一开始使用C编写进入PM代码，因为内嵌汇编太多，不易查询汇编如何出错，所以改用纯汇编编写。

 过程中学到一个小技巧，当赋值的数太大时，编译会报错，这时在数前加个0即可解决。

 程序编译完成，基本过程已然了解，但是在测试机上跑的时候会自动重启。

;;;;;;;;;;;;;;;;;;;程序开始;;;;;;;;;;;;;;;;;;  ;data struct    jump macro selector, offset  db 0eah  dw offset  dw selector  endm    desp struc  segLimit   dw 0; byte0, 1  segBaseLow dw 0; byte2, 3  segBaseMid db 0; byte4  segAttribute dw 0; byte5, 6  segBaseHigh db 0; byte7  desp ends    pgdt struc  segLimit dw 0ffffh; byte 0, 1  segBase  dd 0; byte 2, 3, 4, 5  pgdt ends      ;-------------------------------------------------------------    -------  ; real mode data segment begin  ;.data    .386p; if not p, lgdt is not defined  dseg segment use16   ; gdt segment begin  gdt label byte  DUMMY desp <>; 空描述符！！！  dataSegDesp desp <0ffffh,0h,0h,092h,> ; data segment is     started with: 0x00000000h  dataSegSel = dataSegDesp - gdt     codeSegDesp desp <0ffffh,,,098h,>  codeSegSel = codeSegDesp - gdt    extSegDesp desp <0ffffh,,88h,092h,> ; ext segment is started with: 0x00880000h    extSegSel = extSegDesp -  gdt  gdtLen = $ - gdt  pgdtr pgdt 
     
        ; gdt segment end    ; pmode data segment begin  buffers db 256 dup('@')  bufferLen = $ - offset buffers    msgLen = $ - offset msg  ; pmode data segment end    bufferd db 256 dup('y')  bufferd2 db 256 dup('x')  dseg ends  ; real mode data segment end    ;-------------------------------------------------------------    -------  cseg segment use16  ;.code  assume cs:cseg, ds:dseg    start:  MOV ax, dseg  MOV ds, ax    ;prepare to jump to pmode  ;1>. init gdtr  ;2>. init code descriptor  ;3>. init data descriptor  ;4>. enable a20 address  ;5>. set cr0  ;6>. jump pmode    ;1>. init gdtr  MOV bx, 16  MUL bx  ADD ax, offset gdt  ADC dx, 0  MOV WORD PTR pgdtr.segBase, ax  MOV WORD PTR pgdtr.segBase+2, dx    ;2>. init code descriptor  MOV ax, cs  MUL bx  MOV codeSegDesp.segBaseLow, ax  MOV codeSegDesp.segBaseMid, dl  MOV codeSegDesp.segBaseHigh, dh    ;3>. init data descriptor  MOV ax, ds  MUL bx  ADD ax, offset buffers  adc dx, 0  MOV dataSegDesp.segBaseLow, ax  MOV dataSegDesp.segBaseMid, dl  MOV dataSegDesp.segBaseHigh, dh  lgdt pgdtr    ;4>. enable A20 address  cli  ;call enableA20  PUSH ax  in al, 92h  or al, 2  out 92h, al  POP ax    ;5>. set cr0  MOV eax, cr0  or eax, 1  MOV cr0, eax    ;6>. jump to pmode  ;JMP pmode  jump 
      
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            pmode:  MOV ax, dataSegSel  MOV ds, ax  MOV ax, extSegSel  MOV es, ax    ;;;;;;;访问SPI空间的内容;;;;;;;;;  mov ebx,[0fed1f800h]  mov ds:[0000],ebx  mov dx,0  mov ah,9  int 21h  ;;;;;;;;;;;;;;;;    MOV eax, cr0  and eax, 0fffffffeh  MOV cr0, eax  JMP rmode    rmode:  call disableA20  sti    MOV ax, dseg  MOV ds, ax    disableA20 proc  PUSH ax  in al, 92h  and al, 0fdh  out 92h, al  POP ax  RET  disableA20 endp    MOV ah, 4ch  int 21h  cseg ends  END start