第三章 B代码

顺便解释一下Makefile

################################################### Makefile of pmtestx.asm (x=[1,2,3...])##################################################SRC:=pmtest2.asmBIN:=$(subst .asm,.com,$(SRC))#最终目的：将编译生成的文件复制到软盘镜像当中.PHONY : everythingeverything : $(BIN)sudo mount -o loop pm.img /mnt/floppy/sudo cp $(BIN) /mnt/floppy/ -vsudo umount /mnt/floppy/#编译源文件$(BIN) : $(SRC)nasm $< -o $@

源代码

;分析:; 共有6个节,分别为; 1.[SECTION .gdt] 定义了GDT,lgdt的操作数,对应GDT的选择子三部分数据; 2.[SECTION .data1] 定义了用于显示的数据; 3.[SECTION .gs] 使用times指令定义了堆栈大小,为512字节; 4.[SECTION .s16] 包含进入保护模式前和跳回实模式后的代码,16位代码; 5.[SECTION .s32] 包含进入保护模式后的代码,32位代码,有; TestRead ,读取es段开始的8位数据并显示到屏幕上,同时调用了DispAL函数和DisReturn函数; TestWrite ,将数据写入到es段当中; DispAL ,将AL显示到屏幕对应位置; DispReturn,屏幕换行; 6.[SECTION .s16code] 跳回实模式前的清理过程,32位代码;;%include "pm.inc"org 0100h jmp LABEL_BEGIN[SECTION .gdt] ;GDT节和选择子;GDT定义; 段基址, 段界限,属性LABEL_GDT: Descriptor 0, 0,0 ;空描述符LABEL_DESC_NORMAL:Descriptor 0, 0ffffh,DA_DRW ;Normal描述符LABEL_DESC_CODE32:Descriptor 0, SegCode32Len-1,DA_C+DA_32;32位代码段描述符LABEL_DESC_CODE16:Descriptor 0, 0ffffh,DA_C ;16位代码段描述符LABEL_DESC_DATA: Descriptor 0, DataLen-1,DA_DRW ;数据段描述符LABEL_DESC_STACK: Descriptor 0, TopOfStack,DA_DRW ;堆栈段描述符LABEL_DESC_TEST: Descriptor 0500000h, 0ffffh,DA_DRWLABEL_DESC_VIDEO: Descriptor 0B8000h, 0ffffh,DA_DRW ;显存段描述符;lgdt操作数GdtLen equ $ - LABEL_GDT ;GDT长度GdtPtr dw GdtLen - 1 ;GDT界限 dd 0 ;GDT基地址;GDT 选择子SelectorNormal equ LABEL_DESC_NORMAL - LABEL_GDTSelectorCode32 EQU LABEL_DESC_CODE32 - LABEL_GDTSelectorCode16 EQU LABEL_DESC_CODE16 - LABEL_GDTSelectorData EQU LABEL_DESC_DATA - LABEL_GDTSelectorStack EQU LABEL_DESC_STACK - LABEL_GDTSelectorTest EQU LABEL_DESC_TEST - LABEL_GDTSelectorVideo EQU LABEL_DESC_VIDEO - LABEL_GDT[SECTION .data1];数据节ALIGN 32[BITS 32]LABEL_DATA:SPValueInRealMode dw 0 PMMessage db "In Protect Mode now. ~_~",0 OffsetPMMessage equ PMMessage - $$StrTest: db "ABEDEFGHIJKLMNOPQRSTUVWXYZ",0OffsetStrTest equ StrTest - $$DataLen equ $ - LABEL_DATA[SECTION .gs] ;全局堆栈节ALIGN 32[BITS 32]LABEL_STACK: times 512 db 0TopOfStack equ $ - LABEL_STACK - 1[SECTION .s16];16位代码节,实模式代码[BITS16]LABEL_BEGIN:movax, csmovds, axmoves, axmovss, axmovsp, 0100hmov[LABEL_GO_BACK_TO_REAL+3], axmov[SPValueInRealMode], sp; 初始化 16 位代码段描述符movax, csmovzxeax, axshleax, 4addeax, LABEL_SEG_CODE16movword [LABEL_DESC_CODE16 + 2], axshreax, 16movbyte [LABEL_DESC_CODE16 + 4], almovbyte [LABEL_DESC_CODE16 + 7], ah; 初始化 32 位代码段描述符xoreax, eaxmovax, csshleax, 4addeax, LABEL_SEG_CODE32movword [LABEL_DESC_CODE32 + 2], axshreax, 16movbyte [LABEL_DESC_CODE32 + 4], almovbyte [LABEL_DESC_CODE32 + 7], ah; 初始化数据段描述符xoreax, eaxmovax, dsshleax, 4addeax, LABEL_DATAmovword [LABEL_DESC_DATA + 2], axshreax, 16movbyte [LABEL_DESC_DATA + 4], almovbyte [LABEL_DESC_DATA + 7], ah; 初始化堆栈段描述符xoreax, eaxmovax, dsshleax, 4addeax, LABEL_STACKmovword [LABEL_DESC_STACK + 2], axshreax, 16movbyte [LABEL_DESC_STACK + 4], almovbyte [LABEL_DESC_STACK + 7], ah; 为加载 GDTR 作准备xoreax, eaxmovax, dsshleax, 4addeax, LABEL_GDT; eax <- gdt 基地址movdword [GdtPtr + 2], eax; [GdtPtr + 2] <- gdt 基地址; 加载 GDTRlgdt[GdtPtr]; 关中断cli; 打开地址线A20inal, 92horal, 00000010bout92h, al; 准备切换到保护模式moveax, cr0oreax, 1movcr0, eax; 真正进入保护模式jmpdword SelectorCode32:0; 执行这一句会把 SelectorCode32 装入 cs, 并跳转到 Code32Selector:0 处;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;LABEL_REAL_ENTRY:; 从保护模式跳回到实模式就到了这里 ;重新设置段movax, csmovds, axmoves, axmovss, axmovsp, [SPValueInRealMode]inal, 92h; `.andal, 11111101b; | 关闭 A20 地址线out92h, al; /sti; 开中断movax, 4c00h; `.int21h; / 回到 DOS; END of [SECTION .s16][SECTION .s32]; 32 位代码段. 由实模式跳入.[BITS32]LABEL_SEG_CODE32:movax, SelectorDatamovds, ax; 数据段选择子movax, SelectorTestmoves, ax; 测试段选择子movax, SelectorVideomovgs, ax; 视频段选择子movax, SelectorStackmovss, ax; 堆栈段选择子movesp, TopOfStack; 下面显示一个字符串movah, 0Ch; 0000: 黑底 1100: 红字xoresi, esixoredi, edimovesi, OffsetPMMessage; 源数据偏移movedi, (80 * 10 + 0) * 2; 目的数据偏移。屏幕第 10 行, 第 0 列。cld.1:lodsb ;将PMMessage逐个送入al当中testal, al ;判断al的内容,若为0,则结束循环jz.2mov[gs:edi], ax ;将数据移至显存区addedi, 2jmp.1.2:; 显示完毕callDispReturncallTestReadcallTestWritecallTestRead; 到此停止jmpSelectorCode16:0; ------------------------------------------------------------------------;功能:读取es段起始地址开始8位的数据,并将其显示至屏幕TestRead:xoresi, esimovecx, 8.loop:moval, [es:esi]callDispALincesiloop.loopcallDispReturnret; TestRead 结束-----------------------------------------------------------; ------------------------------------------------------------------------;功能:将StrTest写到以es段起始的地址当中TestWrite:pushesipushedixoresi, esixoredi, edimovesi, OffsetStrTest; 源数据偏移cld.1:lodsbtestal, aljz.2mov[es:edi], alincedijmp.1.2:popedipopesiret; TestWrite 结束----------------------------------------------------------; ------------------------------------------------------------------------; 显示 AL 中的数字; 默认地:;数字已经存在 AL 中;edi 始终指向要显示的下一个字符的位置; 被改变的寄存器:;ax, edi; ------------------------------------------------------------------------DispAL:pushecxpushedxmovah, 0Ch; 0000: 黑底 1100: 红字movdl, alshral, 4movecx, 2.begin:andal, 01111bcmpal, 9ja.1addal, '0'jmp.2.1:subal, 0Ahaddal, 'A'.2:mov[gs:edi], axaddedi, 2moval, dlloop.beginaddedi, 2popedxpopecxret; DispAL 结束-------------------------------------------------------------; ------------------------------------------------------------------------;功能为换行DispReturn:pusheaxpushebxmoveax, edi ;edi为显存偏移,保存显存偏移至eaxmovbl, 160 divbl ;显存偏移/160,ah为显示屏列号,al为显示屏行号andeax, 0FFh ;仅保存行号inceax ;行号+1movbl, 160 mulbl ;行号*160重新转换成偏移movedi, eax ;将显存偏移存回edipopebxpopeaxret; DispReturn 结束---------------------------------------------------------SegCode32Lenequ$ - LABEL_SEG_CODE32; END of [SECTION .s32]; 16 位代码段. 由 32 位代码段跳入, 跳出后到实模式[SECTION .s16code]ALIGN32[BITS16]LABEL_SEG_CODE16:; 跳回实模式: ; 清除选择子数据movax, SelectorNormalmovds, axmoves, axmovfs, axmovgs, axmovss, ax ; 设置cr0,关闭保护模式moveax, cr0andal, 11111110bmovcr0, eaxLABEL_GO_BACK_TO_REAL:jmp0:LABEL_REAL_ENTRY; 段地址会在程序开始处被设置成正确的值Code16Lenequ$ - LABEL_SEG_CODE16; END of [SECTION .s16code]