Create RealToProtectedExampleOsdever.nasm

RealToProtectedExampleOsdever.nasm

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+[BITS 16]       ; We need 16-bit intructions for Real mode
+
+[ORG 0x7C00]    ; The BIOS loads the boot sector into memory location 0x7C00
+
+        cli                     ; Disable interrupts, we want to be alone
+
+        xor ax, ax
+        mov ds, ax              ; Set DS-register to 0 - used by lgdt
+
+        lgdt [gdt_desc]         ; Load the GDT descriptor
+
+        mov eax, cr0            ; Copy the contents of CR0 into EAX
+        or eax, 1               ; Set bit 0
+        mov cr0, eax            ; Copy the contents of EAX into CR0
+
+        jmp 08h:clear_pipe      ; Jump to code segment, offset clear_pipe
+
+
+[BITS 32]                       ; We now need 32-bit instructions
+clear_pipe:
+        mov ax, 10h             ; Save data segment identifyer
+        mov ds, ax              ; Move a valid data segment into the data segment register
+        mov ss, ax              ; Move a valid data segment into the stack segment register
+        mov esp, 090000h        ; Move the stack pointer to 090000h
+
+        mov byte [ds:0B8000h], 'P'      ; Move the ASCII-code of 'P' into first video memory
+        mov byte [ds:0B8001h], 1Bh      ; Assign a color code
+
+hang:
+        jmp hang                ; Loop, self-jump
+
+
+gdt:                    ; Address for the GDT
+
+gdt_null:               ; Null Segment
+        dd 0
+        dd 0
+
+gdt_code:               ; Code segment, read/execute, nonconforming
+        dw 0FFFFh
+        dw 0
+        db 0
+        db 10011010b
+        db 11001111b
+        db 0
+
+gdt_data:               ; Data segment, read/write, expand down
+        dw 0FFFFh
+        dw 0
+        db 0
+        db 10010010b
+        db 11001111b
+        db 0
+
+gdt_end:                ; Used to calculate the size of the GDT
+
+
+
+gdt_desc:                       ; The GDT descriptor
+        dw gdt_end - gdt - 1    ; Limit (size)
+        dd gdt                  ; Address of the GDT
+
+
+
+
+times 510-($-$$) db 0           ; Fill up the file with zeros
+
+        dw 0AA55h                ; Boot sector identifyer