CHAPTER 4
The Central Processing Unit
Chapter Overview
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Microprocessors
Replacing and Upgrading a CPU
Microprocessor Basics
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Components
How microprocessors work
PC microprocessor developments and
features
 The External Data Bus
- Information is transmitted through a computer using binary code
traveling through a bus.
- 8 bit data bus: allowed for the transfer of 1 byte of information at a time.
- The width of the external data bus increaed to 16, and the 32 current
width of conductors. (HW3) is that correct?
The CPU
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Performs operations , decodes and executes
instructions
Controls computer operation
Includes transistors, integrated circuits, and
microprocessors
Note: P4 contains 775 pins
HW3: P5 Contains …… Pins
Microprocessor Design
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Three subsystems
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Control unit (CU):
The control unit of the CPU contains circuitry that uses electrical signals to direct the
entire computer system to carry out, or execute, stored program instructions .
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Arithmetic logic unit (ALU):
The arithmetic/logic unit (ALU) contains the electronic circuitry that executes all
arithmetic and logical operations.
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Input/output (I/O) unit
Fetches data from the outside and passes data back to the external bus
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Registers:
Are temporary memory storage areas used during data manipulation.
- Codes ( ASCII)
 Clock :
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Timing allows the electronic devices in the computer to coordinate and
execute all internal commands in the proper order.
Pulse of voltage is called clock cycle.
Every computer command needs at least 2 clock cycles.
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Clock speed
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The clock rate suggests how many commands can be completed in two
cycles.
Adding two numbers would take 4 commands (__?__ clock cycles).
REGISTERS
Control unit components
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Program counter (PC): Contains the address of an
instruction to be fetched.
Instruction register (IR): Contains the instruction
most recently fetched.
Memory address registers (MAR): Contains the
address of a location in memory.
Memory buffer register (MBR): Contains a word of
data lo be written to memory or the word most
recently read.
PSW ( take home class test)
10
Memory
How Microprocessors Work
Adding Two Simple
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2 + 2 involves more steps than a man :
A = 2 Translated into binary and routed to CU and
sent to X location of memory
When we press + , it is translated into binary and
alerts ALU to be ready for addition
B= 2 is translated into binary again
ALU sums A and B
ALU sends the code for 4 to be stored in an address
register
Pressing = key is the last user action BUT the = is
also translated into machine language
Sum is computed
PC Microprocessor Developments and
Features
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Use the following elements to measure CPU performance:
 Speed : max no. of clock cycles measured in MHz
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Higher the speed, quicker a command will be executed
Transistors: More switches means more power
Registers: Larger the register size, more complicated command
can be processed
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External data bus: if large , more and complex can be executed
Address bus: determines the max amount of memory that can be
addressed by CPU
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Internal cache It is high speed Memory built into the processor.
It is the place to store the most frequently used data instead of
sending it to the slow speed devices ( HDD , RAM)
Virtual Memory
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Is the art of using Hard disk space to hold the
data not immediately required by processor
Possible Upgrade Scenarios
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8086/8088
80286–80486
80386SX
Pentium I
Pentium II
Pentium III
Superscalar Technology
Main components of processor –registers, decoders,
and ALU are called Instruction Pipeline
 To carryout a single instruction, a processor must:
* Read instruction
* Decode instruction
* Fetch (for math function)
* Execute
* Write back the result
Combining all above steps in a single step is called
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Pipelining
Pipelining types
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Superscalar technology allows the Pentium to have
two instruction Pipelines–called U and V.
U Pipeline can execute the full range of Pentium
instructions
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V Pipeline can execute a limited number
Pentium processor breaks a program into small tasks
that are shared b/w pipelines , allowing Pentium to
execute two simple Instructions at the same timeknown as Multithreading
Inserting a CPU
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2.
3.
Low-insertion-force (LIF)
Zero-insertion-force (ZIF)
Single-edge connector (SEC/slot 1)
Chapter Summary
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Understanding processor development and progress
is essential.
The three key elements for measuring a CPU’s
performance are its speed, address bus, and external
data bus.
Several key CPU developments have occurred since
the 80286 processor.
Today’s standard processor is the Pentium III chip.
Replacing a CPU is usually simple.
You must watch for electrostatic discharge (ESD) and
pin damage when handling a CPU.