[bits 16]               ; Set to 16-bit mode
[org 0x7C00]           ; BIOS loads the bootloader at this address

; Define GDT
gdt_start:
    dw 0x0000          ; Limit (low)
    dw 0x0000          ; Base (low)
    db 0x00            ; Base (mid)
    db 0x9A            ; Access byte: Present, ring 0, executable
    db 0xCF            ; Flags: Granularity, 32-bit
    db 0x00            ; Base (high)

    dw 0x0000          ; Limit (low)
    dw 0x0000          ; Base (low)
    db 0x00            ; Base (mid)
    db 0x92            ; Access byte: Present, ring 0, writable
    db 0xCF            ; Flags: Granularity, 32-bit
    db 0x00            ; Base (high)

gdt_descriptor:
    dw gdt_end - gdt_start - 1   ; Limit
    dd gdt_start                   ; Base address

gdt_end:

start:
    cli                     ; Disable interrupts
    lgdt [gdt_descriptor]   ; Load GDT
    mov eax, cr0           ; Read CR0 register
    or eax, 1              ; Set PE bit (Protected Mode Enable)
    mov cr0, eax           ; Write back to CR0

    jmp 0x08:protected_mode_entry ; Far jump to flush the pipeline

protected_mode_entry:
    mov ax, 0              ; Set data segment to the new segment
    mov ds, ax             ; Load data segment register
    mov es, ax             ; Load extra segment register
    mov fs, ax             ; Load fs segment register
    mov gs, ax             ; Load gs segment register

    call clear_screen      ; Clear the screen
    mov si, welcome_message 
    call print_string_protected
    
command_loop:
    mov si, prompt_message
    call print_string_protected

    call read_input

    cmp al, 'h'      ; Check for 'h' command (help)
    je print_help
    cmp al, 'q'      ; Check for 'q' command (quit)
    je quit_terminal
    cmp al, 'f'      ; Check for 'f' command (heckerfetch)
    je heckerfetch
    
    mov si, unknown_command_message
    call print_string_protected
    jmp command_loop  ; Repeat the command loop

print_help:
    mov si, help_message
    call print_string_protected
    jmp command_loop  ; Return to command loop

heckerfetch:
    mov si, heckerfetch_message
    call print_string_protected
    jmp command_loop  ; Return to command loop

quit_terminal:
    mov si, goodbye_message
    call print_string_protected
    jmp hang          ; Hang the system

; Function to read a character from keyboard input
read_input:
    xor eax, eax       ; Clear EAX for reading character input
    int 0x16          ; Call BIOS interrupt to read a character into AL
    ret                ; Return with character in AL

; Function to print a string in protected mode using VGA text buffer
print_string_protected:
    mov edi, 0xb8000     ; VGA text buffer address
.next_char:
    lodsb                ; Load byte at DS:SI into AL and increment SI
    cmp al, 0           ; Check for null terminator
    je .done            ; If null, we are done
    mov [edi], ax       ; Write character and attribute (white on black)
    add edi, 2          ; Move to next character cell
    jmp .next_char      ; Repeat for next character
.done:
    ret                  ; Return from function

; Function to clear the screen in protected mode
clear_screen:
   xor di, di           ; Start from the beginning of the VGA buffer 
   mov cx, 80 * 25      ; Number of characters in a standard text mode screen 
   mov ax, 0x07        ; Attribute for white on black background 
.fill_screen:
   stosw                ; Store word in [ES:DI] and increment DI by 2 
   loop .fill_screen     ; Repeat until CX is zero 
   ret                  ; Return from function

; Data section for messages 
welcome_message db 'Welcome to the HeckerOS!', 0 
prompt_message db 'Enter command (h for help, q to quit, f for heckerfetch): ', 0 
help_message db 'Available commands: h (help), q (quit), f (heckerfetch)', 0 
unknown_command_message db 'Unknown command! Try again.', 0 
goodbye_message db 'Goodbye!', 0 
heckerfetch_message db 'OS: HeckerOS Beta, CPU: HeckerSoft Generic CPU 0GHZ', 'Host: HeckerSoft Unknown Host', 0 

; Bootloader signature (must be present) 
times 510 - ($ - $$) db 0   ; Fill the rest of the sector with zeros 
dw 0xAA55                   ; Boot signature 

hang:
   jmp hang           ; Jump to hang indefinitely