openbsd mbr汇编语言注释

.file#include<machine/asm.h>#include<assym.h>#define /*segmentwhereweareload

二阶段的启动程序（在OpenBSD上对应二进制文件为biosboot，它会被写在活dcateto*/down*/tes*/dcateto*/down*/tes*/ $c,%al;callLchr $s,%si;callLmessage#defineBOOTRELOCSEG /*segmentwherewe#define /*stackstartshere,#define /*eachpartitiontableentryis16#defineCHAR_LBA_READ'.'#defineCHAR_CHS_READ#define /*UseBEL#defineMBR_FLAGS_ _CHS0x0001#ifdefDEBUG#defineCHAR_S /*started*/#defineCHAR_R /*relocated*/#defineCHAR_L /*lookingforbootablepartition*/#defineCHAR_B /*loadingboot*/#define 'G'/* toboot*/#defineDBGMSG(c) $c,%al;callLchr#else/*!DEBUG*/#defineDBGMSG(c)#endif/*!DEBUG

要加载PBR将其第一个扇区的内容读出并到内存的0x7c00MBR到这个位置，所以它首先要把自己到其它地方（0x7a00512），把内存中从0x7c00512PBR。把自 到0x7a00PBRAT&T汇编风格直接数前使用内mov$1,%eax1存入eax寄存器------------------------------------------------------------------------- ax（不加moveax11eax）PCPCX86LowMemory(thefirst| | | | || |1|256|RAM ||||RAM-e(see||480.5|RAM(guarant|YourOS||| |512||| foruse,ifit||0x0009FFFF|1|RAM|| |384||(originallyonafloppyexecutionstartsatthe

/*PC启动时处于实模式，16位/*程 ，如同C语言中的main()

/*原注释。关于内存的布局Memory0x07E00->0x17BFCourstack(toschemetopartitiondisksintomultipleoperatingbootsectorsoftheofcourseexpecttobe

G,G,

此三行代码的功能为设置栈%ss:%sp****0x07C00->0x07DFFweloadPBRhere(at*OFF, *0x07A00,seg:offset*EG, /*BytesinMBR,relocateitall %cs, %cs, %ax,设置若设置了DEBUG，此行打印S设置

loadedatThefirstsectoroftheharddiskisthemasterbootrecord(MBR).sayswhichoneis

%ax, %ax, %si,

movsb

/*设置%cs0x07c0.BIOSMBRat0000:7C00,theloadthepartitionbootbeforeloadingthePBR.

%di,0x200= ljmp

将%si,%di $0x200,cldflagDF $0x200,

0x07c0:00000000:7c00。这两种表示方式中%cs和%ipljmp来确保%cs:%ip的值为/*ljmp语法ljmp的label（ ）1分配的内存地址$1f表示从此语句往下找，第一个名称为1 的地址。f表示查找方Makeit

es:di %ds %ds/*and%dsisatthe%cs %es/*nextbootisatthe ceaswewereloadedmov指令不可以将数据从一个内存地址movsb表示每次移动

%es=%ds=%cs=/*Signalthatorbwasseen_CHS,hiftSHIFTSHIFT键是否被按下。以BIOS16此时需要设置%ah为2，返回 在%al中。按SHIFT表示强制以CHSfourseparatepartitionswasused,toallowmultiplesystemstobeinstalledontheonedisk.ThebootsectorsofoperatingsystemsoneachpartitionwouldofcourseexpecttoloadedatThefirstsectoroftheharddiskisthemasterbootrecordsamelocationwheretheMBRneedstoloadthepartitionrecord(PBR,calledbiosbootinTherefore,theMBRneedstorelocateitselfbeforeloadingtheMakeit testb$0x80,%dljnzdrive_ok

个操作数进行testb运算时会跳转到do_chs

BIOS在跳转到0x7c00地址之前会把启动的磁盘信息存入%dl寄存器。0x80表示第一个磁盘，0x81表示第二个 $0x80,$pt, $0x80,$pt, $0x02, $0x02,%ahint$0x16testb$0x3,%al /*Eithershiftkeydown?*/ #ifdef %ax, $0xe3, $SERIAL, int (%si),%al

定义了OpenBSD 的默这里用$ptptpttestb:Performsabit-wiselogicalANDofthetwooperands.Theresultofabit-wiselogicalANDis1ifthevalueofthatbitinbothoperandsis1;otherwise,theresultis0.testdiscardstheresultsandmodifiestheflags.TheOFandCFflagsarecleared;SF,ZFandPFflagsaresetaccordingtotheresult.cmpb:Subtractsoperand1fromoperand2,butdoesnotstoretheresult;onlychangesthe

loopfind_activecall

Loop把%cx $enoboot,至%cx的值为0.每次循环都把%si $enoboot,Je-jumpifequal(ZF=Jnz-jumpifnotzero(ZF=Jc-jumpifcarryflagisset(CF= %dl, andb$0x0F, addb$'0', %al, $'0'+4, subb%cl, %al, $info,

/*EnsureCtl-Alt-Delwillwork /*(don'trequirepowercycle) jmpNoactivepartition，然后系统就停在那里（可以ctrl-Alt-Delete）来重启以十进制保存磁盘号和分区号。用于系统启动时显示“Using$'0'表示字符0，而非数字0，$'0'等于0的ASCII码00110000(48) 指向启动时要显示的信息（Usingdrive……）,将它的地址载入%si示在终端上。如果定义了串行端口的话就用int0x14 _CHS,flagsjnz1f _CHS,flagsjnzdo_chs这里以及下面会使用BIOS的中这里以及下面会使用BIOS的中LBA方式来读，如果不能就跳转到do_chs 处，以CHS方式 寄存器可参看int0x13的说明 $0xAA55,/*Checkthatbl,bhexchanged*/jnedo_chs/*Ifnot,don'thaveEDDextensions*/testb$0x01,%cl/*Anddowehave"read"available? /*Didthecommandwork?Jumpifnot %dl,(%si) /*Storedriveheretemporarily*/ $0x55AA,%bx $0x41, int (%si), /*Getbackdrivenumber call $0,itworked,itworked,runthepbrwegot $CHAR_LBA_READ, call利用INT0x13以 8(%si), %ecx, /*We'llneed%silater $0x42, int /*(getback%si)flagsunchangedjnc/*若LBA方式无 ，尝试 $CHAR_CHS_READ, call利用INT0x13以 磁 1(%si),%dh /*head*/ 2(%si),%cx /*sect,cyl*/ $0x201, /*functionandnumberofblocks %bx, /*putitat%es:0 int $mand, /*Again,useCHSifnot $DOSMBR_SIGNATURE, jne

MBR跳转，将控制权交给PBR代码跳转到err_stop,同上跳转到err_stop,同上 $eread, jmpjnc $enoos, jmpljmp$0,$BOOTSEG<<转到err_stop,同上lods:lods:loadES:[(E)DI]->LoadsthememorybyteorwordaddressedinthedestinationregisterintotheAL,AX,orEAXregister.Beforeexecutingthelodsinstruction,loadtheindexvaluesintotheSIsource-indexregister.Afterthedataisloaded,thesource-indexregisterisautomaticallyincrementedordecrementedasdeterminedbythevalueofthedirectionflag(DF).TheindexregisterisincrementedifDF=0(DFclearedbyacldinstruction);itisdecrementedifDF=1(DFsetbyastdinstruction).Theincrement/decrementcountis1forabytemove,2foraword,and4foralong.ForablocktransferofCXbytes,wordsorlongs,precedealodsinstructionwitharepprefix;however,lodsisusedmoretypicallywithinaloopconstructwherefurtherprocessingofthedatamovedintoAL,AX,orEAXisusuallyrequired testb%al,%al jmp1b #ifdef

/*%al=*%si++

$0x01,RIAL,RIAL,0e,

INT10h,orINT16isshorthandforBIOSinterruptcall10hex,the17th int int

vectorinanx86-basedcomputersystem.TheBIOStypicallysetsuparealinterrupthandleratthisvectorthatprovides services.Suchservicesincludesettingthe mode,characterandstringoutput,andgraphicsprimitives(readingandwritingpixelsingraphicsmode).Tousethiscall,loadAHwiththenumberofthedesiredsubfunction,loadotherrequiredparametersinotherregisters,andmakethecall.INT10hisfairlyslow,somanyprogramsbypassthisBIOSroutineandaccessthedisyhardwaredirectly.Settingthemode,whichisdoneinfrequently,canbeplishedbyusingtheBIOS,whiledrawinggraphicsonthescreeninagameneedstobedonequickly,sodirectaccesstoRAMismoreappropriatethanmakingaBIOScallforeverypixel. /*sizeofcommandpacket /*sectorstotransfer, /*sectorstotransfer,just1 /*targetbuffer,offset /*targetbuffer,segment 0, /*sectornumber "!Usingdrive" ",partition"

"MBRonfloppyoroldBIOS