GDT_MEMORY_AREA equ 0x7e00
GDT_ENTRY_COUNT equ 3



; Arguments (note: all arguments are 4-byte padded):
;   [Furthest from EBP]
;
;    4. uint32       privilege_level
;       - Only 0 to 3; 0 being kernel and 3 being userland.
;         Everything above 3 is an error.
;       
;    3  uint32      num_pages
;       - Only 20 bits are usable. Everything above will
;         be cut off to be written to the segment descriptor.
;
;    2. ptr32       base_address
;
;    1. uint32      segment_index
;       Note:
;       - Zero is NOT usable and will throw an error.
;         the highest theoretical value for this is 8191,
;         but generally, one should stick to GDT_ENTRY_COUNT.
;
;   [Nearest to EBP]
make_regular_data_segment_in_gdt:
.prolog:
    push esi
    sub esp, 64
    mov esi, esp

    mov [esi + (64 - 4)], eax
    mov [esi + (64 - 8)], ebx
    mov [esi + (64 - 12)], ecx
    mov [esi + (64 - 16)], edx
    mov [esi + (64 - 20)], edi

.get_arguments:
    mov eax, [ebp - 4]  ; segment_index
    mov ebx, [ebp - 8]  ; base_address
    mov ecx, [ebp - 12] ; num_pages
    mov edx, [ebp - 16] ; privilege_level

    mov [esi + 16], eax ; segment_index
    mov [esi + 20], ebx ; base_address
    mov [esi + 24], ecx ; num_pages
    mov [esi + 28], edx ; privilege_level

.validate_arguments:
    cmp eax, 8191
    ja .segment_index_too_tall
    cmp eax, 0
    je .segment_index_is_zero

    cmp edx, 3
    ja .privilege_level_too_large

.calculate_work_address:
    shl eax, 3 ; Entry offset from table start
    add eax, GDT_MEMORY_AREA ; Entry address
    mov [esi + 12], eax

.write_segment_limit:
    mov [eax], cx
    shr ecx, 16
    and ecx, 0x0f
    mov [eax + 6], cl

.write_base_address:
    mov [eax + 2], bx
    shr ebx, 16
    mov [eax + 4], bl
    mov [eax + 7], bh

.set_default_flags:
    or [eax + 6], byte (0b1100 << 4)

.set_data_segment_specific_access_byte:
    mov al, 0b1001
    ; OR the privilege level into the access byte
    mov dl, [ebp - 16]
    shl dl, 1
    and dl, 0b0110
    or al, dl
    shl al, 4 ; Move the 4 bits of general flags to their place

    ; Data-segment specific part
    or al, 0b0010 ; DATA | HEAP | READ_WRITE | NOT_ACCESSED

    mov ebx, [esi + 12]
    mov [ebx + 5], al ; Write to the final destination

.epilog:
    mov edi, [esi + (64 - 20)]
    mov edx, [esi + (64 - 16)]
    mov ecx, [esi + (64 - 12)]
    mov ebx, [esi + (64 - 8)]
    mov eax, [esi + (64 - 4)]

    add esp, 64
    pop esi
    ret

.segment_index_too_tall:
.segment_index_is_zero:
.privilege_level_too_large:
    ; todo(logs): Write debug message.
    cli
    hlt



; Arguments (note: all arguments are 4-byte padded):
;   [Furthest from EBP]
;
;    4. uint32       privilege_level
;       - Only 0 to 3; 0 being kernel and 3 being userland.
;         Everything above 3 is an error.
;       
;    3  uint32      num_pages
;       - Only 20 bits are usable. Everything above will
;         be cut off to be written to the segment descriptor.
;
;    2. ptr32       base_address
;
;    1. uint32      segment_index
;       Note:
;       - Zero is NOT usable and will throw an error.
;         the highest theoretical value for this is 8191,
;         but generally, one should stick to GDT_ENTRY_COUNT.
;
;   [Nearest to EBP]
make_code_segment_in_gdt:
.prolog:
    push esi
    sub esp, 64
    mov esi, esp

    mov [esi + (64 - 4)], eax
    mov [esi + (64 - 8)], ebx
    mov [esi + (64 - 12)], ecx
    mov [esi + (64 - 16)], edx
    mov [esi + (64 - 20)], edi

.get_arguments:
    mov eax, [ebp - 4]  ; segment_index
    mov ebx, [ebp - 8]  ; base_address
    mov ecx, [ebp - 12] ; num_pages
    mov edx, [ebp - 16] ; privilege_level

    mov [esi + 16], eax ; segment_index
    mov [esi + 20], ebx ; base_address
    mov [esi + 24], ecx ; num_pages
    mov [esi + 28], edx ; privilege_level

.validate_arguments:
    cmp eax, 8191
    ja .segment_index_too_tall
    cmp eax, 0
    je .segment_index_is_zero

    cmp edx, 3
    ja .privilege_level_too_large

.calculate_work_address:
    shl eax, 3 ; Entry offset from table start
    add eax, GDT_MEMORY_AREA ; Entry address
    mov [esi + 12], eax

.write_segment_limit:
    mov [eax], cx
    shr ecx, 16
    and ecx, 0x0f
    mov [eax + 6], cl

.write_base_address:
    mov [eax + 2], bx
    shr ebx, 16
    mov [eax + 4], bl
    mov [eax + 7], bh

.set_default_flags:
    or [eax + 6], word (0b1100 << 4)

.set_code_segment_specific_access_byte:
    mov al, 0b1001
    ; OR the privilege level into the access byte
    mov dl, [ebp - 16]
    shl dl, 1
    and dl, 0b0110
    or al, dl
    shl al, 4 ; Move the 4 bits of general flags to their place

    ; Code-segment specific part
    or al, 0b1010 ; CODE | NON-CONFORMING | READABLE | NOT_ACCESSED

    mov ebx, [esi + 12]
    mov [ebx + 5], al ; Write to the final destination

.epilog:
    mov edi, [esi + (64 - 20)]
    mov edx, [esi + (64 - 16)]
    mov ecx, [esi + (64 - 12)]
    mov ebx, [esi + (64 - 8)]
    mov eax, [esi + (64 - 4)]

    add esp, 64
    pop esi
    ret

.segment_index_too_tall:
.segment_index_is_zero:
.privilege_level_too_large:
    ; todo(logs): Write debug message.
    cli
    hlt



; [Furthest to EBP]
;   1)  Return Address
; [Nearest to EBP]
load_flat_gdt:
.prolog:
    push esi
    sub esp, 64
    mov esi, esp

    mov [esi + (64 - 4)], eax
    mov [esi + (64 - 8)], ebx
    mov [esi + (64 - 12)], ecx
    mov [esi + (64 - 16)], edx
    mov [esi + (64 - 20)], edi

    ; mov eax, [ebp - 4]
    ; mov [esi], eax

.set_segments:
    push ebp
    mov ebp, esp
    push dword 2            ; Segment Index
    push dword 0            ; Start Offset
    push dword 0x000fffff   ; Page Count
    push dword 0            ; Required privilege level
    call make_regular_data_segment_in_gdt
    mov esp, ebp
    pop ebp

    push ebp
    mov ebp, esp
    push dword 1            ; Segment Index
    push dword 0x0000       ; Start Offset
    push dword 0x000fffff   ; Page Count
    push dword 0            ; Required privilege level
    call make_code_segment_in_gdt
    mov esp, ebp
    pop ebp

    mov bx, 0
    mov ds, bx
    mov es, bx
    mov fs, bx
    mov gs, bx
    mov ss, bx
    lgdt [.gdtr]

    mov ebx, cr0
    or ebx, 1
    mov cr0, ebx

    jmp dword (1 << 3):.epilog

bits 32
.epilog:
    mov bx, (2 << 3)
    mov ds, bx
    mov es, bx
    mov fs, bx
    mov gs, bx
    mov ss, bx

    mov edi, [esi + (64 - 20)]
    mov edx, [esi + (64 - 16)]
    mov ecx, [esi + (64 - 12)]
    mov ebx, [esi + (64 - 8)]
    mov eax, [esi + (64 - 4)]

    jmp dword stage2_protected_entry

align 16
.gdtr:
.gdtr.length:
    dw 23
.gdtr.pointer:
    dd GDT_MEMORY_AREA

align 1