boot.asm

; Time to enter long mode
    xchg bx, bx
    ; First disable paging
    mov eax, cr0                                   
    and eax, 01111111111111111111111111111111b     ; Clear the PG-bit, which is bit 31.
    mov cr0, eax
    
    ; Clear page tables
    mov edi, 0x1000    ; Set the destination index to 0x1000.
    mov cr3, edi       ; Set control register 3 to the destination index.
    xor eax, eax       ; Nullify the A-register.
    mov ecx, 4096      ; Set the C-register to 4096.
    rep stosd          ; Clear the memory.
    mov edi, cr3       ; Set the destination index to control register 3.

    mov DWORD [edi], 0x2003      ; Set the uint32_t at the destination index to 0x2003.
    add edi, 0x1000              ; Add 0x1000 to the destination index.
    mov DWORD [edi], 0x3003      ; Set the uint32_t at the destination index to 0x3003.
    add edi, 0x1000              ; Add 0x1000 to the destination index.
    mov DWORD [edi], 0x83        ; 2 MiB RW @ 0x0
    add edi, 8                   ; Move over 8 bytes
    mov DWORD [edi], 0x200083    ; 2 MiB RW @ 0x200000

    ; Identity map the first 4 MiB
    ; Switch to PSE and only map a single page?
;    mov ebx, 0b00000011          ; Set the B-register to 0b10000011.
;    mov ecx, 2                   ; Set the C-register to 2.
;.SetEntry:
;    mov DWORD [edi], ebx         ; Set the uint32_t at the destination index to the B-register.
;    add ebx, 0x200000            ; Add 0x200000 to the B-register.
;    add edi, 8                   ; Add eight to the destination index.
;    loop .SetEntry               ; Set the next entry.

    ; Enable PAE paging
    mov eax, cr4                 ; Set the A-register to control register 4.
    or eax, 1 << 5               ; Set the PAE-bit, which is the 6th bit (bit 5).
    ; PSE is on by default for PAE
    or eax, 1 << 4               ; Set the PSE-bit, which is the 2nd bit (bit 1).
    mov cr4, eax                 ; Set control register 4 to the A-register.

    ; Set LongMode bit
    mov ecx, 0xC0000080          ; Set the C-register to 0xC0000080, which is the EFER MSR.
    rdmsr                        ; Read from the model-specific register.
    or eax, 1 << 8               ; Set the LM-bit which is the 9th bit (bit 8).
    wrmsr                        ; Write to the model-specific register.

    ; Enable paging
    mov eax, cr0                 ; Set the A-register to control register 0.
    or eax, 1 << 31              ; Set the PG-bit, which is the 32nd bit (bit 31).
    mov cr0, eax                 ; Set control register 0 to the A-register.
    
    ; Load 64-bit gdt
    lgdt [gdt64.Pointer]         ; Load the 64-bit global descriptor table.
    jmp gdt64.Code:entry64       ; Set the code segment and enter 64-bit long mode.