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// this function is in physical addr 0x0A00 ~ 0x1000.
// inline asm is turbo asm.
// C compiler is Borland C++ 3.1
// Real Mode Data Segment Value is 0x0000.
// Real Mode Code Segment Value is 0x0000.
// Real Mode Stack Segment Value is 0x0000.
// Stack is valid in this function.( SP is initialized 0xFFFF in other function.)

// this is linked with following.asm 
//.model tiny
//.code
//org 0A00h
//end

#define jmpn(offset)         db 0xEB , offset;
#define jmpf(segment,offset) db 0xEA; dw offset; dw segment;
#define HALT                 db 0xEB , 0xFE;

void protected_mode()
{

    asm cli;

    init_GDT(0x600);// initialize and load( asm lgdt ) GDT at physical addr 0x600
                          // index 0 : null descriptor 
                          // index 1 : 16bit code descriptor , addr 0x00 ~ 0xFFFF, excute_read
                          // index 2 : 16bit data descriptor , addr 0x00 ~ 0xFFFF, read_write

    asm {
    first_real_16 label;

        smsw    ax;
        or      ax,0x0001;
        lmsw    ax;
        jmpn    (0x00);
        jmpf    (0x0008,protected_16); // use selector on segment arg

    protected_16 label;

    ready_real label;

        mov     ax, 0x0010;    	       // selector index 2
        mov     ds, ax;
        mov     es, ax;
        db 0x8E, 0xE0;                 // mov fs, ax;
        db 0x8E, 0xE8;                 // mov gs, ax;
        mov     ss, ax;

        lidt    real_mode_IDTR;        // size 1024(4*256), addr 0x00000000

        smsw    ax;
        and     ax, 0xFFFE;
        lmsw    ax;

        jmpf    (0x0000, second_real_16);  /*  system is rebooted this line */

        HALT; // for debug , if problem is not , halt.

    second_real_16 label;

        mov     ax, 0;
        mov     ds, ax;
        mov     es, ax;
        db 0x8E, 0xE0;                 // mov fs, ax;
        db 0x8E, 0xE8;                 // mov gs, ax;
        mov     ss, ax;
    }

    asm sti;
}