these ports and their functions.
EXTEXERNAL PORT
Keyboard Port
PS/2 Mouse Port
Video Display Signal Port
Serial Port (RS-232-C)
Parallel Printer Port
Small System Computer Interface Port
Universal Serial Bus Port
High-Speed Universal Serial Bus Port
Firewire
Infra Red Link Port
Game /MIDI Port
Internal Modem Modular Phone Port (4 conductors)
Personal Computer Memory Card International
Association
Ethernet Network Twisted Pair Port (8 conductors)
TERM USED
KYBD
Bus Port
VGA, SVGA, XGA
COM1:,COM2:,COM3:,COM4:
LPT1:LPT2:, PRN
SCSI
USB
USB 2.0
IEEE-1394
IrDA
MIDI
Modular Jack
CONNECTOR
DIN 5F
DIN 5F
HDB15F
DB9M or DB25M
DBM25F
Centronics 36F
Series A USB
Series A USB
Special
Wireless
DB15F
RJ-11F
PCMCIA
Modular Jack
Header
RJ-45F
Universal Serial Bus (USB) and Firewire
These are bus ports. They are growing in popularity due to their ease of use, availability and hot-swap capability. These
ports also act as external buses, supporting multiple devices, when used in conjunction with a hub. These standards
allow equipment to have compatibility among different platforms and types of computers. These ports support a large
variety of devices, and even permit networking, without additional equipment. USB2 allows 100 mbts./sec. Transmissions
and will replace USB.
IrDA ,900 mhz and Bluetooth Connections
These standards for wireless connections of components employ small transceivers for very short distance (within a few
feet) communication between devices.
IrDA employs modulated invisible light in the infra-red spectrum (like a TV remote) and works “line of sight”. While it will
bounce off surfaces and pass through some materials like paper and fabric, it will not penetrate structural material so is
limited to the same room. 900 Mhz and Bluetooth use modulated radio frequency and will work through obstructions for
short distances. However, these frequencies do not bounce off surfaces efficiently.