1 Investigating CPU Instructions

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InvestigatingCPUInstructions

Objectivesofthistutorial

Attheendofthislabyoushouldbeableto:

 EnterCPUinstructionsusingtheCPUsimulator

 DescribetheeffectofcompareinstructiononCPUstatusflagsZandN

 ConstructaloopandexplainjumpinstructionsthatuseZandNflags

 Usedirectaddressingtoaccessmemorylocation(s)

 Useindirectdirectaddressingtoaccessmemorylocation(s)

 Constructasubroutineandcallit

 Passparameter(s)tothesubroutine



Specialinstructions

 Saveyourworkatregularintervals.



 TheCPUsimulatorisworkinprogresssoitmaycrashfromtimetotime.Ifthishappens

thenrestartitandloadyourlatestcode,hencetheimportanceoftheabovestatement!



 Askifyouneedhelpwithanyaspectofthistutorial.



 Thesimulatoristheretohelpyouunderstandthetheorycoveredinlecturessomake

themostofit.

1

Exercises

Learningobjectives:ToenterCPUinstructionsanddescribeeffectofcompareinstructionon

CPUstatusflagsZandN.

IntheoryInpractice

TheCPUinstructionsetsareoften

groupedtogetherintocategories

containinginstructionswithrelated

functions.

Thisexercisefamiliarisesyouwith

thewaytheinstructionsareentered

usingthesimulator.

Themostencounteredinstructions

involvedatamovements,e.g.the

MOVinstruction.

Anothercommoninstructionisthe

comparisoninstruction,e.g.CMP.

Thisinstructionsetsorresetsthe

statusflagsZandNasarecordof

theresultofthecomparison

operation.

1. Enter the following code and run it:

MOV #1, R01

MOV #2, R02

ADD R01, R02

2. Add the following code and note the states of the status

flags Z and N after each compare instruction is executed:

CMP #3, R02 Z  N 

CMP #1, R02 Z  N 

CMP #4, R02 Z  N 

Explainyourobservationsofthestatesofthestatus

flagsafterthecompareinstructionsabove:





** Now save the above code! **

Learningobjectives:ToconstructaloopandexplainjumpinstructionsthatuseZandNflags

IntheoryInpractice

Thecomputerprogramsoften

containloopswherethesame

sequenceofcodeisrepeatedly

executeduntilorwhilecertain

conditionismet.Loops(oriterative

statements)arethemostuseful

featuresofprogramminglanguages.

Attheinstructionlevel,loopsuse

conditionaljumpinstructionsto

jumpbacktothestartoftheloopor

tojumpoutoftheloop.

Thisexerciseiscreatedto

demonstratetheuseofthejump

instructionwhichoftenusesthe

resultofacompareinstruction.Such

jumpinstructionsusetheZandthe

Nstatusflagstojumpornotjump.

3. Add the following code and run it (ask your tutor how to

enter a label):

MOV #0, R01

Label1

ADD #1, R01

CMP #3, R01

JLT $Label1

Summarize what the above code is doing in plain

English:

4. Modify the above code such that R01 is incremented by 1

until it reaches the value of 4. Copy the code below:

** Now save the above code! **

2

Learningobjectives:Tousedirectaddressingtoaccessmemorylocation(s)

IntheoryInpractice

Althoughinstructionsmovingdata

toorfromregistersarethefastest

instructions,itisstillnecessaryto

movedatainoroutofthemain

memory(RAM)whichisamuch

slowerprocess.

Examplesofinstructionsusedto

storeintoorgetdataoutofmemory

areexploredhere.Themethodused

hereusesthedirectaddressing

method,i.e.thememoryaddressis

directlyspecifiedintheinstruction

itself.

5. Add the following code and run:

STB #h41, 16

LDB 16, R03

ADD #1, R03

STB R03, 17

Make a note of what you see in the program’s data area:

What is the significance of h in h41 above?

Modify the above code to store the next two characters in

the alphabet. Write down the modified code below:

** Now save the above code! **

Learningobjective:Touseindirectaddressingtoaccessmemorylocation(s)

IntheoryInpractice

Therearecircumstanceswhichmake

directaddressingunsuitableand

inflexiblemethodtouse.

Inthesecasesindirectaddressingis

amoresuitableandflexiblemethod

touse.Inindirectaddressing,the

addressofthememorylocationis

notdirectlyincludedinthe

instructionbutisstoredinaregister

whichistheusedintheinstruction.

Thisexerciseintroducesanindirect

methodofaccessingmemory.Thisis

calledregisterindirectaddressing.

Thereisalsothememoryindirect

addressingbutthisisleftasan

exerciseforyou.

6. Add the following code and run:

Label2

MOV #16, R03

MOV #h41, R04

Label3

STB R04, @R03

ADD #1, R03

ADD #1, R04

CMP #h4F, R04

JNE $Label3

Make a note of what you see in the program’s data area:

Explain the significance of @ in @R03 above:

** Now save the above code! **

3

Learningobjective:Toconstructasubroutineandcallit

IntheoryInpractice

Anotherveryusefulfeatureof

programminglanguagesis

subroutines.Thesecontain

sequencesofinstructionswhichcan

beexecutedmanytimes.Ifa

subroutinewasnotavailablethe

samesequenceofinstructions

wouldhavebeenrepeatedmany

timesincreasingthecodesize.

Atinstructionlevel,asubroutineis

calledbyaninstructionsuchasCAL.

Thiseffectivelyisajumpinstruction

butitalsosavestheaddressofthe

nextinstruction.Thisislaterusedby

asubroutinereturninstruction,e.g.

RETtoreturntotheprevious

sequenceofinstructions.

7. Add the following code but do NOT run it yet:

MSF

CAL $Label2

Now convert the code in (6) above into a subroutine by

inserting a RET as the last instruction in the subroutine.

Make a note of the contents of the PROGRAM STACK

after the instruction MSF is executed (see tutor what to

do to facilitate this observation):

Make a note of the contents of the PROGRAM STACK

after the instruction CAL is executed:

What is the significance of the additional information?

** Now save the above code! **

Learningobjective:Topassparameter(s)tothesubroutine

IntheoryInpractice

Usefulastheyarethe

subroutinesarenotvery

flexiblewithouttheuseofthe

subroutineparameters.These

arevaluespassedtothe

subroutinetobeconsumed

insidethesubroutine.

Thecommonmethodof

passingparameterstothe

subroutinesisusingthe

programstack.The

parametersarepushedonthe

stackbeforethecall

instruction.Thesearethen

poppedoffthestackinsidethe

subroutineandconsumed.

Thisexerciseiscreatedto

demonstratethismechanism.

8. Let’s make the above subroutine a little more flexible. Suppose

we wish to change the number of characters stored when

calling the subroutine. Modify the calling code in (7) as below:

MSF

PSH #h60

CAL $Label2

Now modify the subroutine code in (6) as below and run the

above calling code:

Label2

MOV #16, R03

MOV #h41, R04

POP R05 Å

Label3

STB R04, @R03

ADD #1, R03

ADD #1, R04

CMP R05, R04 Å

JNE $Label3

RET

Add a second parameter to change the starting address of the

data as a challenge and write the code in the box above!

** Now save the above code! **

4