; Copyright 2022 Mattia Giambirtone & Contributors
;
; Licensed under the Apache License, Version 2.0 (the "License");
; you may not use this file except in compliance with the License.
; You may obtain a copy of the License at
;
; http://www.apache.org/licenses/LICENSE-2.0
;
; Unless required by applicable law or agreed to in writing, software
; distributed under the License is distributed on an "AS IS" BASIS,
; WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
; See the License for the specific language governing permissions and
; limitations under the License.

; Utilities to switch to protected mode and load our GDT

[bits 16]
switch_to_protected_mode:
    cli ; Now we disable interrupts

    lgdt [gdt_descriptor] ; Time to load the GDT descriptor
    ; We set 32-bit mode in CR0. Almost done!
    mov eax, cr0
    or eax, 0x1
    mov cr0, eax
    jmp CODE_SEG:switch32 ; At last, we perform a far jump into a different segment


[bits 32]
switch32: ; We're not in 32 bit mode, yay!
    mov ax, DATA_SEG ; Time to update the segment registers with their new values
    mov ds, ax
    mov ss, ax
    mov es, ax
    ; Bonus: we have two more user-defined
    ; segment registers when in 32-bit mode
    mov fs, ax
    mov gs, ax

    mov ebp, 0x1f8400 ; We also make the stack larger (2MiB)
    mov esp, ebp
    call BEGIN_32BIT ; We call back into mbr.s which loads the kernel