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LittleManComputer
AdditionalNotesonCompilerandInterpreters
1 UnderstandingCompilersandInterpreters
1.1 WhatweLearnfromAssemblyCode
Learningaboutassemblycode,remindusthat:
1. Variablescorrespondtomemorylocations.
2. Thememoryofacomputercontainsbothdataandcode.
3. IfstatementsandloopsarecreatedbychangingtheProgramCounter.
1.2 Compilers
Acompilerisatranslatorfromahighlevellanguagetotheassemblycodeofaparticular
CPU.Acompiledprogramworkson
•
•
theparticularCPUand
OperatingSystem
thatitwascompiledfor.Internally,thecompilerhasseveralstages:
1. Aparserchecksthatthesourcecodefollowsthesyntaxofthelanguage.Atreeis
constructedrepresentingtheprogramcode.Atthisstage,syntaxerrorsare
generated.
2. Thetypecheckerchecksthattheexpressionsintheprogramarecorrectlytyped
andhowmuchspaceisneedforeachvariable.Atthisstage,theerrorsgenerated
concernvariables(andothernames)thatarenotdeclaredandcodethatis
incorrectlytypes.
3. Thecodegeneratorthentranslatestheprogramtoassemblycode.Compilers
usuallyincludeanassemblersotheoutputisusuallyinbinary(callobjectcode)
ratherthanassemblycode.Thetwomaintasksarei)decidingwhichregisterto
use(as,unlikeLMC,modernCPUshavemanyregisters)andii)choosetheCPU
instructions.
Thefirsttwostepsaredeterminedbythelanguagebeingcompiled;thefinalstepis
determinedbytheprocessorbeingtargeted.Moderncompilershaveamodular
structure,whichfront-endsfordifferentsourcelanguagesandback-endsfordifferent
CPUs.Inaddition,moderncompilersincludeoptimisersthatmakethegeneratedcode
fasterwithoutchangingitsmeaning.Theseoptimiserstypicallyoperateonan
intermediatelanguage,whichisusedforallsourcelanguagesandallCPUs.
1.3 Interpreter
Aninterpreterissimplerthanacompiler.Itincludestheparserbutinsteadofthecode
generator,theinterpretergoesthroughtheinternalrepresentationofthesourcecode
(suchasanabstractsyntaxtree)and‘executes’thecodedirectly.
Althoughinprincipleanylanguagecanbecompiledorinterpreted,languagesthatare
usuallycompiledtendtobedynamicallytypedandscoped,whilecompiledlanguages
arestaticallytypedandlexicallyscoped.
Dynamicvstatictyping:indynamictyping,thetypeofavariabledependsonits
useandmaychangeatdifferentpointsintheprogram.Sincethetypeisnotknow
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inadvance,theoperation(e.g.integerversusfloatingpointarithmetic)cannotbe
determinedeither,whichisinconvenientforacompiler.
Dynamicsscoping:scopingisaboutmatchingnamestovariables(ormemory
locations).Inalexicallyscopedlanguage,suchasC,thecompilermatchesnames
tovariables.InamoredynamiclanguagelikePython,namesare‘resolved’atrun
timeandtheprocessdependsonthevariablesthatexistwhenareferencetoa
nameisexecuted.Manylanguagesincludeaspectsofbothapproaches.
2 WritinganLMCInterpreter
OnewaytounderstandhowaninterpreterworksistowriteonefortheLMC.(Note:an
interpreterforaCPUisoftencalledanemulatororasimulator).Below,weoutlinehow
anLMCinterpretercanbewritten.Hereisanexampleofapossiblesolutionbeingused.
Load (L) Run(R) Stop(S) > R
MAR = 0 MDR = 0 ACC = 0 PC =
Program halted
Load (L) Run(R) Stop(S) > L
Location = 0 Enter value (or
Location = 1 Enter value (or
Location = 2 Enter value (or
Location = 3 Enter value (or
Location = 4 Enter value (or
Location = 5 Enter value (or
Location = 6 Enter value (or
Load (L) Run(R) Stop(S) > R
MAR = 0 MDR = 0 ACC = 0 PC =
Press enter to continue
MAR = 4 MDR = 11 ACC = 11 PC
Press enter to continue
MAR = 5 MDR = 17 ACC = 28 PC
Press enter to continue
MAR = 6 MDR = 28 ACC = 28 PC
Program halted
Load (L) Run(R) Stop(S) >
0
'.')
'.')
'.')
'.')
'.')
'.')
'.')
504
105
306
0
11
17
.
0
= 1
= 2
= 3
2.1 DesignSuggestions
Herearesomefragmentsofsuchaprogramtoillustratetheidea:
Representthestateofthesystem.TheLMCstateisitsregistersandmemory.
acc = 0
mdr = 0
mar = 0
pc = 0
memory = [504,105,306, 0, 11, 17,...]
Updatethestate:thismeansfollowingtherulesoftheCPU.IntheLMC,thewaythe
datamovesbetweenregistersdependsontheopcode:
def execute(memory, opcode, arg):
global acc, mar, mdr, pc
if opcode == ADD:
mar = arg
readMem(memory)
acc = acc + mdr
elif opcode == SUB:
mar = arg
readMem(memory)
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acc = acc - mdr
elif opcode == STO:
mar = arg
mdr = acc
writeMem(memory)
elif opcode == LDA:
mar = arg
readMem(memory)
acc = mdr
elif opcode == BR:
pc = opcode
elif ...
Someoftheadditionalfunctionsneededtocompletetheinterpreterareshownbelow:
def readMem(memory):
global mdr
mdr = memory[mar]
def writeMem(memory):
memory[mar] = mdr
def fetch(memory):
global pc, mar
mar = pc
pc = pc + 1
readMem(memory)
2.2 JavaandVirtualMachine
Manysystemscombineaspectsofbothcompilersandinterpreters.Anotableexampleis
JavaandthesimilarapproachtakenbytheMicrosoft.netlanguagefamily.
JavaisacompiledlanguagebutitisnotcompiledforrealCPUs.Instead,thecompiled
codeisforaJavaVirtualMachine(JVM).AstherearenorealJVMCPUs,theyare
emulated.Thisapproachhasmanyadvantages:forexample,onlyonecompiledversion
ofaprogramisneededanditcanberunonanymachinewithanemulator,butitis
muchfasterthanapureinterpreter.
3 Appendix1:LMCVersions
LittleManComputerisawidelyusedsimulatorofa(verysimple)computer.Therearea
numberofimplementations.
1. Anexcellentversioninjavascript:
a. Availablefromhttp://peterhigginson.co.uk/LMC/
b. Ithasadequatedocumentationat
http://peterhigginson.co.uk/LMC/help.html
c. TheMDRisnotvisible.
2. AMSWindowsversion,requiring.NETandfunctionallythesameas1.Excellent
ifyoucanrunit.
a. Availablefromhttp://www.gcsecomputing.org.uk/lmc/lmc.html
3. Awebapplet,usingJava,withinstructionsandexamples(seehomepage)from
YorkUniversityinCanada.
a. Homepage:http://www.yorku.ca/sychen/research/LMC/
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b. Webapplet:http://www.yorku.ca/sychen/research/LMC/LittleMan.html
c. Theappletisgoodifitwillruninyourbrowser;Ifindthatonlyonedigit
ofthememoryisdisplayed.Theword‘compile’isusedinsteadof
assemble.Increasingincompatiblewithmodernbrowsers.
4.
5.
6.
7.
AnotherJavaversionfromtheUniversityofMinnesota;alsoavailableasaweb
applet:
a. Seefromhttp://www.d.umn.edu/~gshute/cs3011/LMC.html
b. Aminorissue:thisversiondoesnoshowtheMARandMDRregistersand
doesnotsimulatetheseparatestagesofthefetch-executecycle.
AversionfromDurhamUniversityforMacorWindows
a. Seehttp://www.dur.ac.uk/m.j.r.bordewich/LMC.html
b. Thisversioncallsthe‘assembler’a‘compiler’whichisunfortunate,butit
isotherwisegood.
Aspreadsheetversion
a. http://www.ictcool.com/2011/12/16/download-lmc-simulation-v-1-5-2requires-microsoft-excel/
b. Thisversiondoesnotincludeanassembler:youcanenterthe3-digit
codesinstead.
Aflashversion
a. AvailableasaCASresourcefrom
http://community.computingatschool.org.uk/resources/1383
b. Thisversionismainlyusedfordemonstrationratherthanprogramming;
thereisnoassembler.Thewebpageisuseful.
ThereisalsoaninformativeWikipediapage
http://en.wikipedia.org/wiki/Little_man_computer
Appletversion Javaversion
Enter
program
here
Enter
program
here
Importantnote:theappletusesthemnemonic'STA'whereastheJavaversionuses
'STO'forstoreaccumulator.
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