LMC Simulation

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How the CPU WorksHow the CPU Works

The “Little Man Computer”The “Little Man Computer”VersionVersion

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Download the tutorialDownload the tutorialYou can download your own copy of the Little You can download your own copy of the Little Man Computer simulation from the University of Man Computer simulation from the University of HertfordshireHertfordshire Start the download nowStart the download now Save it to your hard driveSave it to your hard drive We will use it shortlyWe will use it shortly

The link: The link: http://www.herts.ac.uk/lis/ltdu_temp/ltdu/projectshttp://www.herts.ac.uk/lis/ltdu_temp/ltdu/projects/mm5/#/mm5/#You can access this link directly from my You can access this link directly from my websitewebsite

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The Little Man ComputerThe Little Man Computer

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LMC Instruction CodesLMC Instruction Codes

LOAD instruction – op code 1LOAD instruction – op code 1 The Little Man goes to the mailbox specifiedThe Little Man goes to the mailbox specified Reads the three-digit number in the mailboxReads the three-digit number in the mailbox Puts that into the CalculatorPuts that into the Calculator

Note:Note: The number in the mailbox is unchangedThe number in the mailbox is unchanged The number in the calculator is replaced by The number in the calculator is replaced by

the new numberthe new number

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STORE instruction – op code 2STORE instruction – op code 2 The Little Man goes to the Calculator and reads the The Little Man goes to the Calculator and reads the

number therenumber there He writes the number down on a slip of paper and He writes the number down on a slip of paper and

puts it into the mailbox specified in the instructionsputs it into the mailbox specified in the instructions

Note:Note: The number in the Calculator does not changeThe number in the Calculator does not change The number in the Mailbox is replaced with the new The number in the Mailbox is replaced with the new

valuevalue

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ADD instruction – op code 3ADD instruction – op code 3 The Little Man walks over to the mailbox The Little Man walks over to the mailbox

address specified in the instructionsaddress specified in the instructions He reads the number and He reads the number and adds this number to adds this number to

the number already in the calculatorthe number already in the calculator..

Note:Note: The number in the mailbox is unchangedThe number in the mailbox is unchanged

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SUBTRACTION – op code 4SUBTRACTION – op code 4 Same as the ADD instruction, except the Same as the ADD instruction, except the

number is subtractednumber is subtractedFor this model we will assume that the Little Man For this model we will assume that the Little Man handles negative numbers correctly, though we will handles negative numbers correctly, though we will not deal with it here.not deal with it here.

Note:Note: The number in the mailbox is left unchangedThe number in the mailbox is left unchanged

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INPUT (or READ) – op code 5, INPUT (or READ) – op code 5, “address” 00 (the 00 is ignored for this “address” 00 (the 00 is ignored for this special instruction code)special instruction code) The Little Man gets a slip of paper in from the The Little Man gets a slip of paper in from the

in-basketin-basket He types this number into the calculatorHe types this number into the calculator Each INPUT instruction handles only 1 slip of Each INPUT instruction handles only 1 slip of

paperpaper The Little Man will ignore the address portion The Little Man will ignore the address portion

of this codeof this code

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OUTPUT (or PRINT) – op code 6, OUTPUT (or PRINT) – op code 6, “address” 00 (the 00 is ignored for this “address” 00 (the 00 is ignored for this special instruction code)special instruction code) The Little Man walks to the calculator and The Little Man walks to the calculator and

writes down the number on a slip of paperwrites down the number on a slip of paper This slip is put in to the out basketThis slip is put in to the out basket Each OUTPUT instruction places 1 piece of Each OUTPUT instruction places 1 piece of

paper in the out basketpaper in the out basket The Little Man will ignore the address portion The Little Man will ignore the address portion

of this codeof this code

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COFFEE BREAK (or HALT) – COFFEE BREAK (or HALT) – op code 7op code 7 The Little Man takes a break.The Little Man takes a break. He will ignore the address portion of the He will ignore the address portion of the

instructioninstruction

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A Simple ProgramA Simple Program

A few itemsA few items We need to store the instructions somewhereWe need to store the instructions somewhere We need a method to tell the Little Man where We need a method to tell the Little Man where

to find the particular instructions that he is to find the particular instructions that he is supposed to perform at any given timesupposed to perform at any given time

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Assumed SolutionsAssumed Solutions Assume that the instructions are stored in the Assume that the instructions are stored in the

mailbox starting at mailbox number 00mailbox starting at mailbox number 00 The Little Man will perform instructions by The Little Man will perform instructions by

looking at the value in the instruction location looking at the value in the instruction location counter and execute the instruction found in counter and execute the instruction found in the mailbox whose address has that valuethe mailbox whose address has that value

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Assumed SolutionsAssumed Solutions Each time the LM completes and instruction, Each time the LM completes and instruction,

he will increment the Instruction Location he will increment the Instruction Location Counter and do the instruction specified by Counter and do the instruction specified by the counterthe counter

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Look at a program where the USER uses Look at a program where the USER uses the Little Man Computer to add two the Little Man Computer to add two numbers.numbers.

The User will place two numbers in the in The User will place two numbers in the in basketbasket

The sum of the two will appear as a result The sum of the two will appear as a result in the out basketin the out basket

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INPUTINPUT 500500 The first step is to give the LM access to the The first step is to give the LM access to the

datadata This instruction has the LM read the first This instruction has the LM read the first

number from the in basket in to the calculatornumber from the in basket in to the calculator

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STORE 99STORE 99 299299 It is not possible for the LM to simply read It is not possible for the LM to simply read

another number into the calculator. another number into the calculator. It would destroy the first numberIt would destroy the first number We must save the first number somewhereWe must save the first number somewhere Mailbox 99 is chosen so that we don’t have Mailbox 99 is chosen so that we don’t have

any conflicts with other data or programsany conflicts with other data or programsDiscuss thisDiscuss this

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INPUT INPUT 500500 The LM reads the second number into the The LM reads the second number into the

calculatorcalculator

ADDADD 399399 We do not need to save the 2We do not need to save the 2ndnd number in the number in the

example, but we could have.example, but we could have. The 1The 1stst number from mailbox 99 is added to number from mailbox 99 is added to

the 2the 2ndnd number in the calculator number in the calculator

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OUTPUTOUTPUT 600600 The LM puts the result in to the out basketThe LM puts the result in to the out basket

COFFEE BREAKCOFFEE BREAK700700 The program terminatesThe program terminates

Note:Note: These instructions are stored sequentially starting These instructions are stored sequentially starting

from mailbox 00. from mailbox 00. This is so we could run the program againThis is so we could run the program again

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A Simple ProgramA Simple ProgramCode SummaryCode Summary

MailboxMailbox InstructionInstruction

StepStep codecode descriptiondescription

0000 500500 INPUTINPUT

0101 299299 STORE DATASTORE DATA

0202 500500 INPUT 2INPUT 2NDND # #

0303 399399 ADD 1ADD 1STST # TO it # TO it

0404 600600 OUTPUT RESULTOUTPUT RESULT

0505 700700 STOPSTOP

9999 DATADATA

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Run the CPU SimulatorRun the CPU SimulatorNow you try it.Now you try it.Do the sample program included with the Do the sample program included with the simulationsimulationWrite your own program (and show me that Write your own program (and show me that they work)they work) Write a program to add 3 numbersWrite a program to add 3 numbers Write a program that correctly calculates Write a program that correctly calculates

x + y + z – a – b (Ex. 1 + 2 + 3 – 4 – 5 = -3)x + y + z – a – b (Ex. 1 + 2 + 3 – 4 – 5 = -3)

For the bold and intelligent ones:For the bold and intelligent ones: Write the last program only using 2 mailboxes!Write the last program only using 2 mailboxes!

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How the CPU WorksHow the CPU Works

A More Complex ViewA More Complex View

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OverviewOverview

We saw in the “Little Man Computer” a We saw in the “Little Man Computer” a simplified version of how a CPU works.simplified version of how a CPU works. Memory, the Control Unit, the Accumulator Memory, the Control Unit, the Accumulator

(calculator) were all located in the same area(calculator) were all located in the same area Decimal system used for communication of Decimal system used for communication of

instructionsinstructions Memory was limited to 100 “mailboxes”Memory was limited to 100 “mailboxes”

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CPU ComponentsCPU Components

3 Major Parts3 Major Parts Arithmetic/Logic Unit (ALU)Arithmetic/Logic Unit (ALU) Control Unit (CU)Control Unit (CU) MemoryMemory

The ALU & CU are known as the Central The ALU & CU are known as the Central Processing UnitProcessing Unit

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Block Diagram of SystemBlock Diagram of System

Memory

CPU

ALU

I/O

Control UnitProgram Counter

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Direct Comparison of Direct Comparison of LMC & CPULMC & CPU

ALU

I/OInterface

ControlUnit

Memory

ProgramCounter

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Arithmetic/Arithmetic/Logic Unit (ALU)Logic Unit (ALU)

The component where data is held The component where data is held temporarilytemporarilyCalculations occur hereCalculations occur hereIt knows how to perform operations such It knows how to perform operations such as ADD, SUB, LOAD, STORE, SHIFT. as ADD, SUB, LOAD, STORE, SHIFT. It knows the commands that make up the It knows the commands that make up the machine language of the CPUmachine language of the CPUIt is the calculatorIt is the calculator

Memory

CPU

ALU

I/O


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Control UnitControl Unit

Interprets and controls the execution of Interprets and controls the execution of instructionsinstructions

This control corresponds to the “fetch-execute” This control corresponds to the “fetch-execute” cyclecycle The The fetch cyclefetch cycle gets the instructions gets the instructions The The execute cycleexecute cycle does the work specified in the does the work specified in the

instructioninstruction

Note

Memory

CPU

ALU

I/O


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RegistersRegisters

““AA Register Register is a single, permanent storage is a single, permanent storage location within the CPU used for a location within the CPU used for a PARTICULAR, defined purpose.”PARTICULAR, defined purpose.”

““A register is used to hold a A register is used to hold a binarybinary value value temporarily for storage, for manipulation, temporarily for storage, for manipulation, and/or for simple calculations.”and/or for simple calculations.”

Registers have special addressesRegisters have special addresses

Memory

CPU

ALU

I/O


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The End…The End…

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LMC Simulation