中斷服務程序對編譯器來說首要需要考慮的就是現場寄存器的入棧保護。也就是說，一段在主程序內正在運行的代碼，向R1內寫了0xFF做為中間變量。而此時，突然來了中斷，那么為了保護原有數據不被破壞，就必須將原R1壓入堆棧。而PUSH動作是要花費指令周期去執行的，為了提高代碼執行速度，若一開始就規劃好主程序使用第0和第1組寄存器，而中斷服務程序則指定為第2組。那么，編譯器在編譯的時候就可以做優化，不必再執行原PUSH動作（應為兩組寄存器的物理地址不同）。
所以，上述使用分組寄存器的辦法是一種代碼優化手段。
下附KEIL的HELP文檔，作為參考。
//-----------------------------------------

RegisterBanks
Inallmembersofthe8051family,thefirst32bytesofDATAmemory(0x00-0x1F)isgroupedinto4banksof8registerseach.ProgramsaccesstheseregistersasR0-R7.Theregisterbankisselectedbytwobitsoftheprogramstatusword,PSW.

Registerbanksareusefulwhenprocessinginterruptsorwhenusingareal-timeoperatingsystembecausetheMCUcanswitchtoadifferentregisterbankforataskorinterruptratherthansavingall8registersonthestack.TheMCUcanthenrestoreswitchbacktotheoriginalregisterbankbeforereturning.

Theusingfunctionattributespecifiestheregisterbankafunctionuses.Forexample:

voidrb_function(void)using3
{
.
.
.
}

Theargumentfortheusingattributeisanintegerconstantfrom0-3.Expressionswithoperatorsarenotallowed.Theusingattributeisnotallowedinfunctionprototypes.Theusingattributeaffectstheobjectcodeofthefunctionasfollows:

Thecurrentlyselectedregisterbankissavedonthestackatfunctionentry.
Thespecifiedregisterbankisset.
Theformerregisterbankisrestoredbeforethefunctionisexited.
Thefollowingexampleshowshowtospecifytheusingfunctionattributeandwhatthegeneratedassemblycodeforthefunctionentryandexitlookslike.

stmtlevelsource

1
2externbitalarm;
3intalarm_count;
4externvoidalfunc(bitb0);
5
6voidfalarm(void)using3{
71alarm_count++;
81alfunc(alarm=1);
91}


ASSEMBLYLISTINGOFGENERATEDOBJECTCODE

;FUNCTIONfalarm(BEGIN)
0000C0D0PUSHPSW
000275D018MOVPSW,#018H
;SOURCELINE#6
;SOURCELINE#7
00050500RINCalarm_count+01H
0007E500RMOVA,alarm_count+01H
00097002JNZ?C0002
000B0500RINCalarm_count
000D?C0002:
;SOURCELINE#8
000DD3SETBC
000E9200EMOValarm,C
00109200EMOV?alfunc?BIT,C
0012120000ELCALLalfunc
;SOURCELINE#9
0015D0D0POPPSW
001722RET
;FUNCTIONfalarm(END)

Inthepreviousexample,thecodestartingatoffset0000hsavestheinitialPSWonthestackandsetsthenewregisterbank.Thecodestartingatoffset0015hrestorestheoriginalregisterbankbypoppingtheoriginalPSWfromthestack.

Note

Theusingattributemaynotbeusedinfunctionsthatreturnavalueinregisters.Youmustexerciseextremecaretoensurethatregisterbankswitchesareperformedonlyincarefullycontrolledareas.Failuretodosomayyieldincorrectfunctionresults.Evenwhenyouusethesameregisterbank,functionsdeclaredwiththeusingattributecannotreturnabitvalue.
Theusingattributeismostusefulininterruptfunctions.Usuallyadifferentregisterbankisspecifiedforeachinterruptprioritylevel.Therefore,youcouldassignoneregisterbankforallnon-interruptcode,asecondregisterbankforthehigh-levelinterrupt,andathirdregisterbankforthelow-levelinterrupt.

CopyrightKeil,AnARMCompany.Allrightsreserved.

假定你跟你老板共用一張辦公桌,每次你老板來了你就得讓開桌子給他用,等他用完,桌上原來的擺放已經亂七八糟了.

高人出了個主意,桌子仍共用,但桌面上的板子可整塊拆走,老板來了,換另一塊桌板就好了,等他走了,把你的板搬回來.

51這張桌子上一共配了4張桌板,使用usingn來拆換編號為n的桌板.