CS2630ComputerOrganization

Meeting6:functionsinMIPSBrandonMyers

UniversityofIowa

http://qz.com/726338/the-code-that-took-america-to-the-moon-was-just-published-to-github-and-its-like-a-1960s-time-capsule/

lotsmoreinfo:http://tcf.pages.tcnj.edu/files/2013/12/Apollo-Guidance-Computer-2009.pdf

Wherewearegoing

Instructionsetarchitecture(e.g.,MIPS)

Compiler

Memorysystem I/OsystemProcessor

Datapath &Control

Digitallogic

translatingsourcecode(CorJava)ProgramstoassemblylanguageAndlinkingyourcodetoLibrarycode

HowthesoftwaretalksTothehardware

HowaprocessorrunsMIPSPrograms!

Howswitches(1or0)canbeusedtobuildInterestingfunctions:fromintegerarithmetictoprogrammablecomputers

Representationsofaprogram

Compiler

int x = arr[1];arr[2] = x + 10;

lw $t0, 4($r0)addi $t0, $t0, 10sw $t0, 8($r0)

Assembler

100011100000100000000000000001000010000100001000000000000000101010101110000010000000000000001000

Highlevellanguageprogram(humanreadable)

assemblyprogramastext(CS2630studentreadable)

assemblyprogramasbinary(machinereadable)

Storinganinstructionasbits• eachMIPSinstructionsis32bits,storedin“instructionmemory”

8e 13 00 00

8e 14 00 04

02 74 90 20

ae 12 00 08

22 10 00 04

22 31 ff ff

1e 20 ff f9

000000040008000C001000140018

lw $s3, 0($s0)

lw $s4, 4($s0)

add $s2, $s3, $s4

sw $s2, 8($s0)

addi $s0, $s0, 4

addi $s1, $s1, -1

bgtz $s1, loop

address contents meaning

PseudoinstructionsNotalltheMIPSinstructionsare“real”(i.e.,understoodbytheprocessor)TAL – thetrueassemblylanguage;MIPSprocessorsknowalloftheseinstructionsMAL – slightlymoreconvenientlanguage;eachMALinstructionisusuallytranslatedto1-3TALinstructionsbytheassembler

Pseudoinstructions(MAL)

lui $t2, 0x0012ori $t3, $t3, 0x3456addu $t1, $t2, $t3

assemblertranslatesitto

Example:howmanybitsarerequiredfortheimmediateinthisinstruction?addiu $t1, $t2, 0x123456

lui =“loadupperimmediate”

Morepseudoinstructions(MAL)• otherI-typeinstructionswithtoo-largeimmediate• move $rd, $rs• li $rd,[32-bitimmediate]• la $rd,[32-bitimmediate]• branchinstructionswith,=

• reallyuseslt (setiflessthan),followedbybeq to0or1

• Foryoutotry:usetheseinstructionsinaMIPSprogram,thenseewhatMARStranslatesthemto

Peerinstruction• Putthefollowinginto• (A)onlyinMAL• (B)TAL

Whataboutprocedures?• Firsttry:usethenameoftheprocedureasalabelandjustjumptoit

• Whataresomelimitationsofthisproposal?(freeresponsewithclickers)

Functioncalls:only 6steps1. storeargumentssomewherethecallee codecanaccess

them2. savecurrentlocationandthenjumptofunction3. allocatelocalstorageneeded(savingregisters)for

runningthefunction4. performthecodeinthefunction5. storeresultsomewherethecaller codecanaccessitand

restoresavedregisters6. jumpbacktolocationofthefunctioncall

MIPSregistersusedforfunctioncallsRegisterNumber ConventionalName Usage$0 $zero hard-wiredto0

$1 $at reservedforassembler

$2- $3 $v0,$v1 returnvaluesfromfunctions

$4- $7 $a0- $a3 arguments

$8- $15 $t0- $t7 temporary registers

$16- $23 $s0- $s7 savedregisters

$24- $25 $t8- $t9 temporary registers

$26- $27 $k0- $k1 reservedforOS

$28 $gp globalpointer$29 $sp stackPointer$30 $fp framePointer$31 $ra returnAddress

Administrivia• reminder:HW2duetonight11:59pm• Midtermscheduled

• inclassonOctober6

Secondtry:allowmultiplecallsites bysavingreturnaddressin$raint mult_by_2(int x) {

return x*2;}…int z = mult_by_2(5);

reference:$ra returnaddress$v0 returnvalue$a0 argument

Convenienttip:usejal tosave$ra foryou!int mult_by_2(int x) {

return x*2;}…int z = mult_by_2(5);

reference:$ra returnaddress$v0 returnvalue$a0 argument

Problem:nomodularity!everyprocedureneedstoknowwhatregistersarebeingusedbytherestoftheprogramL

Conventiontotherescue!RegisterNumber ConventionalName Usage$0 $zero hard-wiredto0

$1 $at reservedforassembler

$2- $3 $v0,$v1 returnvaluesfromfunctions

$4- $7 $a0- $a3 arguments

$8- $15 $t0- $t7 temporary registers

$16- $23 $s0- $s7 savedregisters

$24- $25 $t8- $t9 temporary registers

$26- $27 $k0- $k1 reservedforOS

$28 $gp globalpointer$29 $sp stackPointer$30 $fp framePointer$31 $ra returnAddress

temporary registersmighthaveadifferentcontentsafteraprocedurecallastheydidbefore

saved registershavethesamecontentsafteraprocedurecallastheydidbefore

Treatsavedregisterslikeyoutreatacampground

Before LaborDayWeekend

After LaborDayWeekend

DuringLaborDayWeekend

Howtocleanupyourcampground• Beforeyouuseasavedregister,saveitsvalue.• Whenyouarefinished,restorethevalue.

howdowesaveandrestoreregisters?

Callingprocedureswithinprocedures• Makesureoneoftheregistersyousave/restoreis$ra!

• Refertothe“preservedacrossacall”columnoftheREGISTERtableontheMIPSreferencecard

• “No”meansthecallerneedstosaveitiftheywantitlater

• “Yes”meansthecallee needsto“cleanthecampground”beforereturning

Peerinstruction• Whatmust besavedbeforetheprocedurerunsandrestoredbeforeitreturns?

multipleanswera) $t0b) $a0c) $zerod) $s0e) $s1f) $s2g) $rah) $v0

Memoryorganizationofprograms

local variables, return addressesRW

dynamically allocated memory like Java objectsRW

globaldata(initializedwhenprocessstarts)RW

assembledcode(initializedwhenprocessstarts)RX legend:

R=readableW=writeableX=executable

Thestack• Thecall-stack(orjuststack)hasonepart,or“frame”,foreachactiveprocedure

• anactiveprocedureisaJavamethod,Pythonfunction,etc thathasnotyetreturned

• Theframestoresthestateneededbytheprocedurecall,includingreturnaddress,localvariablesthatdon’tfitinregisters,andsavedregisters

• InMIPS,weuseregister$sp (stackpointer)tostoretheaddressofthecurrentframe

Example:usingthestack

yoo frame

who frame

amI frame

amI frame

amI frameprocedureamI isrecursive(callsitself)

stack

Example:usingthestackinMIPS

Templateforaproceduredefinition