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AmnotabletocallC++functionpointersfrominlineassembly
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2yearsago
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Duetosomecuriosity,IwastryingtousesomeassemblywithmyArduinoMEGA2560.
Iamnotabletoinvokeafunctionfromac++functionpointerarray.
WhenIuncommentthecallfunction,func_aruns.However,whenIalmostreproducetheassembly,itdoesntwork.
Example:
extern"C"void__attribute__((used,noinline,noreturn))func_a();
typedefvoid(*volatilefunc_ptr)();
func_ptrtasks[MAX_TASKS]={&func_a};
extern"C"
void__attribute__((used,noinline))call(func_ptr*ptr){
(*ptr)();
}
void__attribute__((noreturn,used))setup(){
//call(tasks);
asmvolatile(
"ldsr26,(tasks)\n"
"ldsr27,(tasks+1)\n"
"ldr30,X+\n"
"ldr31,X\n"
"eijmp\n"
);
}
Generatedassemblyrecoveredfromavr-objdump
000001c6:
1c6:a0910002ldsr26,0x0200;0x800200
1ca:b0910102ldsr27,0x0201;0x800201
1ce:ed91ldr30,X+
1d0:fc91ldr31,X
1d2:1994eijmp
1d4:0895ret
000001d6:
1d6:dc01movwr26,r24
1d8:ed91ldr30,X+
1da:fc91ldr31,X
1dc:1994eijmp
AnspecificresourcesIshouldreferforthingslikethis?
Ifitmatters,I'musingplatformioonalinuxsystemforcompilation.
c++atmega2560assembly
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askedJun18,2020at10:48
darkspinedarkspine
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3
Itseemsyougotconfusedbythepointerindirections,whichis
confoundedbytheimplicitindirectionsmadebythecompiler.
Here:
extern"C"
void__attribute__((used,noinline))call(func_ptr*ptr){
(*ptr)();
}
Theparameterptrisnotafunctionpointer:it'sapointertoa
functionpointer.Youhavetodereferenceittwiceinordertogetthe
function:(**ptr)();.Notethatthecompilerimplicitlydereferencesa
functionpointerifyoucallitlikeafunction:that'swhyyougetno
errorbyexplicitlydereferencingthepointeronlyonce.Inthe
generatedassembly,thelines
ldr30,X+
ldr31,X
dereferenceptrinordertogettheaddressofthefunctiontocall.
Youprobablywantedtowrite
extern"C"
void__attribute__((used,noinline))call(func_ptrptr){
ptr();
}
Andthenuseitas:
call(tasks[0]);
Ifyoujustcall(tasks),thearrayidentifierdecaystopointer,and
thecompilerhandlesthisasashortcutforcall(&tasks[0]);,which
turnsouttobecompatiblewithyourinitialimplementationofcall().
Now,inyourinlineassemblyyouhave
ldsr26,(tasks)
ldsr27,(tasks+1)
Theassemblytreatstasksasasymbolrepresentingtheaddressof
thetasksarray.Thelinkerreplacesthosesymbolsbytheactual
address(0x0200).ThoseinstructionsreadtheRAMatthisandthe
followingaddress.TheseslotsofRAMholdthefirstitemofthearray,
namely&func_a.SonowyouhavethisaddressintheXregisterpair.
NoneedtoreadRAMagain:youcanmovwr30,r26anddotheindirect
jump.Or,betteryet,loadintoZtobeginwith:
ldsr30,(tasks)
ldsr31,(tasks+1)
eijmp
SeealsoMajenko'sanswer:hereIamassumingEINDissomehow
correctlysetuptobeginwith.OnanUno,youwouldjustuseijmp
insteadofeijmpandnotworryaboutEIND.
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answeredJun18,2020at13:17
EdgarBonetEdgarBonet
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Thatkindofeplainedthings.WhenIused(tasks)intheasm,Iexpectedittoloadtheaddressofthefirstelementofthearrayandnotthevaluepointedtobyit.Imnotsurehowtodothat.MaybeIwillhavetounderstandtheextendedasmsyntax.
– darkspine
Jun19,2020at5:18
@darkspine:Youcanloadtheaddressoftaskswith,e.g.,ldir26,lo8(tasks)\nldir27,hi8(tasks).Unlikelds,theldiinstructiondoesnotaccesstheRAM,astheaddressisknownatbuildtimeandfilled-inbythelinker.
– EdgarBonet
Jun19,2020at7:33
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IwillstartoffbysayingthatIamnoexpertinAVRassembly,soIhavenoideaifanyofthisisactuallycorrect.HoweverfromreadingthevariousdocumentsonthesubjectIhavegleaned:
OntheATMega2560EIJMPusesacombinationofboththeZregisterandtheEINDregistertoformafulladdress.
ThatmeansthataswellassettinguptheZregisterforthelower16bitsoftheaddressyoualsohavetosetuptheEINDregisterfortheupper5bitsoftheaddress.
IndirectjumptotheaddresspointedtobytheZ(16bits)PointerRegisterintheRegisterFileandtheEINDRegisterintheI/Ospace.Thisinstructionallowsforindirectjumpstotheentire4M(words)Programmemoryspace.
Itthengoesontoshowanexample:
ldir16,$05;SetupEINDandZ-pointer
outEIND,r16
ldir30,$00
ldir31,$10
eijmp;Jumpto$051000
HowyoudeterminewhattheEINDregistershouldbe?Well,ingeneralyoudon't.Youwouldn'tuseEIJMP.FromwhatIunderstandforfunctionsoutsidethelower64kwordsofaddressspacethecompilerbuildsa"jumptable"whichisstoredinthelowerareaofmemory.YoujumptotheentryinthattablewithIJMPandthattablethenjumpstothefinaldestinationwithEIJMPforyou.
Doingthiskindofjumpinginassemblyisn'treallyanicethingtotryandfigureout.It'smessy.BettertoletthecompilerdoitforyouanduseCratherthanassembly.TheAVR-GCCguysputalotofworkintomakingfunctionpointersworkcorrectly(fromwhatIhavereaditwasn'teasyevenforthem),andyou'rejustre-inventingthewheel.
Attheveryleastyoushouldimplementyourcode100%inCtobeginwiththenlookatthecompiledoutputtoseehowthecompilerthinksitshouldbedone.
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answeredJun18,2020at12:58
Majenko♦Majenko
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Iswitchedtoeijmpbecausethatswhatgccwasusing.Theproblemturnedouttobethattasksisdereferencedwhenusingitintheasm.Also,thanksforthedocument,ItsmuchmoreverbosethantheATMega2560datasheet.
– darkspine
Jun19,2020at5:20
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