format MZ heap 0 entry fos_seg:start segment fos_seg use32 int32_routine: db 66h,0E8h ; 16 bit call(equivalent to push 16-bit int number) rb 255*4 dw 0 int32_vect: rb 256*8 use32 int0_hand: ; int 0 had to be put here push word 0 ; because the lowest call statement would return 4 intx_jmp: cli inc word [gs:0] ; Flicker something on the screen push eax mov ax,2 shl 3 ; Set es to code segment mov es,eax pop dword [es:setax-4] ; Save current eax pop word [es:teax2-2] ; Save interrupt number pop dword [es:ijump-6] ; Save return jump pop word [es:ijump-2] pop ax ; Discard hi word from seg mov [es:setsp-4],esp ; Save stack pointer pop word [es:setf-2] ; Save 16 bit flags jmp 3 shl 3:pm16_start pm16_start: use16 mov eax,cr0 dec ax ; Switch back to real mode mov cr0,eax jmp fos_seg:pm16_end ; Set cs to 16-bit real mode pm16_end: lidt [cs:real16_IDTR] ; Load old interrupt table mov ax,cs ; Setup 16bit stack mov ss,ax mov ax,stend mov sp,ax mov eax,0 ; Retreive save eax setax: push word 0 ; Retreive calling flags setf: push cs ; Emulate 16 bit interrupt push word teax2.real16 push dword [0] ; Jump to 16 bit vector teax2: retfw .real16: pushfw ; Save return flags(software ints) cli mov [cs:setax2-4],eax ; Save return eax pop word [cs:setf2-2] ; Save flags for later mov eax,cr0 inc ax ; Switch to protected mode mov cr0,eax jmp 1 shl 3:pm32_start2 ; Jump back to 32-bit protected mode pm32_start2: use32 mov ax,4 shl 3 ; Restore ds, es, and ss mov ds,eax ; these and ss are the only segs mov es,eax ; that we care about mov ss,eax mov esp,0 setsp: mov ax,0 setf2: mov [esp],ax ; Restore return flags lidt [cs:real32_IDTR] ; Lets go back to 32 bit ints popf ; Restore flags mov eax,0 ; Restore return eax setax2: jmp 0:0 ; Switching from real/protected makes ijump: ; the cpu forget what typ of int, so ; we have to do a jump instead align 8 real32_GDT: dw 5*8-1 ; limit of GDT real32_GDT_address dd real32_GDT ; linear address of GDT dw 0 ; padding for null descriptor dw 0FFFFh,0,9A00h,0CFh ; 4 GB 32-bit fos_seg code descriptor dw 0FFFFh,0,9200h,0CFh ; 4 GB 32-bit fos_seg data descriptor dw 0FFFFh,0,9A00h,08Fh ; 4 GB 16-bit fos_seg code descriptor dw 0FFFFh,0,9200h,0CFh ; 4 GB data descriptor base 0 label real32_IDTR pword ; pointer to 32 bit int vector real32_IDT_limit dw 7FFh real32_IDT_base dd int32_vect label real16_IDTR pword ; pointer to 16 bit int vector real16_IDT_limit dw 3FFh real16_IDT_base dd 0 use16 start: mov ax,7202h ; check for 386 push ax popf pushf pop bx cmp ax,bx je processor_ok call no_386 db '80386 or better required$' no_386: push cs pop ds pop dx mov ah,9 ; Print Message int 21h mov ax,0x4CFF ; Quit program int 21h processor_ok: xor eax,eax mov ax,cs ; calculate linear address of GDT mov ds,ax ; Set DS to fos_seg shl eax,4 or [real32_GDT+10],eax ; Adjust GDT pointer to fos_seg or [real32_GDT+10+8],eax or [real32_GDT+10+16],eax add [real32_GDT_address],eax add [real32_IDT_base],eax ; IDT has to be adjusted as well add [initstack-4],eax ; As well as the stack mov cx,100h-1 mov bx,int32_vect mov word [bx],int0_hand ; Fill int 0 vector mov word [bx+2],1 shl 3 ; Fos_seg mov dword [bx+4],0x8e00 ; Present, DPL=0, 32 bit, 0 for 16-31 of offset xor di,di mov eax,0e8660000h+intx_jmp-4 ; 16 bit call-code init_interrupts: add bx,8 mov word [bx],di ; Offset of currect int routine mov word [bx+2],1 shl 3 ; Fos_seg mov dword [bx+4],0x8e00 ; Present, DPL=0, 32 bit, 0 for 16-31 of offset mov [di+2],eax ; Our call routine code add di,4 ; Next vector sub ax,4 ; Adjust call code loop init_interrupts ; Next interrupt mov ax,0xb800 ; Used for storing on screen mov gs,ax cli lidt [cs:real32_IDTR] ; load 32 bit int table lgdt [cs:real32_GDT] ; load GDT register mov eax,cr0 ; switch to protected modes inc ax mov cr0,eax jmp 1 shl 3:pm32_start ; jump to protected mode pm32_start: use32 mov eax,4 shl 3 ; load 32-bit data descriptor mov ss,eax mov esp,stend2 initstack: mov es,eax ; set our datasegment resgisters mov ds,eax sti ; restore the interrupts and go for it .l9: inc word [gs:2] bt word [0x417],1 ; Check if left shift is pressed jnc .l9 mov ax,0x4c00 ; If so, end program int 21h ststart: ; Our 16 bit stack rb 4024 stend: ; Our 32 bit stack rb 4024 stend2: