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[求助]求解答一下杨季文老师黑皮书第10章保护模式T10-5.asm看不懂的几个地方，多谢
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2019-9-18 16:13
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首先，所贴的代码用masm5.0或者tasm2.0均可以编译生成t10-5.obj，然后再用tlink.exe /3 t10-5.obj可以正确生成exe文件。
问题都在代码t10-5.asm的注释区用“看不懂的地方”标出来了 ，为了便于理解，我把书上对这个例子的说明也贴出来，这样方便大家快速了解整个代码，把时间集中在我提的几个问题上。
;T10-5.ASM ;演示任务切换和任务内特权级变换 include 386SCD.ASM .386P ;全局描述符表 GDTSEG SEGMENT PARA USE16 GDT LABEL BYTE dummy descriptor <> normal descriptor <0ffffh,0,0,atdw,0> normal_sel=normal-gdt effgdt label byte demotss descriptor <demotsslen-1,demotssseg,,at386tss,> demotss_sel=demotss-gdt demoldtab descriptor <demoldtlen-1,demoldtseg,,atldt,> demoldt_sel=demoldtab-gdt ;临时任务的任务状态段描述符 temptss descriptor <temptsslen-1,temptssseg,,at386tss+dpl2,> temptss_sel=temptss-gdt ;临时任务代码段 tempcode descriptor <0ffffh,tempcodeseg,,atce,> tempcode_sel=tempcode-gdt ;子程序代码段描述符 subr descriptor <subrlen-1,subrseg,,atce+d32,> subr_sel=subr-gdt+rpl3 videobuff descriptor <0ffffh,0,0,0f00h+atdw+dpl3,0> video_sel=videobuff-gdt gdnum=($-effgdt)/(size descriptor) gdtlen=$-gdt gdtseg ends ;演示任务的任务状态段 DemoTSSSEG segment para use16 dd 0 ;链接字 dd DemoStack0LEN ;0级堆栈指针 dw DemoStack0_SEL,0 dd 0 ;1级堆栈指针 dw ?,0 dd DemoStack2LEN ;2级堆栈指针 DW Demostack2_sel,0 dd 0 ;cr3 dw demobegin,0 ;eip dd 0 ;eflags dd 0 ;eax dd 0 ;ecx dd 0 ;edx dd 0 ;ebx dd DemoStack2LEN ;esp dd 0 ;ebp dd 0 ;esi dd 0B8000H ;edi dw video_sel,0 ;es dw DemoCode_sel,0 ;cs dw DemoStack2_sel,0 ;ss dw DemoData_sel,0 ;ds dw ToDLDT_SEL,0 ;fs dw ToTTSS_SEL,0 ;gs dw DemoLDT_SEL,0 ;ldtr dw 0 dw $+2 ;I/O许可位图指针 db 0ffh ;I/O许可位图结束字节 DemoTSSLEN=$ DemoTSSSEG ends ;演示任务的局部描述符表LDT DemoLDTSEG segment para use16 demoldt label byte demostack0 descriptor <demostack0len-1,demostack0seg,,atdw+d32,> demostack0_sel=(demostack0-demoldt)+til demostack2 descriptor <demostack2len-1,demostack2seg,,atdw+d32+dpl2,> demostack2_sel=(demostack2-demoldt)+til+rpl2 ;演示代码段描述符 democode descriptor <democodelen-1,democodeseg,,atce+d32+dpl2,> democode_sel=(democode-demoldt)+til+rpl2 ;演示数据段描述符 demodata descriptor <demodatalen-1,demodataseg,,atdw+d32+dpl3,> demodata_sel=(demodata-demoldt)+til ;把LDT作为普通数据段描述的描述符（DPL=2） todldt descriptor <demoldtlen-1,demoldtseg,,atdw+dpl2,> todldt_sel=(todldt-demoldt)+til ;把tss作为普通数据段描述的描述符（DPL=2） TOTTSS descriptor <temptsslen-1,temptssseg,,atdw+dpl2,> tottss_sel=(tottss-demoldt)+til demoLDNUM=($-demoldt)/(size descriptor) ;指向子程序subrb的调用门（dpl=3） tosubr gate <subrb,subr_sel,0,at386cgat+dpl3,0> tosubr_sel=(tosubr-demoldt)+til+rpl2 ;指向临时任务temp的任务门（DPL=3） totempt gate <0,temptss_sel,0,attaskgat+dpl3,0> totempt_sel=(totempt-demoldt)+til demoldtlen=$-demoldt demoldtseg ends ;演示任务的0级堆栈 DemoStack0SEG segment para use32 demostack0len=1024 db demostack0len dup(0) demostack0seg ends DemoStack2SEG segment para use32 demostack2len=512 db demostack2len dup(0) demostack2seg ends ;演示任务的数据段 DemoDataSEG segment para use32 message db 'Value=',0 demodatalen=$ demodataseg ends subrseg segment para use32 assume cs:subrseg subrb proc far push ebp mov ebp,esp pushad mov eax,[ebp+12] mov esi,eax mov ah,7 jmp short subr2 subr1: stosw subr2: lodsb or al,al jnz subr1 mov edx,[ebp+16] mov ecx,8 subr3: rol edx,4 mov al,dl call htoasc stosw loop subr3 popad pop ebp ret 8 subrb endp htoasc proc and al,0fh add al,90h daa adc al,40h daa ret htoasc endp subrlen=$ subrseg ends DemoCodeSEG segment para use32 assume cs:DemoCodeSEG DemoBegin: mov fs:tosubr.dcount,2 ;?看不懂的地方2，这里的段寄存器为啥是fs push dword ptr gs:temptask.treip ;?看不懂的地方3，这里的段寄存器为啥是gs push offset Message call32 tosubr_sel,0 ;这个子函数看得懂 assume ds:temptssseg push gs pop ds ;看不懂的地方4，为啥要push gs然后pop ds mov ax,normal_sel mov temptask.trds,ax mov temptask.tres,ax mov temptask.trfs,ax mov temptask.trgs,ax mov temptask.trss,ax jump32 totempt_sel,0 ;看不懂的地方5，这里的totempt_sel选择子对应的是段temptssseg，而段temptssseg是个没赋初值的结构 ;体，怎么跳转过去呢？ DemoCodeLEN=$ DemoCodeSEG ends temptssseg segment para use16 temptask taskss <> db 0ffh temptsslen=$ temptssseg ends tempcodeseg segment para use16 assume cs:tempcodeseg Virtual: mov BX,tempTSS_SEL LTR BX jump16 demotss_sel,0 ;这里进行了切换到任务DemoTSSSEG，因为EIP是demobegin，这里跳转到demobegin toreal: clts mov eax,cr0 and eax,0fffffffeh mov cr0,eax jump16 <seg real>,<offset real> tempcodelen=$ tempcodeseg ends RDataSEG segment para use16 VGDTR PDESC <GDTLEN-1,> SPVAR DW ? SSVAR DW ? RDataSEG ends RCodeSEG segment para use16 assume cs:rcodeseg,ds:rdataseg,es:rdataseg ;看不懂的地方1，以前没见到过一个段同时和两个段寄存器对应的代码 start: mov ax,Rdataseg mov ds,ax cld call init_gdt mov ax,DemoLDTSEG mov fs,ax mov cx,DemoLDNUM mov SI,OFFSET DemoLDT call init_ldt mov ssvar,ss mov spvar,sp lgdt qword ptr vgdtr cli mov eax,cr0 or eax,1 mov cr0,eax jump16 <tempCode_sel>,<offset virtual> Real: mov ax,RDataSEG mov ds,ax lss sp,dword ptr spvar sti mov ax,4c00h int 21h init_gdt proc near push ds mov ax,gdtseg mov ds,ax mov cx,gdnum mov si,offset effgdt initg: mov ax,[si].basel movzx eax,ax shl eax,4 shld edx,eax,16 mov [si].basel,ax mov [si].basem,dl mov [si].baseh,dh add si,size descriptor loop initg pop ds mov bx,16 mov ax,gdtseg mul bx mov word ptr vgdtr.base,ax mov word ptr vgdtr.base+2,dx ret init_gdt endp init_ldt proc ildt: mov ax,fs:[si].basel movzx eax,ax SHL eax,4 shld edx,eax,16 mov fs:[si].basel,ax mov fs:[si].basem,dl mov fs:[si].baseh,dh add si,size descriptor loop ildt ret init_ldt endp RCodeSEG ends end start
把386scd.asm也贴上来，供参考
;符号常量等的定义 ;存储段描述符/系统段描述符结构类型的定义 descriptor struc limitl dw 0 basel dw 0 basem db 0 attributes dw 0 baseh db 0 descriptor ends ;门描述符结构类型的定义 gate struc offsetl dw 0 selector dw 0 dcount db 0 gtype db 0 offseth dw 0 gate ends ;伪描述符结构类型的定义 pdesc struc limit dw 0 base dd 0 pdesc ends ;TSS结构类型的定义 taskss struc trlink dw ?,0 tresp0 dd ? trss0 dw ?,0 tresp1 dd ? trss1 dw ?,0 tresp2 dd ? trss2 dw ?,0 trcr3 dd ? treip dd ? treflag dw ?,? treax dd ? trecx dd ? tredx dd ? trebx dd ? tresp dd ? trebp dd ? tresi dd ? tredi dd ? tres dw ?,0 trcs dw ?,0 trss dw ?,0 trds dw ?,0 trfs dw ?,0 trgs dw ?,0 trldt dw ?,0 trflag dw 0 triomap dw $+2 taskss ends ;存储段描述符类型值说明 ATDR=90h ATDW=92H ATDWA=93H ATCE=98H ATCER=9AH ATCCO=9CH ;存在的只执行一致代码段类型值 ATCCOR=9EH ;系统描述符和门描述符类型值说明 ATLDT=82h ;局部描述符表段值类型 ATTASKGAT=85H AT386TSS=89H ;?386TSS类型值 AT386CGAT=8CH ;386调用门 AT386IGAT=8EH AT386TGAT=8FH ;DPL和RPL值说明 DPL1=20H DPL2=40H DPL3=60H RPL1=01H RPL2=02H RPL3=03H IOPL1=1000H IOPL2=2000H IOPL3=3000H ;其他常量值说明 D32=4000H ;在描述可执行段的描述符中，表示32位代码段 TIL=04H ;描述符表标志 VFML=0002H IFL=0200H ;32位偏移的段间转移宏指令 JUMP32 MACRO selector,offsetv DB 0eaH DW offsetv ;32位偏移 DW 0 DW selector ENDM CALL32 MACRO selector,offsetv db 09ah dw offsetv dw 0 dw selector endm JUMP16 MACRO selector,offsetv db 0eah dw offsetv dw selector endm call16 MACRO selector,offsetv db 9ah dw offsetv dw selector endm
再次感谢各位大大！
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最后于 2019-9-18 16:16
被lishua编辑
，原因： 对原贴中罗嗦的地方进行删除。
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