The Central Processing
Unit
Chapter 3
What Goes on Inside the
Computer
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1
Functions of Computer
Systems
Recall that there are four main
functions of computer systems:
Secondary
Storage
Input
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Processing
Output
2
Processing
In this chapter, we will focus on the
central processing unit (CPU) in
more detail.
Secondary
Storage
Input
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Processing
Output
3
The CPU
The CPU interacts
closely with
memory (primary
storage).
CPU
Memory, however,
is not part of the
CPU.
Memory
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4
Parts of the CPU
The CPU consists of a variety of parts
including:
• Control unit
Control
Unit
ALU
Registers
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• Arithmetic/logic
unit (ALU)
• Registers
5
The Control Unit…
directs the other parts of the computer
system to execute stored program
instructions.
Control
Unit
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The control unit
communicates with the
ALU and memory.
6
The Arithmetic/Logic Unit
(ALU)…
performs mathematical operations as
well as logical operations.
ALU
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Mathematical Operations
The ALU can perform four kinds of
mathematical calculations:
•
•
•
•
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addition
subtraction
multiplication
division
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Logical Operations
The ALU can perform logical
operations.
Logical operations can test for these
conditions:
• Equal-to (=)
• Less-than (<)
• Greater-than (>)
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Equal-to Condition
In a test for this condition, the ALU
compares two values to determine if
they are equal.
If
=
Then
=
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Less-than Condition
In a test for this condition, the ALU
compares values to determine if one
value is less than another.
If
=
Then
<
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Greater-than Condition
In a test for this condition, the ALU
compares values to determine if one
value is greater than another.
If
=
Then
>
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Registers…
are temporary
storage areas
for data or
instructions.
Registers
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Data held
temporarily in
registers can be
accessed at
greater speeds
than data stored
in memory.
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Memory (Primary Storage)
Memory is the
part of the
computer that
stores data and
program
instructions for
processing.
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CPU
Memory
14
Memory…
is also referred to as
RAM (randomaccess memory).
RAM is temporary,
finite, and more
expensive than
secondary storage.
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CPU
Memory
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Executing Program
Instructions
Before the CPU can execute a
program, program
instructions and data must
be placed into memory from
an input device or storage
device.
Input
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Secondary
Storage
Processing
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Executing Program
Instructions
Once the necessary data and
instructions are in memory, the CPU
performs the following steps for each
instruction:
•
•
•
•
Fetching
Decoding
Executing
Storing
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CPU
Memory
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Fetching Instructions
Control
Unit
ALU
Registers
The control unit
fetches (gets)
the
instruction
from memory.
Memory
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18
Decoding Instructions
The control unit
decodes the
instruction
and directs
that the
necessary data
be moved from
memory to the
ALU.
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Control
Unit
ALU
Registers
Memory
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Executing Arithmetic/Logic
Operations
Control
Unit
ALU
Registers
The ALU performs
the arithmetic
or logical
operation on the
data.
Memory
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Storing Results
The ALU stores
the result of
its operation
on the data in
memory or in
a register.
Control
Unit
ALU
Registers
Memory
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Executing Program
Instructions
Secondary
Storage
Control
Unit
ALU
Registers
Eventually, the
control unit sends
the results in
memory to an
output device or
secondary storage.
Memory
Output
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22
Instruction Time
The time it takes to fetch an instruction
and decode it is called instruction time.
Control
Unit
Memory
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Control
Unit
ALU
+
Memory
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Execution Time
The time it takes to execute an ALU
operation and then store the result is
called execution time.
ALU
ALU
+
Registers
Memory
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Machine Cycle
The combination of I-time and E-time
is called the machine cycle.
Itime
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Etime
Machine
Cycle
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Memory Locations and Addresses
The control unit
can find data
and
instructions
because each
location in
memory has an
address.
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Control
Unit
Memory
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Storage Locations
Each location in
memory is
identified by an
address.
Memory
Each location has
a unique address.
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Symbolic Addresses
The choice of the
location in
memory is
arbitrary.
Addresses can
only hold one
number or word.
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$
%
Memory
Pat
364
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Data Representation
The system in which all computer data
is represented and manipulated is
called the binary system.
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Binary System
The binary system has only two digits
to represent all values.
This corresponds to the two states of
a computer’s electrical system —on
and off.
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Off/On Switches
The computer can represent data by
constructing combinations of off or
on switches.
off
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or
on
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Zero or One?
The binary system can also be
represented by the digits zero and one.
0
or
1
Zero (off) and one (on) make up the
two digits in the binary system.
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The Bit
one bit
two bits
three bits
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Each 0 or 1
in the
binary
system is
called a
bit.
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The Byte
A group of 8 bits is called a byte.
0
1
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0
0
1
0
1
0
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One Character of Data
Each byte represents one character of
data (a letter, digit, or special
character).
0
1
0
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0
1
0
1
0
= J
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Storing Bytes
Storage and memory capacity is
expressed in the number of bytes
they can hold:
1 kilobyte = 210 or 1024 bytes
1 megabyte = 220 or 1,048,576 bytes
1 gigabyte = 230 or 1,073,741,824
bytes
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Computer Word
A computer word is defined as the
number of bits that constitute a
common unit of data.
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Computer Word Length
Word length varies by computer. For
example:
8 bits = 1 byte = one word length
64 bits = 8 bytes = one word length
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Coding the Computer
A code for determining which group of
bits represent which characters on
a keyboard is called ASCII.
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ASCII
ASCII has been adopted, as the
standard, by the U.S. government and
is found in a variety of computers.
0 1 0 0 1 0 1 0 =
ASCII-8 code
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J
Keyboard character
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Inside the Computer
The flat board within the personal
computer that holds the computer
circuitry is the motherboard.
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The Motherboard
The motherboard includes:
• Microprocessor
• Memory components
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The Microprocessor…
is the CPU of the personal computer.
It looks like a computer chip.
The microprocessor
contains tiny
electronic switches
(transistors).
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Microprocessor Switches
If electric current passes through a
transistor, the switch is on.
If no current can pass, the switch is
off.
A combination of transistors can
stand for a combination of bits.
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Memory…
components are also chips.
This form of semiconductor
memory stores data in binary
form (off/on or zero/one).
Storage can be temporary (RAM)
or permanent (ROM).
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RAM…
means random-access memory.
RAM chips can be accessed
easily and fast—regardless of
where the data is stored.
RAM is usually volatile storage in that if
the power is shut off, the contents are lost.
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ROM…
is read-only memory.
ROM chips contain programs
and data that are permanently
etched on the chip.
The contents do not disappear when
the power is turned off.
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Computer Speed and
Power
Speed and power are determined by:
•
•
•
•
•
•
Microprocessor speed
Bus lines
Cache
Flash memory
RISC
Parallel processing
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Microprocessor Speeds
Microprocessor speeds can be
measured in a variety of ways:
• Megahertz
• MIPS
• Megaflops
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Megahertz
One measure of microprocessor speed
is megahertz (MHz) which is one
million machine cycles per second.
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MIPS
Another measure of microprocessor
speed is MIPS which is one million
instructions per second.
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Megaflops
Megaflops, or one million floatingpoint operations per second, is still
another measure of microprocessor
speed.
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Bus Lines
A bus line is a set of parallel
electrical paths. A bus is like a
mode of transportation for data.
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Bus Width
The amount of data that can be
carried at one time is bus width
(wider = more data).
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Cache
Cache is a relatively small block of
very fast memory.
The data and instructions stored in
cache are those that are most
recently or most frequently used.
Cache speeds up the internal transfer
of data and software instructions.
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Flash Memory
Accessing data and instructions from
within the CPU is fast but not when
it comes from a secondary storage
device.
Flash memory is nonvolatile (like
secondary storage) but allows fast
access (like RAM).
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RISC
New RISC computers have chips with
fewer instructions as a means of
making them run faster.
Older, slower machines have chips
(called CISC) with instructions that
are seldom or never used.
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Parallel Processing
Using several processors at the same
time (in parallel) greatly increases
processing speed.
When parallel processing, the
computer can be starting other
tasks before the sequence of fetchdecode-execute-store is complete.
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Conclusion
Regardless of the design and
processing strategy of a computer,
its goal is the same: to turn raw
data into useful information.
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