switch_to_pm.asm

[bits 16]
switch_to_pm:
    cli ; disable interrupts (BIOS Interrupts are no longer valid in 32 bit protected mode)
    lgdt [gdt_descriptor]

    mov eax , cr0 ; To make the switch to protected mode , we set
    or eax , 0x1 ; the first bit of CR0 , a control register
    mov cr0 , eax ; Update the control register

    ; Make a far jump (i.e. to a new segment) to our 32-bit
    ; code. This also forces the CPU to flush its cache of
    ; pre -fetched and real -mode decoded instructions , which can
    ; cause problems.
    jmp CODE_SEG:init_pm 

[bits 32]
; Initialise registers and the stack once in PM.
init_pm:
    mov ax, DATA_SEG ; Now in PM, our old segments are meaningless ,
    mov ds, ax ; so we point our segment registers to the
    mov ss, ax ; data selector we defined in our GDT
    mov es, ax
    mov fs, ax
    mov gs, ax

    mov ebp , 0x90000 ; Update our stack position so it is right
    mov esp , ebp ; at the top of the free space.
    
    call BEGIN_PM