extend.c

来自「windows汇编语言程序设计 吴中平著 一书所附所有源代码。 该书重点讲解」· C语言 代码 · 共 75 行

#include "bios.h"
#include "dos.h"
#include "stdio.h"
#define ATTDW 0x92
#define ATTDR 0x90
#define SELECTOR_1 8
#define SELECTOR_2 16
#define GDTNUM 3
asm .386p
struct descriptor
{
   int  limitl;
   int  basel;
   char basem;
   int attr;
   char baseh;
};
struct pdesc
{
   int  limit;
   long base;
};
struct descriptor gdt[]={{0,0,0,0,0},{0xffff,0xffe0,0x10,ATTDW+0x8f00,0},{0xffff,0xf000,0xff,ATTDR,0}};
struct pdesc vgdt;
void opena20()
{
asm  push ax
asm  in al,92h
asm  or al,2
asm  out 92h,al
asm  pop ax
}

void envir_4gb()
{
    asm cli
    asm push ds
    asm mov word ptr vgdt.limit,GDTNUM*8-1
    asm xor eax,eax
    asm mov ax,ds
    asm shl eax,4
    asm xor ebx,ebx
    asm lea bx, gdt
    asm add eax,ebx
    asm mov dword ptr vgdt.base,eax
    asm lgdt qword ptr vgdt
    asm mov eax,cr0
    asm or al,1
    asm mov cr0,eax
    asm jmp virtual
virtual:
    asm mov ax,SELECTOR_1
    asm mov gs,ax
    asm mov ax,SELECTOR_2
    asm mov fs,ax
    asm and al,0feh
    asm mov cr0,eax
    asm jmp real
real:
   asm pop ds
   asm sti
}
main()
{unsigned long i,temp;
opena20();
envir_4gb();
for (i=0;i<0xf;i+=4)
{
asm mov ebx,i
asm mov eax,fs:[ebx]
asm mov temp,eax
printf("%lx   ",temp);
}
}