the central processing unit

1Copyright © Prentice Hall 2000

The Central Processing Unit

Chapter 3Chapter 3

What Goes on Inside the What Goes on Inside the ComputerComputer


Functions of Computer Systems

Recall that there are four main Recall that there are four main functions of computer systems:functions of computer systems:

Input Processing Output

SecondaryStorage


Processing

In this chapter, we will focus on the In this chapter, we will focus on the central processing unit central processing unit (CPU) in (CPU) in more detail.more detail.

Input Processing Output

SecondaryStorage


The CPU

The The CPUCPU interacts interacts closely with closely with memorymemory (primary (primary storage).storage).

CPU

MemoryMemory, Memory, however, is not however, is not part of the CPU.part of the CPU.


Parts of the CPU

The CPU consists of a variety of parts The CPU consists of a variety of parts including:including:

Control Unit ALU

Registers

• Control unitControl unit

• Arithmetic/logic Arithmetic/logic unit (ALU)unit (ALU)

• RegistersRegisters


The Control Unit…

directsdirects the other parts of the the other parts of the computer system to execute stored computer system to execute stored program instructions.program instructions.

The control unit The control unit communicates with the communicates with the ALU and memory.ALU and memory.

Control Unit


The Arithmetic/Logic Unit (ALU)…

performs performs mathematical mathematical operations operations as well as as well as logicallogical operations. operations.

ALU


Mathematical Operations

The ALU can perform four kinds of The ALU can perform four kinds of mathematical calculations:mathematical calculations:

• additionaddition• subtractionsubtraction• multiplicationmultiplication• divisiondivision


Logical Operations

The ALU can perform logical The ALU can perform logical operations.operations.

Logical operations can test for these Logical operations can test for these conditions:conditions:

• Equal-to (=)Equal-to (=)• Less-than (<)Less-than (<)• Greater-than (>)Greater-than (>)


Equal-to Condition

In a test for this condition, the ALU In a test for this condition, the ALU compares two values to determine compares two values to determine if they are if they are equalequal..

If =

Then =


Less-than Condition

In a test for this condition, the ALU In a test for this condition, the ALU compares values to determine if one compares values to determine if one value is value is less thanless than another. another.

If =

Then <


Greater-than Condition

In a test for this condition, the ALU In a test for this condition, the ALU compares values to determine if compares values to determine if one value is one value is greater thangreater than another. another.

If =

Then >


Registers…

are temporary are temporary storage areas storage areas for data or for data or instructions.instructions.

Registers

Data held Data held temporarily in temporarily in registers can be registers can be accessed at accessed at greater speeds greater speeds than data stored than data stored in memory.in memory.


Memory (Primary Storage)

Memory is the Memory is the part of the part of the computer that computer that stores data stores data and program and program instructions for instructions for processing.processing.

CPU

Memory


Memory…

is also referred to is also referred to as as RAMRAM (random- (random-access memory).access memory).

CPU

MemoryRAM is RAM is temporarytemporary, , finitefinite, and more , and more expensiveexpensive than than secondary storage.secondary storage.


Executing Program Instructions

Before the CPU can execute Before the CPU can execute a program, program a program, program instructions and data must instructions and data must be placed into memory be placed into memory from an from an inputinput device or device or storagestorage device. device.

Input Processing

SecondaryStorage



Once the necessary data and Once the necessary data and instructions are in memory, the CPU instructions are in memory, the CPU performs the following steps for each performs the following steps for each instruction:instruction:

CPU

Memory

• FetchingFetching• DecodingDecoding• ExecutingExecuting• StoringStoring


Control Unit

ALU

Registers

Fetching Instructions

The control unit The control unit fetchesfetches (gets) (gets) the the instruction instruction from from memory.memory.

Memory


Decoding Instructions

The control unit The control unit decodes the decodes the instruction and instruction and directs that directs that the necessary the necessary data be data be moved from moved from memory to the memory to the ALU.ALU.

Control Unit ALU

Registers

Memory


Executing Arithmetic/Logic Operations

The ALU The ALU performs the performs the arithmetic or arithmetic or logical logical operation on operation on the data.the data.

Control Unit

ALU

Registers

Memory


Storing Results

The ALU stores The ALU stores the result of the result of its operation its operation on the data on the data in memory or in memory or in a register.in a register.

Control Unit

ALU

Registers

Memory



Eventually, the Eventually, the control unit sends control unit sends the results in the results in memory to an memory to an ououttput device or put device or secondary storage.secondary storage.

Output

SecondaryStorage

Control Unit

ALU

Registers

Memory


Instruction Time

The time it takes to fetch an instruction and The time it takes to fetch an instruction and decode it is called decode it is called instruction time.instruction time.

Memory

Control Unit

+Memory

Control Unit ALU


Execution Time

The time it takes to execute an ALU The time it takes to execute an ALU operation and then store the result is operation and then store the result is called called execution time.execution time.

+ALU

Memory

ALU

Registers


Machine Cycle

The combination of I-time and E-The combination of I-time and E-time is called the machine cycle.time is called the machine cycle.

E-time

Machine Cycle

I-time


Control Unit

Memory Locations and Addresses

The control unit The control unit can find data can find data and and instructions instructions because each because each location in location in memory has an memory has an addressaddress..

Memory


Storage Locations

Each location in Each location in memory is memory is identified by an identified by an addressaddress..

MemoryEach location has Each location has a unique address.a unique address.


Symbolic Addresses

The choice of the The choice of the location in location in memory is memory is arbitrary.arbitrary.

Memory

17

364

Pat

$

%

Addresses can Addresses can only hold oneonly hold one number number oror word. word.


Data Representation

The system in which all computer data is The system in which all computer data is represented and manipulated is called represented and manipulated is called the the binary system.binary system.


Binary System

The binary system has only The binary system has only two two digitsdigits to represent all values. to represent all values.

This corresponds to the This corresponds to the two statestwo states of a computer’s electrical system —of a computer’s electrical system —on and off.on and off.


Off/On Switches

The computer can represent data by The computer can represent data by constructing combinations of constructing combinations of off off oror on on switches.switches.

off onor


Zero or One?

The binary system can also be represented The binary system can also be represented by the digits by the digits zerozero and and oneone..

0 1or

Zero (off) and one (on) make up Zero (off) and one (on) make up the two digits in the binary system.the two digits in the binary system.


The Bit

Each 0 or 1 Each 0 or 1 in the in the binary binary system is system is called a called a bitbit..

one bit

two bits

three bits


The Byte

A group of 8 bitsA group of 8 bits is called a is called a bytebyte. .

0 0 0 0 01 1 1


One Character of Data

Each byte represents Each byte represents one character of one character of datadata (a letter, digit, or special (a letter, digit, or special character).character).

0 0 0 0 01 1 1

J=


Storing Bytes

Storage and memory capacity is Storage and memory capacity is expressed in the expressed in the number of bytesnumber of bytes they can hold:they can hold:

1 1 kilokilobyte = 2byte = 21010 or 1024 bytes or 1024 bytes1 1 megamegabyte = 2byte = 22020 or 1,048,576 bytes or 1,048,576 bytes1 1 gigagigabyte = 2byte = 23030 or 1,073,741,824 or 1,073,741,824 bytesbytes


Computer Word

A computer wordA computer word is defined as the is defined as the number of bits that constitute a number of bits that constitute a common unit of data.common unit of data.


Computer Word Length

Word length Word length varies by computer. For varies by computer. For example:example:

8 bits = 1 byte = one word length8 bits = 1 byte = one word length

64 bits = 8 bytes = one word length 64 bits = 8 bytes = one word length


Coding the Computer

A code for determining which group A code for determining which group of bits represent which characters of bits represent which characters on a keyboard is called on a keyboard is called ASCIIASCII..


ASCII

ASCII has been adopted, as the ASCII has been adopted, as the standard, by the U.S. government and standard, by the U.S. government and is found in a variety of computers.is found in a variety of computers.

0 0 0 0 01 1 1 J=ASCII-8 code Keyboard

character


Inside the Computer

The flat board within the personal The flat board within the personal computer that holds the computer computer that holds the computer circuitry is the circuitry is the motherboardmotherboard..


The Motherboard

The motherboard includes:The motherboard includes:• MicroprocessorMicroprocessor• Memory componentsMemory components


The Microprocessor…

is the CPU of the personal computer. is the CPU of the personal computer. It looks like a computer chip.It looks like a computer chip.

The microprocessor The microprocessor contains tiny contains tiny electronic switches electronic switches (transistors).(transistors).


Microprocessor Switches

If electric current passes through a If electric current passes through a transistor, the switch is transistor, the switch is onon..

If no current can pass, the switch is If no current can pass, the switch is offoff..

A combination of transistors can A combination of transistors can stand for a combination of bits.stand for a combination of bits.


Memory…

components are also chips.components are also chips.

This form of semiconductor This form of semiconductor memory stores data in binary memory stores data in binary form (off/on or zero/one).form (off/on or zero/one).

Storage can be Storage can be temporarytemporary (RAM) (RAM) or or permanentpermanent (ROM). (ROM).


RAM…

means random-access memory.means random-access memory.

RAM chips can be accessed RAM chips can be accessed easily and fast—regardless easily and fast—regardless of where the data is stored.of where the data is stored.

RAM is usually volatile storage in that if RAM is usually volatile storage in that if the power is shut off, the contents are the power is shut off, the contents are lost.lost.


ROM…

is read-only memory.is read-only memory.

ROM chips contain programs ROM chips contain programs and data that are permanently and data that are permanently etched on the chip.etched on the chip.

The contents do not disappear when The contents do not disappear when the power is turned off.the power is turned off.


Computer Speed and Power

Speed and power are determined Speed and power are determined by:by:

• Microprocessor speedMicroprocessor speed• Bus linesBus lines• CacheCache• Flash memoryFlash memory• RISCRISC• Parallel processingParallel processing


Microprocessor Speeds

Microprocessor speeds can be Microprocessor speeds can be measured in a variety of ways:measured in a variety of ways:

• MegahertzMegahertz• MIPSMIPS• MegaflopsMegaflops


Megahertz

One measure of microprocessor speed One measure of microprocessor speed is is megahertzmegahertz (MHz) which is one (MHz) which is one million machine cycles per second.million machine cycles per second.


MIPS

Another measure of microprocessor Another measure of microprocessor speed is speed is MIPSMIPS which is one million which is one million instructions per second.instructions per second.


Megaflops

Megaflops,Megaflops, or one million floating- or one million floating-point operations per second, is still point operations per second, is still another measure of another measure of microprocessor speed.microprocessor speed.


Bus Lines

A bus line is a set of parallel A bus line is a set of parallel electrical paths. A bus is like a electrical paths. A bus is like a mode of transportation for data.mode of transportation for data.


Bus Width

The amount of data that can be The amount of data that can be carried at one time is bus width carried at one time is bus width (wider = more data).(wider = more data).


Cache

Cache is a relatively small block of Cache is a relatively small block of very fast memory.very fast memory.

The data and instructions stored in The data and instructions stored in cache are those that are cache are those that are most most recently or most frequently usedrecently or most frequently used..

Cache speeds up the internal Cache speeds up the internal transfer of data and software transfer of data and software instructions.instructions.


Flash Memory

Accessing data and instructions from Accessing data and instructions from within the CPU is fast but not when within the CPU is fast but not when it comes from a secondary storage it comes from a secondary storage device.device.

Flash memory is Flash memory is nonvolatilenonvolatile (like (like secondary storage) but allows secondary storage) but allows fast fast accessaccess (like RAM). (like RAM).


RISC

New RISC computers have chips with New RISC computers have chips with fewer instructions as a means of fewer instructions as a means of making them run faster.making them run faster.

Older, slower machines have chips Older, slower machines have chips (called CISC) with instructions that (called CISC) with instructions that are seldom or never used.are seldom or never used.


Parallel Processing

Using several processors at the Using several processors at the same time (in parallel) greatly same time (in parallel) greatly increases processing speed.increases processing speed.

When parallel processing, the When parallel processing, the computer can be starting other computer can be starting other tasks before the sequence of fetch-tasks before the sequence of fetch-decode-execute-store is complete.decode-execute-store is complete.


Conclusion

Regardless of the design and Regardless of the design and processing strategy of a computer, processing strategy of a computer, its goal is the same: to turn raw its goal is the same: to turn raw data into useful information.data into useful information.

the central processing unit

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