通常在进入保护模式时要关闭所有的中断，把IDTR的界限设置为0，CPU自动关闭所有中断，包括NMI，返回实模式后恢复IDTR并开中断。
另外A20地址线的控制对于正确访问整个内存也很重要，在进入保护模式前要让8042打开A20地址线。
在这个例子里FS段寄存器设成可访问4GB内存的基址和界限，由于在DOS中很少有程序会用到GS、FS这两个386增加的段寄存器，当要读写4GB范围中的任一个地方都可通过FS段来达到，直到FS在实模式下被重装入冲掉为止。
这个例子在386SX、386DX、486上都运行通过。例子里加有十分详细的注释，由于这一程序是用BC 3.1编译连接的，而其连接器不能为DOS程序处理32位寄存器，所以直接在代码中加入操作码前缀0x66和地址前缀0x67，以便让DOS实模式下的16位程序可用32位寄存器和地址。程序的右边以注释形式给出等效的32位指令。要注意16位的指令中mov al, byte ptr [BX]的指令码正好是32位的指令mov al, byte ptr[EDI]。
读者可用这个程序验证BIOS是否同时在两个区域出现。如果有线性定址能力的VESA显示卡(如TVGA9440)还可进一步验证线性显示缓冲区在1MB之上的工作情况。
#include
unsigned long GDT-Table[]=
{0,0, //NULL - 00H
0x0000FFFF,0x00CF9A00, //Code32 - 08h Base=0 Limit=4G-1 Size=4G
0x0000FFFF,0x00CF9200 //Data32 - 10h Base=0 Limit=4G-1 Size=4G
};
unsigned char OldIDT [6]={0}; //Save The IDTR before Enter Protect Mode.
unsigned char pdescr-tmp [6]={0}; //NULL The IDTR s Limit=0 CPU will
// disable all Interrupts, include NMI.
#define KeyWait() {while(inportb(0x64) &2);}
void A20Enable(void)
{
keyWait ();
outportb(0x64,0xD1);
KeyWait();
outportb(0x60,0xDF); //Enable A20 with 8042.
KeyWait();
outportb(0x64,0xFF);
KeyWait ();
}
void LoadFSLimit4G(void)
{
A20Enable (); //Enable A20
//***
Disable ints & Null IDT
//***
asm {
CLI //Disable inerrupts
SIDT OldIDT //Save OLD IDTR
LIDT pdescr-tmp //Set up empty IDT.Disable any interrupts,
} // Include NMI.
//***
Lodd GDTR
//***
asm{ // The right Code is Real, But BC++ s Linker NOT
// Work with 32bits Code.
db 0x66 //32 bit Operation Prefix in 16 Bit DOS.
MOV CX,DS //MOV ECX,DS
db 0x66 //Get Data segment physical Address
SHL CX,4 //SHL ECX,4
MOV word ptr pdescr-tmp [0],(3*8-1)
//MOV word ptr pdescr-tmp [0], (3*8-1)
db 0x66
XOR AX,AX //XOR EAX,EAX
MOV AX,offset GDT-Table
// MOV AX,offset GDT-Table
db 0x66
ADD AX,CX //ADD EAX,ECX
MOV word ptr pdescr-tmp [2], AX
//GDTR Base low16 bits
db 0x66
SHR AX,16 //SHR EAX,16
MOV word ptr pdescr-tmp [4],AX
//GDTR Base high16 bits
LGDT pdescr-tmp //Load GDTR
}
//****
//* Enter 32 bit Flat Protected Mode
//****
asm{
mov DX,0x10 // The Data32 Selector
db 0x66,0x0F,0x20,0xC0 // MOV EAX,CR0
db 0x66
MOV BX,AX // MOV EBX,EAX
OR AX,1
db 0x66,0x0F,0x22,0xC0
//MOV CRO,EAX // Set Protection enable bit
JMP Flsuh
} //Clear machine perform cache.
flush: // Now In Flat Mode, But The CS is Real Mode Value.
asm { //And it s attrib is 16Bit Code Segment.
db 0x66
MOV AX,BX //MOV EAX,EBX
db 0x8E,0xE2 //MOV FS,DX
//Load FS Base=0 Size=4G now
db 0x66,0x0F,0x22,0xC0 //MOV CRO,EAX
//Return Real Mode.
LIDT OldIDT //LIDT OldIDT //Restore IDTR
STI // STI //Enable INTR
}
}
unsigned char ReadByte (unsigned long Address)
{
asm db 0x66
asm mov di,word ptr Address // MOV EDI, Address
asm db 0x67 //32 bit Address Prefix
asm db 0x64 //FS:
asm mov al,byte ptr [BX] // =MOV AL, FS: [EDI]
return -AL;
}
unsigned char WriteByte(unsigned Long Address)
{
asm db 0x66
asm mov di,word ptr Address //MOV EDI, Address
asm db 0x67 //32 bit Address Prefix
asm db 0x64 //FS:
asm mov byte ptr [BX],al //=MOV FS: [EDI],AL
return -AL;
}
//////// Don t Touch Above Code ///
# include
void Dump4G (unsigned long Address)
{
int i;
int j;
for (i=0; i